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Article

Geochemistry, Mineralogy and Textural Properties of the Lower Globigerina Limestone Used in the Built Heritage

by
Lino Bianco
1,2
1
Faculty for the Built Environment, University of Malta, MSD 2080 Msida, Malta
2
Faculty of Architecture, University of Architecture, Civil Engineering and Geodesy, 1 Hristo Smirnenski Blvd., 1046 Sofia, Bulgaria
Minerals 2021, 11(7), 740; https://doi.org/10.3390/min11070740
Submission received: 17 May 2021 / Revised: 22 June 2021 / Accepted: 25 June 2021 / Published: 7 July 2021

Abstract

:
The Lower Globigerina Limestone Member, the oldest member of the Globigerina Limestone Formation, outcrops over most of the Maltese archipelago, notably Malta. It has provided the islands’ main building material since the Neolithic period. This paper makes available a corpus of findings relating to the geochemistry, mineralogy and textural properties of this limestone—mostly unpublished and undertaken nearly three decades ago—which provide a useful source to understand its behavior. Bulk chemistry and mineralogy showed that non-carbonate and clay content is higher in limestone of inferior quality. Textural analyses gave insight into the fabric of the matrix, including inter- and intra-particle porosity. These analyses were supplemented by an array of petrophysical tests, including color (a parameter which has a correlation with density and Fe2O3 content), ultrasonic pulse velocity and compressive strength. The findings not only give insight into the composition of the limestone, using insoluble residue content of ≥5% as the threshold of inferior quality lithotype, but provided an insight into the physico-mechanical bonding present, a characteristic which has a bearing on the deterioration of this limestone.

1. Introduction

What primarily impressed the English poet Samuel Taylor Coleridge (1772–1834), when he travelled to Malta in 1804 for a two-year posting as undersecretary (and later public secretary) to the Civil Commissioner of Malta, was the shining buildings erected in “sand-free stone” around the Grand Harbour of Malta [1]. As Blouet recounts, “As he starts to explore Valletta and the urban environment around the harbors, Coleridge is bewildered at the Maltese use of limestone for every kind of construction—house, street, and wall—a centuries-old tradition born from the lack of other natural resources on the island [2]” [3] (p. 6).
The sand-free stone which Coleridge makes reference to is the Lower Globigerina Limestone (LGL), the honey-colored stone in which the built legacy of the Maltese Archipelago is realized (Figure 1). A description of the islands penned by Johannes Quintinus D’Autun (1500–1561) and published in Lyon in 1536, noted that masons in Malta “make good use of the island’s stone for building purposes The Maltese stone is … remarkable for its softness; it is worked easily, but it is not strong enough against moistures and the sea-breeze” [4] (pp. 37–39).
The LGL is one of the soft limestones outcropping in the Mediterranean Basin. Often referred to as “Malta stone”, it is the main industrial mineral terrestrial resource of the Maltese Islands. It is easy to quarry—it is “soft, and easy to cut into any shape” [5] (p. 46), and outcrops over two thirds of mainland Malta [6], the main island of the Maltese Archipelago. With a superficial area of approximately 316 km2, this archipelago is located circa 93 km south of Sicily and 288 km north of Libya, and comprises several islands, the inhabited ones being Malta, Gozo and Comino.

1.1. Utilization of Lower Globigerina Limestone in Buildings

LGL has been quarried since the Neolithic period (3600–2500 BC), and is still the principal source of limestone for buildings on the Maltese Islands—whether for the construction of contemporary architectural and infrastructural works or for the restoration of built heritage. It was utilized by builders flourishing during one of the oldest Neolithic civilizations of the Mediterranean Basin [7,8,9,10,11]. Archeological structures dating from this period illustrate that those who built them differentiated between the softer, less durable LGL and the harder, denser, stronger and more durable varieties of Coralline Limestone (CL) used for external walls, which were uncarved. In contemporary minerals planning practice in Malta, LGL and CL are referred to softstone and hardstone, respectively.
Malta’s numerous free-standing megalithic structures dating from the Neolithic period, a number of which are listed as UNESCO World Heritage sites [12] (Figure 2a) are examples of anthropogenic geodiversity par excellence [13]. The softness of LGL implies that it is easily hewed; as a result, there are many hypogea and catacombs carved into outcrops throughout the islands, most notably the Ħal Saflieni Hypogeum (c. 2500 BC) [14] and St Paul’s Catacombs (early centuries of Christianity and used until seventh or eighth century AD) [15].
LGL was used in Mdina, the old capital city which originally formed part of the ancient of city of Melite (Greek: Μελίτη; Maleth under the Phoenicians [16]), and in Valletta, the Late Renaissance capital, which was founded in 1566. Laid out as a Hippodamian grid on a peninsula between two natural harbors [17], Valletta boasts an ensemble of 320 monuments within a superficial area of 55 ha (0.56 km2) (Figure 2b). This entire area is registered as a UNESCO World Heritage Site [18]. LGL was used in Malta not only in buildings and monuments (see, for example, [19,20]), but also in the erection of fortifications surrounding Mdina [21], Valletta [22,23] and Cottonera [23]. The Hospitaller Order of St John (1530–1798) made extensive use of Globigerina limestone from local quarries throughout the duration of their stay [24,25].
The structural engineering properties of LGL were tested to the limit with the erection of the Neoclassical Rotunda of Mosta, one of the largest unsupported masonry domes in the history of building construction [26,27]. During the early years of the Order’s presence in Malta, CL was used in the production of small works such as mill stones, with one such quarry operating in Birgu [28]. By the late seventeenth century, it was used at sites exposed to sea-spray. For example, the exterior walls of the bastions facing Marsamxett Harbor were erected in CL, as directed by the Flemish architect and military engineer Carlo Grunenburg [22]. The lower part was constructed in dimension stones from highly durable CL extracted from the San Leonardo’s Beds [29]. Hardstone from these beds was used for the drums of the columns of the gateways to the Grandmaster’s Palace (Figure 2b), which were added in the mid-eighteenth century [29].
During the British occupation of the islands (1800–1964), LGL was exported to other countries bordering the Mediterranean [31,32,33]. Noting the importance of this limestone for restoration and conservation of the cultural heritage of the islands, when applying for Malta’s accession to the European Union, the Government of the Republic of Malta—which had prohibited the export of Maltese stone—put forward a case for the retention of the status quo in this regard [34]. An internal policy on the utilization of Malta stone was subsequently drafted—but never implemented—to conserve and regulate in situ mineral reserves of LGL suitable for the preservation of cultural patrimony [35]. Following a nomination by the University of Malta [36], LGL was designated as a Global Heritage Stone Resource in 2019, a designation supported by the International Union of Geological Sciences. Traditionally, the waste generated in quarrying and forming LGL dimension stones was negligible, as all was utilized as a product in the building industry: roughly cut stone as infill for walls, for levelling land and for sub-bases, whilst LGL powder was used in the production of mortar. LGL obtained from demolitions was often recycled for same uses. Today, it is utilized in polymer cement mortar [37,38] and reconstituted masonry products as an alternative to natural limestone and concrete blocks [39].

1.2. Aims of the Study

The author was the first who comprehensively studied Malta’s LGL, as a topic for his Master’s thesis in 1992–93 at the University of Leicester, UK, under the academic supervision of Dr Hugh Martyn Pedley [40]. The present article makes available the corpus data from this postgraduate research project, which has never been published in its entirety. It specifically addresses the geochemical, mineralogical, textural and physico-mechanical properties of LGL—all vital to understanding the behavior of this limestone. These properties are “useful for an integrated, holistic approach to natural limestone selection, a critical factor to identify sources either for stone replacement in … architecture or determine the preservation and/or conservation interventions required. They establish existing limestone resources compatible with the fabric used in a given monument” [41] (p. 9). They are of paramount importance in determining which limestone should be utilized in any conservation intervention on cultural heritage [42,43]. The aims of the study were to establish empirically whether (i) clay weakens the fabric and encourages weathering, (ii) bioturbation is a useful indicator to establish the quality of LGL, (iii) bioturbation introduces weaknesses and unstable material into the fabric, and (iv) color and sound are indicators of quality of LGL. The study included discussions on natural weathering (subsequently published as [44]) and on the potential applications of limestone.

2. Geological Setting

2.1. General Lithostratigraphy

The earliest literature on the stratigraphy of the archipelago is from the nineteenth century [45,46,47,48]. This information was later restated in elementary geological texts [49,50,51]. Knowledge in this area was further refined in the second half of the twentieth century [52,53,54,55]. A new lithostratigraphical and palaeoenvironmental interpretation of the Coralline Limestone formations was published by Pedley [56].
The islands form part of the Pelagian Block, which extends from the Maltese Islands to eastern Tunisia and to the west of the Ionian Sea [57,58,59]. Situated on the eastern edge of the North African Pelagean Shelf, which is mainly calcareous and upper Triassic to late Tertiary in age, they lie on a submarine plateau that starts due south of the Ragusa Peninsula and extends to the coast of Libya and Tunisia. The depth of the shelf varies from around 90 m between Malta and Sicily, to over 1000 m before reaching the North Africa [55,60]. The shallower part of the shelf formed an epicontinental terrestrial link during the first epoch of the Quaternary, facilitating the migration of now extinct exotic fauna during the Pleistocene [61].
The stratigraphy of the islands is composed of five, mid-Tertiary, shallow marine carbonates, frequently fissured, Oligo-Miocene lithological formations, and facies capped with Quaternary deposits [6]. Ordered chronologically, starting from the earliest, the stratigraphy is composed of the following formations: Lower Coralline Limestone (LCL), Globigerina Limestone, Blue Clay, Greensand and the Upper Coralline Limestone. The stratigraphical sequence—which follows Rizzo [49] and Pedley [56]—and the corresponding thickness of the relative geological strata outcropping in Malta, is given in Table 1. The geological map of the Maltese Archipelago is reproduced in Figure 3. The LGL, the oldest member of the Globigerina Limestone Formation, is Aquitanian in age [53,62]. Isopachyte maps of the LGL are given in Pedley [63] and Pedley et al. [64]. A synthesis of the stratigraphic features of the archipelago is given in Baldassini and Di Stefano [65].

2.2. Lithostratigraphic Outline of LGL

Lithological and paleontological features of LGL vary both laterally and vertically. A comprehensive and concise description of the lithostratigraphy is given in Baldassini et al. [66]. It is composed of massively bedded, pale yellow-brown biodetrital limestones (globigerinid biomicrosparites and biomicrites). Pale cream to yellow planktonic foraminiferal packstones quickly develop into wackestones above its base [6]. It consists mainly of globigerinid planktonic foraminifera. The color is less pale in the lower part of this member, which is coarser grained, yellow-brown and strongly bioturbated, in contrast with the upper part which is fine-grained, marly and light yellow. Macrofossils are abundant: bryozoans and the echinoid Scutella in the lower part, and pectinid bivalves (predominantly Flabellipecten) and echinoids (such as Schizaster) in the upper part [66,67]. Scutella biofacies (also referred to as “Scutella Bed” or “transitional Bed”, consisting of a massive density of large flat echinoids, are present in the lowest part of the LGL member, with Schizaster/Hemiaster biofacies in the remaining part [68]. These biofacies mark the threshold between the LGL and the LCL formations, overlaying the latter. Recent research on the bio-chronostratigraphy of the LGL [66] follows the interpretation of a number of authors [53,69,70,71] who position this threshold on the upper surface of a ubiquitous hardground below the Scutella biofacies. A phosphatic bed—referred to as the “Basal Globigerina Limestone Phosphatic Bed” in Carbone et al. [70] and the “Terminal Lower Coralline Limestone Hardground” in Bennett [69,72]—covers this upper surface.
The threshold between the LGL and the overlying Middle Globigerina Limestone Member (MGL) is marked by another ubiquitous hard bed, less than 1 m thick and rich in brown phosphatic nodules (“C1”, or “Lower main conglomerate” in Pedley [54] and Pedley et al. [64]; “Qammieh bed” in Bennett [69]; “Lower Main Phosphorite Conglomerate Bed” in Pedley and Bennett [74]; “Qammieh Conglomerate Bed” in Rose et al. [71]). It marks an interruption in sedimentation [74] and occurs at the top of the LGL and at the base of the MGL [74,75]. New preliminary data regarding the bio-chronostratigraphy of the LGL is based on calcareous plankton was published by Foresi et al. [67], placing the deposition of this member between 25.1 and 24.3 Ma [66].

2.3. Geochemistry, Mineralogy and Texture of LGL

A main scientific publication addressing this theme was penned by the eminent British oceanographer, Sir John Murray (1841–1914) [48]. His analysis and interpretation of the geology of the Malta Islands was the main source in all publications appearing until the late 1960s (e.g., [19,50]. In his work, Murray relied heavily on information furnished to him by Civil Engineer Charles Henry Colson (1864–1939) who, at the time, was engaged in the construction of a new dock in Malta, which became the theme of a paper—co-authored with his father Charles Colson—presented at the 1893–1894 session of the Institution of Civil Engineers [76]. Based on the communication with Colson, classifications of the numerous beds of Lower Coralline Limestone, Globigerina Limestone, Greensand and Upper Coralline Limestone were drawn up and cited as Murray [48] by Rizzo [49] when the latter tabulated the varying terminology coined for the stratigraphic beds by different authors [46,77], including his own, which was a refined version of Murray’s. Based on this information from Colson, a classification of the strata occurring within the Globigerina Formation—inclusive of their diagnostic properties and respective uses in the building industry—was drawn up by Hughes [19]. The same information was used to tabulate, for all formations, a classification of all beds present, and their distinguishable characteristics and uses in Bianco [40] and published as Bianco [78].
The samples analyzed by Murray [48] from the Globigerina Formation had a CaCO3 content between 63.20% and 94.73%; the upper beds had 30% to 40%. In most beds, MgCO3 was present. All contained traces of Ca3(PO4)2; it was 3% to 4% in some samples. The insoluble residue consisted of clayey matter and iron oxides, glauconite, and small mineral grains—namely, quartz splinters, feldspars, augite, zircon, tourmaline, rutile and hornblende—with sizes rarely >0.1 mm in diameter. These minerals are present in insignificant quantities in the Upper Globigerina Limestone Member [54]. No feldspar, augite and hornblende were identified in the LGL.

2.4. Terminology Associated with LGL Used in the Building Trade

Through the centuries, stone masons developed their own terminology to distinguish and classify LGL, locally known in the building trade as “ġebla tal-franka”, the Maltese translation for “freestone”. Franka is a generic name. Grading LGL in terms of first- and second-quality lithotypes, a classification still widely used in the local quarrying industry, is rudimentary. Stone masons distinguish between franka and inferior quality, darker in color, less durable lithostratigraphical beds occurring within the LGL member [78]. Good quality LGL “rings” when hit by a mallet [79]. The inferior LGL is known as “sol” (or “soll”, from the old Maltese word “saul” [48]), a limestone with “ħafna frak tal-ħġieġ” (literally translated as “many glass fragments”) [79]. Such beds are present at approximately 12 m intervals in the good quality LGL [80]. Sol occurs in two varieties: “sol aħmar” (red sol) and “sol ikħal” (blue sol); the latter is also known as “ġebla l-kaħla” or “ħadra”, which translates as “the blue stone” or “green”. LGL has a specific blue-colored bed of sol ikħal, which is dark grey in color when freshly extracted but dries rapidly to pale grey when exposed [48]; it is suitable only for use in foundations and as fill [79]. Similarly colored lenticular patches are also present in this member. Based on a study of a site in Msida (UTM coordinates: 453212E, 3972483N) (Figure 1b), the mineralogy and geochemistry of these patches is quantitatively different from that of sol ikħal; their SiO2 content is >10% whilst that of sol ikħal <10% [81]. The texture of the limestone forming these patches alternates from wackestone to mudstone to bioclastic wackestone. Petrographically, these variations are linked to process/es other than limestone texture. Whilst similar to sol ikħal, they differ from sol aħmar, in which the micrite is often recrystallized to microspar [82]. For the scope of this paper, inferior LGL, also referred to as second-quality lithotype, refers to sol aħmar.

3. Materials and Methods

3.1. Sampling

The main sampling site is Tal-Warda softstone quarry at Qrendi (UTM coordinates: 451481E, 3966547N), officially known as Softstone Quarry No. 55 (Figure 1b). It is located adjacent to the Chapel of St Catherine. The site location, bordering with the locality of Mqabba, is plotted on the official aerial imagery, dated 1998, available at the Mapping Unit of the Planning Authority, Malta [83] (Figure 4). This open-pit mining site was selected because it was possible to correlate its position within the LGL. The top of the quarry is close to upper horizon of the LGL, that is, the base of the lower main phosphorite bed, a fact confirmed by infrastructure works which were being carried out on an adjacent road. The quarry was accessed via a ramp along its perimeter walls, so a visual inspection of its faces and the subsequent extraction of samples could be carried out with ease. The fault present in the upper part fell outside the sampling area. Based on a visual assessment, a number of beds were identified. The face of the quarry was classified in beds depending on varying bio-retexturing activity. Such classification—new to the local quarrying and building industry—was proposed as a way to explore the hypothesis that bioturbation is a reliable method to assess the quality of LGL. The relative position of the samples from the top of the quarry and their respective thickness were noted; thickness varied depending on the extent of burrowing activity in the host rock. In total, 16 samples from the host rock were analyzed; burrows were avoided. A summary of the sedimentological descriptions of the beds and of the hand specimens are given in Table 2. Sample locations were identified with the assistance of Dr Hugh Martyn Pedley. A lithostratigraphic column with respect to the general stratigraphy of the Maltese Islands is given in Figure 5.
To comprehend what constitutes first-quality LGL, samples were taken from (i) the seventeenth-century Baroque Church of Santa Maria at Birkirkara (UTM coordinates: 451543E, 3972435N, Figure 1b), the subject of a recent publication [85] (Figure 6a) and (ii) Piccolo Fewda quarry, officially known as Softstone Quarry No. 32 (Figure 1b), in Mqabba (UTM coordinates: 447091E, 3584992N) which, at the time, was providing LGL for the restoration of the church [79,86]. Although in partial ruin until restoration works commenced in 1969, the original carvings on the main elevation of the church have weathered well, and the acanthus capitals are in optimal condition (Figure 6b). Thus, the limestone in which the church was erected arguably represents the best quality lithotype available at that time, which, according to tradition, was quarried from the limits of “Tal Balal” (UTM coordinates: 452223E, 3967432N). Old quarries in this locality were reclaimed for agrarian use, so the precise location of the site from which the original dimension stones were extracted is difficult to trace. Samples from the Piccolo Fewda quarry were collected with the assistance of the quarry owner Salvatore Bondin, a third-generation quarryman with over six decades of quarrying experience and a former President of the Softstone Quarries Association (of Malta).
The terminology outlined in Section 2.4 was used when referring to the quarry at Mqabba. However, when referring to the quarry at Qrendi, the terms used refer to a visual criterion based on bioturbation—first-quality LGL has minimal burrowing, and burrowing increases with decreasing quality.

3.2. Characterization Techniques

The characterization of the samples was established through geochemical, mineralogical, textural and physico-mechanical analysis. Chemical analysis was determined through (i) loss-on-ignition (LOI), (ii) X-ray fluorescence (XRF) analysis and (iii) electron microprobe analysis. LOI—a measure of percentage loss in weight of an ignited sample due to the release of CO2, water and other volatiles—was used to estimate the organic and carbonate content of sediments [87]. Standard-size polished thin sections coated with a thin carbon layer were used for the microprobe. To determine the bulk chemistry, an ARL 8420+ X-ray fluorescence spectrometer was used on pressed powder pellets [88]. To obtain an accurate elemental analysis of fossil fragments and grains, an electron probe microanalyzer was used. To establish qualitatively the mineralogy of the whole rock, the non-carbonate content and the clay fraction, a Philips PW1729 X-ray generator (XRD) was used. The mineralogy of the non-carbonate content was quantitatively established through acid insoluble residue (IR) in 10% HCl, whilst for the clay fraction, an oriented mount technique was used, since the d001 peaks are enhanced [89]. A glass mount and a piece of unused filter paper were analyzed as controls. Both XRF and X-ray diffraction analyses were undertaken at the former Department of Geology, now integrated in the School of Geography, Geology and the Environment, University of Leicester.
Optical and scanning electron microscopy (SEM) (for both rock specimens and insoluble residue) and mercury intrusion porosimetry (MIP) were used to obtain information on the texture. The cementing fabric, porosity and permeability were studied through thin section analysis; sections were impregnated with blue-dyed araldite for ease of reading the pore sizes and structure. To identify petrographically the distinction between first- and second-quality LGL as understood by the local quarrymen, samples Q from the Mqabba quarry and samples C from the Old Church were analyzed. They were re-examined by Professor Elena Koleva-Rekalova using a Zeiss Axioskop 40 transmitted light microscope, which was equipped with a digital camera for taking microphotographs in both plane-polarized and cross-polarized light [90]. The classification of the limestone was performed according to Dunham’s scheme [91]. Texture, cement fabric and pores, as well as the non-carbonate fraction, were analyzed using a Hitachi S-520 scanning electron microscope equipped with an energy dispersive analyzer [92,93]. To avoid contamination, fragments (measuring circa 5 × 5 × 5 mm) were freshly cut from the sample retained following thin section preparation. To ensure good quality imaging, these fragments were handled to evade contact with skin oil which might out-gas in the SEM vacuum system. Furthermore, a film of conductive paint was applied to ensure electrical contact between the fragment and the stub and, to ensure good quality imaging, a thin gold coating was applied. MIP was used to establish the volumes and radii of pores within the fabric [94,95]. A Quantachrome SP-33B mercury intrusion porosimeter was used. All analyses were undertaken at the University of Leicester except for MIP, which was carried out at the Department of Civil Engineering of the Faculty of Architecture and Civil Engineering, the forerunner of the Faculty for the Built Environment, of the University of Malta, Msida, Malta.
To evaluate the physico-mechanical properties, the following testing regimes were used: (i) apparent density, (ii) compressive strength (CS), using the Avery-Denison model, (iii) ultrasonic pulse velocity (UPV), using the PUNDIT model, and (iv) color, using (EEL) Abridged Reflectance Spectrometer model. For regimes (i) to (iii), tests were undertaken on oven-dried (temperature 105+/−5 °C) and saturated (fully submerged for 24 h) samples. With the exception of color analysis, which was undertaken at the Department of Geology, University of Leicester, all other testing regimes were carried out at the Department of Civil Engineering, University of Malta. This decision was undertaken to cut down on the bulk handling of the large samples required for the petrophysical testing regimes.

4. Results

4.1. Geochemistry

The XRF analyses for major oxides and the LOI are given in Table 3. LOI was <44%, the theoretical value for pure CaCO3. Samples Q1_4 and Q1_6, identified in the field as second-quality LGL (Table 2), have low CaCO3, whilst samples Q1_8 and Q1_9 have a CaCO3 content comparable to first-quality lithotype. A lower CaCO3 content is present in Q1_15. Burrow infills at the stratigraphic level from which Q1_8 was extracted had a similar CaCO3 content to first quality, while similar infills at the lithostratigraphic beds from which Q1_11 and Q1_14 were extracted had a similar CaCO3 content to second-quality lithotype [84], which indicates that burrowing activity introduced material different from the host rock.
The main non-carbonate fraction is SiO2 (Figure 7); its presence is inversely proportional to the CaCO3 content. The variation in Al2O3, Fe2O3, K2O and TiO2, although less pronounced, is similar to SiO2 (Figure 8). Second-quality LGL is characterized by higher content of these oxides; the variation between beds 9 and 15 is gradual. The presence of MgO, which does not correlate with any noted variation in the quality of LGL, may be due to change in allochem mineralogy [96]. Na2O and MnO are present in traces. Electron probe analysis of the host rock and burrow infills showed alteration in the composition due to iron oxide and glauconite breakdown.
Analysis of samples from Mqabba quarry and church converge to similar results. CaCO3 content decreases progressively as the quality of the LGL decreases [85]. The principal non-carbonate oxides—SiO2, Al2O3, Fe2O3 and K2O—are minimally present in the first-quality LGL.
Sol ikħal has a lower CaCO3 content than sol aħmar which, in turn, has a lower CaCO3 content than first-quality LGL [81,85]. Based on first-quality LGL from both sites, LGL with an IR content of ≥5% is of inferior quality.
XRF is often used to verify the XRD data and vice versa. Analysis of the chemical composition of geological samples using the semi-quantitative analysis method can be used to determine elemental composition of rocks. The data on oxides obtained via current methods are useful for geochemical characterizations of the limestone and for corresponding interpretations. These oxides can be attributed to quartz, clays, K-feldspar, muscovite and some iron oxide/s. These data are not useful, however, for determining the mineral composition. For example, it is not possible to estimate how much SiO2 comes from quartz, kaolinite, etc.

4.2. Mineralogy

The mineralogical analysis, including the fraction of the IR, is given in Table 4. The various types of XRD traces are given in Figure 9.
Further to calcite (cal), quartz (qtz) was identified in the whole-rock analysis in all samples except for a few of the first-quality samples (Figure 9a). Analysis of the insoluble residue indicates that these samples are characterized by the absence of muscovite (ms); quartz and K-feldspar (Kfs) are present in all samples (Figure 9b,c). Kaolinite (kln), illite (ill) and smectite (sme) are sometimes present. Burrows present in the beds from which samples Q1_8 and Q1_11 were extracted contain goethite [84]. Inferior-quality LGL from Mqabba quarry contains geothite. This mineral—typically formed under oxidizing conditions and includes material frequently grouped as limonite—occurs as a weathering product of iron bearing minerals such as pyrite. Gypsum (gp), albite (al) and rutile (rt) are occasional. Kaolinite, illite, smectite, quartz and K-feldspar make up the mineralogy of the clay fraction (Figure 9d–f). Second-quality beds have higher non-carbonate content. Although identified as being first quality, sample Q1_15 has an IR content similar to second-quality lithotype. The variation in the non-carbonate content from samples Q1_9 to Q1_15 is gradual.

4.3. Texture

4.3.1. Optical Microscopy

Samples analyzed are of intra- and inter-particle porosity type, the former more frequent than the latter. They consist of fine-grained Globigerina-bioclastic wackestone, which alternates with Globigerina-bioclastic packstone until the latter becomes the main fabric, from bed 11 and lower. The sediment is comprised of planktonic and benthonic foraminifera; rare echinoid fragments are also present. Undamaged microfossils have unfilled chambers. Allochems are cemented by fine-grained sparry calcite; they imperfectly fill the inter-particle voids. Porosity along grain boundaries is predominant in the host rock compared to the burrow infill. Quartz grains—elongated or rounded and dispersed through the matrix—glauconite, and some iron oxide/s are also present. Staining due to breakdown of iron-rich minerals is present.
The petrographical characteristics of the C and Q samples are given in Table 5. Samples C1 and C2 are from the undeteriorated and deteriorated surface, respectively. Sample Q1 is first-quality LGL, Q2 is identical to Q1 but seasoned for 12 years, and Q3 is second-quality LGL. Irrespective of the quality of the LGL, all samples are Globigerina wackestone. In Q1, the micrite is darker in color; only in some places it is recrystallized to microspar and spar. Small planktonic Globigerina foraminifera occur. Single and strongly altered benthic foraminifera are presented. The bioclasts of echinoids and bivalves are very rare, altered and difficult to discern. In Q2, the micrite in the matrix is commonly recrystallized to microspar. Small planktonic Globigerina foraminifera predominate. Single and strongly altered benthic foraminifera are present. The bioclasts of echinoids are rare, with sizes less than 1.0 mm; some glauconite grains are visible. In Q3, the micrite is commonly recrystallized to microspar. It is preserved only in some places that resemble micrite intraclasts. Planktonic Globigerina foraminifera occur. Rare bioclasts and single benthic foraminifera are present. Only one bivalve fragment, 1.2 × 0.03 mm in size, is present; glauconite grains are sporadic.
Unlike the Q samples, in which terrigenous components are rare, they are sporadic in the C samples. In C1, the micrite in the matrix is preserved only in some places (Figure 10). It is frequently recrystallized to microspar and rarely to spar. In the more recrystallized areas, more pores are observed. In some places the recrystallization obliterates the primary texture of the limestone. Small planktonic Globigerina foraminifera dominate and most often their chambers represent pores. Bioclasts are very rare and predominantly they are represented by echinoids, whose sizes do not exceed 0.5 mm. There are sporadic benthic foraminifera. In C2, the micrite matrix displays darker coloration (Figure 11); micrite is frequently recrystallized to microspar and in these places the pores are of an insignificant quantity. Small planktonic Globigerina foraminifera predominate. Commonly their chambers are empty. Bioclasts are very rare, and their sizes are less than 0.5 mm. Single echinoid fragments are with syntaxial overgrowths. One bioclast (0.45 × 0.3 mm) is silicified.

4.3.2. Scanning Electron Microscopy and Porosimetry

Three samples were analyzed through SEM: Q1_1 and Q1_6, first- and second-quality LGL, respectively, and a sample from the church. Microphotographs are given in Figure 12. The pore structure, the physico-mechanical interlocking and the fine-grained cement are shown in Figure 12a,b; fine-grained sparry calcite, which fills the inter-particle voids, cements most allochems. Although more samples are required for robust conclusive evidence, it is evident that the post-depositional, compact, low permeability LGL might have originally been very porous. The passage of water through the matrix encouraged the growth of the post-depositional interlocking calcite crystal fabric present in the church sample (Figure 12b). Such interlocking crystals provide a physico-mechanical bond which produces a more durable building stone.
Table 6 summarizes the interpolated intruded volume for each sample analyzed through MIP. When comparing the results from the Qrendi quarry with those from the Mqabba quarry and the church samples, variations in the distribution of the pores can be observed, which is indicative of petrographical differences arising from the provenance of the limestone [85]. In the latter two sites, the volume of pores is higher in first-quality LGL. A similar correlation, although less defined, is present at Tal-Warda quarry.

4.4. Petrophysical Properties

The relative density varies: lower quality LGL has higher density, an indication that particle porosity is higher in better-quality LGL (Table 7). Subject to a constant loading rate set at 0.15 N/mm2, the CS of 100 × 100 × 100 mm samples varies between 15.6–30.2 N/mm2 for dry states and 8.3–17.4 N/mm2 for saturated states; this is within the range stated in the national structural handbook [97]. The host rock of sample Q_11 is first quality, but the presence of burrows reduces its quality. Inferior LGL characteristically has higher mean dry compressive strength (fk) and lower porosity. According to the Centre Technique de Matériaux Naturels de Construction [98], LGL is definitely not a hard stone (fk > 40 N/mm2) [99]; some lithostratigraphic beds qualify as a compact (10 ≤ fk ≤ 40 N/mm2) rather than as a soft stone (fk < 10 N/mm2). No significant variation in CS between the two lithotypes is present. An anomaly is present in sample Q1_11, a first-quality LGL with visually pronounced dark burrow infill (Table 2). UPV values follow a similar pattern, both perpendicular and parallel to the bedding plane. Correlation is present between UC and UPV values, a result in line with Vasanelli et al. [100]. Applying the classification for UPV [101], velocities in the range of 2.5–3.5 km/s were considered low, and wet samples yielded lower values [100].
For all samples, the hue was yellow as the color wavelength varied between 580 and 582 nm. (Table 8). Variations were present in wavelength, purity (or chroma) and brightness (or value, tone). A possible correlation between density and color—a third-order polynomial—was found: the darker the color, the higher the density.

5. Discussion

5.1. Lithology

Composed of skeletal and shell fragments, variations in non-carbonate content in the lithostratigraphy of LGL may be due to the Miocene environmental changes that took place during its formation or due to changes in the source area of the sediments. Sediment input appears to have changed at the same time that small quantities of non-carbonate minerals intruded into areas where carbonates were being deposited.
According to the widely used Institute of Geological Sciences classification of limestone purity based on CaCO3 content [102], the LGL is of medium (93.5% < CaCO3 < 97.0%) to high (97.0% < CaCO3 < 98.5%) purity, and purity increases with quality. Only some beds of the first-quality lithotype are of high purity. Principal limestone characteristics are imperative for effective damage diagnostics [103,104].
Until the completion of Bianco [40], the quality criteria used to differentiate between the two LGL lithotypes were based on consideration of CS, porosity, “ringing tone” and “glass fragments”. The best-quality LGL does indeed have marked lithophonic characteristics when hit by a mallet, whilst inferior-quality stone is richer in quartz splinters.
The mineralogy determines the properties of LGL to a large extent, whilst the pore size controls strength, durability and the movement of moisture. Durability and CS are not interchangeable, an error often made by quarrymen [41,85]. Whilst it is indicative of the stone’s quality, the durability of both lithotypes depends not only on the strength of the constituents making up the fabric but on the physico-mechanical interlocking and the sparry calcite cement where the grains come into contact. Although quartz, the main non-carbonate mineral, is considerably hard, it is cemented with the softer calcite, thus the overall hardness and strength are weak. The cement bonding the minerals and other organic fragments is a controlling factor in strength and durability. This cement also influences the tensile strength of the fabric. Generally, brittle fractures start at grain boundaries; the grains will loosen and become detached as stresses increase until tension is released on failure. The failure path along both LGL lithotypes is illustrative of the weakest path along which stresses are transmitted. Failure along this path requires minimal energy.

5.2. Compressive Strength

Compressive strength (CS) is an indicator of the strength of LGL but is not indicative of durability. Both factors depend on the kind of cement and the degree of cementation. In the inferior lithotype (Figure 12c,d), an occasional film of precipitated calcite crystals coats the individual fragments and cements the fragments. CS is provided by well-distributed calcite cement. No cement is present in the interstices between the grains. The first-quality LGL supports more extensive re-crystallization.
The non-carbonate content includes minerals of varying coefficients of thermal expansion. Differential expansion of these minerals generates stress at both inter- and intra-particle pores and causes the breakdown of the cementing fabric and the development of micro-cracks within the sediment. It is important that stone cutting and dressing should be done in such a way that the dimension stone produced is free from micro-cracking, as damage can be induced by bad workmanship. Low CS values may arise from micro-cracks caused by using the mallet too heavily when dressing the stone or by mishandling the dimension stone during building works.

5.3. Porosity

Given that the porosity along grain boundaries is predominant, the type and percentage of pores are key to both the mechanical behavior and durability of LGL. The results seem to demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the limestone fabric faster compared to other porosity structures. The matrix to allochems ratio decreases with decreasing quality. The micrite in the matrix is commonly recrystallized to microspar, which obliterates the primary texture of the LGL; it is preserved only in some places that resemble micrite intraclasts. Overall porosity and pore space distribution depend on the heterogeneity of the formation. Generally, for carbonates, the depositional porosity varies from 40% to 70% when uncemented. After lithification it may vary from 5% to 15% [105]. The porosity of first quality LGL is 39+/−1% [106], a value endorsed by Morris [107]. Both LGL lithotypes predominately contain pores with radii ranging from 40 to 40,000 Å, although the inferior lithotype is less porous. The lower the volume of pores, the lower the “life-span” of the LGL, and the more likely it is to have higher thermal conductivity.
The effect of carbonic acid, an important solvent agent in limestone, increases with increasing micro porosity. The dissolving action is prolonged by capillary action which, in turn, ensures that water is retained in the micropores for a longer duration. Variation in effective porosity in both lithotypes requires further investigation.
SEM imagery shows euhedral interlocking calcite crystals, which suggests that the first-quality LGL might originally have been very porous. Porosity is modified by post-depositional processes. The passage of water through the matrix encouraged the growth of the post-depositional dog-tooth calcite crystal fabric present in the first quality LGL (Figure 12a,b). Calcite mud in the second-quality LGL (Figure 12c,d) prevents the flow of water through the fabric, thus it lacks the euhedral dog-tooth interlocking calcite crystals present in the first-quality lithotype. These crystals provide a bonding agent which produces a more durable building stone. Therefore, the diagenetic history of the limestone is important in its eventual quality.

5.4. Weathering

The weathering and durability properties of LGL have long been a topic of interest, with references found throughout history [4,108,109]. Systematic research into the limestones of Malta, including LGL, was conducted by the Building Research Station [110,111,112], and models describing the weathering and decay of the Globigerina Limestone have been published notably by Fitzner et al. [113] and Sandrolini et al. [114]. Weathering of LGL is also a theme within an ongoing project run by the Geological Institute of the Bulgarian Academy of Sciences [115].
Limestone matrix varies with the environment of deposition and the subsequent lithostratigraphic history. Chemical processes of weathering are aided by physical breakdown, which increases the surface area of fragment surfaces, thus providing easier access to oxygen and moisture which further perpetuate and accelerate chemical breakdown. The presence of oxygen leads to oxidation, while the presence of moisture leads to solution or hydration. With the advent of industrialization, the rate of decay of limestone increased dramatically. Photographs taken in the mid-nineteenth century indicate that the fortifications along the Valletta seafront started to deteriorate at a high rate following the advent of the first coal-operated ships frequenting the harbor [116]. This may have been due to sulfur emissions in the air causing acid attack to the limestone.
Porosity is related to the weathering resistance of limestone. Pore structure has a bearing on weathering [117]: (i) the shape, size and nature of pores, (ii) how they are connected, and (iii) the degree of micro-porosity (pores < 0.025 mm) are important parameters. The importance of these three factors is illustrated by photographs of buildings utilizing CL. The weathered limestone of the bastions facing Marsamxett Harbor has large voids [29]. The larger the weathered voids, the easier it is for water to move in and out of the fabric without causing major damage, even though the surface area on which weathering can occur is increased.
Results from XRF (Table 3), and IR and XRD (Table 4) analysis converge to show that non-carbonate impurities are lower in first-quality LGL. The principal non-carbonate components are attributed to quartz, clays, feldspars, muscovite and some iron oxide mineral/s. Quartz and mica are resistant to weathering. Feldspars weather slowly to kaolinite and illite. Illite and smectite are structurally related to micas; most of the inter-layer water of smectite is lost on heating to 335 °C. Illite is a clay mica similar to kaolinite; it differs from muscovite in that it has less potassium and more silica [118] and may be formed during diagenesis by alteration of other clay species. Illite may result from post-depositional weathering of muscovite and silicates, in particular feldspars. Kaolinite absorbs water without swelling; it is sometimes a byproduct of the weathering of feldspars and other silicates. Smectite is also an end product of weathering; unlike kaolinite, it is a swelling clay. These mineralogical changes are more significant in the inferior lithotype. Ferric compounds present in thin sections may be due to weathering of glauconite. Goethite and limonite are weathering end products. The latter may be considered as goethite plus amorphous ferric hydroxide.
Alveolar weathering characterizes LGL; this is due to (i) selective intra-burrow cementation and (ii) preferential erosion of the adjacent weakly cemented sediment. The “marki tas-swaba” (literally “finger marks) [79] are the profiles of preferential weathering in areas where burrows are present. Salt crystallization, long identified as the primary cause of LGL decay [4,108], is more a cause of damage to fine-grained limestone than salts washed by the rain [119]. On crystallization, salts generate stresses within pores of the sediment, which may be in the range of 2 to 20 N/mm2 [120].

5.5. Color

Color is caused by oxidizing or reducing processes at the time of sedimentation. Oxidation during the process of fragmentation might have changed the color of LGL from an original bluish-grey to yellow and ochre-brown, as illustrated by the blue lenticular patches at Msida [81,82]. Ferric solutions entering along cracks, marked with yellow bands on either side, create a contrast with the original green-grey calcite matrix [81]. The color changed from bluish-green to yellow due to post-depositional oxidation resulting from water flowing through the cracks.
Minerals such as quartz, feldspars, augite, mica may be of igneous origin. Terrigenous components in LGL are rare or sporadic and are very small in size, mainly below 0.1 mm. This makes it difficult to accurately determine under a microscope the type of feldspars (plagioclase and K-feldspar). Quartz grains are mostly monocrystalline and some of them have undulose extinction. Other LGL-forming clay minerals are formed by the weathering of non-clay minerals. The color of pure calcium carbonate and pure clay minerals is white with occasional tints of grey and green. The introduction of pigments such as ferrous-ferric iron hydroxides or limonite minerals can generate green or yellow-ochre hues, respectively. Soluble hydrocarbons give a grey-to-black color. Iron-based minerals are more abundant in second-quality LGL. Such minerals have the strongest pigment. These pigments are usually unstable if exposed to light and weathering. Most of the iron has been formed either by precipitation or during geological processes related or unrelated to lithification. From thin section studies, iron minerals have been washed into the formation as iron in solution or freed through decaying organic material. There is a reasonable correlation of color with the Fe2O3 content.
Color changes according to whether the dimension stones are freshly cut or weathered. In the former, color is important as a quality criterion. Color on drying is an imperative visual consideration when selecting building stones. It is important that dimension stones in buildings, in particular monumental and official structures, weather as desired.

5.6. Bioturbation

Formed within soft unconsolidated sediments, burrows exhibit patterns, shapes and well-defined ichnofabrics [121]; their compaction can be computed in terms of Ricken [122]. Bioturbation is a process whereby sediments are remixed by organisms, affecting the porosity and permeability of the host sediment [123]. Bioturbated and non-bioturbated areas have an impact on the weathering of the fabric [84]. Bio-retexturing is a process which destroys the primary depositional fabrics and masks inorganic process-related structures [123]. Photos taken on site indicate bio-retexturing and weathering in inferior LGL. Burrowing activity introduces unlithified sediment of variable permeability in the host rock [84,124]. The XRF and XRD analyses indicate that the mineralogy of burrow infill is quantitatively different from the host rock. When rich in goethite, the infills have a characteristic dark yellow ochre-like color.

6. Conclusions

Composed of calcium carbonate limestone of predominantly medium purity (only few first-quality lithobeds are of high purity), LGL was subject to varying lithification and compaction processes during diagenesis. LGL with a high quartz and clay content is of the inferior lithotype (and marginally more expensive on the tungsten carbide tips used in circular saws to quarry [79]). It is less durable, although its CS is comparable to the first-quality lithotype. The following conclusions were drawn [40]:
  • IR content is indicative of the quality of LGL
Bulk chemistry and mineralogical show that non-carbonate and clay content increase with decreasing quality in LGL; thus a lower IR content suggests higher-quality LGL. An anomaly is present in the designation of the quality of two samples (Q1_8 and Q1_9) based on the visual presence of bioturbation. An IR threshold of 5% was introduced in 1994 by the author, then a Minerals Planning Officer within the Planning Authority (Malta), as a benchmark to evaluate the quality of the mineral resources prior to issuing development planning consent for new LGL quarries or extensions to existing sites. An IR content of <5% denotes first-quality LGL; IR content ≥5% is indicative of inferior lithotypes. The reliability of this rudimentary mineralogical method as a mode of quality control—an empirical grounding to the alternative of visual assessment—was confirmed by research carried out several years later [125].
2.
Bioturbation is a reliable method to assess the quality LGL
Bio-retexturing introduces unlithified sediment of variable permeability into the host rock [124]. The mineralogy of burrow infill is quantitatively different from the host rock. Sometimes, such burrows are richer in goethite. Bioturbation is pronounced in the inferior lithotype [84].
3.
First-quality LGL is more porous than inferior lithotypes
Measuring porosity is a traditional field method used to differentiate between two lithotypes. The inferior lithotype is richer in clay minerals, which may be a cause of weakness in the fabric, reducing its durability. This research established that in first-quality LGL, the movement of water through the matrix encourages the growth of the post-depositional interlocking calcite crystals whose physico-mechanical bonding enhances the durability of the lithotype.
4.
Other correlations
Interpolation of results indicates that there is a correlation between density and color, density and CS, and color and Fe2O3 content.
Research on restoration of built heritage focuses on employing techniques that prohibit the ingress of moisture into the fabric. This approach ignores self-sustaining reactions—both mineralogical and otherwise—which continue to take place within the fabric. Studying the mechanics and processes inherent to the nature of the fabric is vital in order to understand and predict the likely behavior of the fabric when a particular restoration technique is used. Interfering with natural weathering processes hinders the course of the action of the elements.

Funding

This research was funded through a scholarship from the Oil Exploration Directorate, Office of the Prime Minister, Malta, to undertake postgraduate studies under the academic supervision of Hugh Martyn Pedley at the University of Leicester, UK.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The author would like to thank his academic tutor Hugh Martyn Pedley for sharing his invaluable academic and field experience on the geology of the Maltese Islands; the late Anselm Dunham (former Head of the Department of Geology, University of Leicester) for his methodical approach in industrial mineralogy and his unwavering scholarly advice; Godwin Debono and Saviour Xerri (both formerly of the Oil Exploration Directorate, Office of the Prime Minister, Malta); Godwin Cassar and the late Fredrick Camilleri (former Director General and Head of the Minerals Planning Unit, Planning Authority, Malta); Alex Torpiano (Dean, Faculty of the Built Environment, University of Malta; then Head of the Building and Civil Engineering Department at the same university); and the late Salvatore Bondin (former President of Għaqda Sidien tal-Barrieri, Quarry Owners Association, Malta), a third generation quarryman with over 60 years of experience in open-pit mining in Malta. A note of gratitude to Elena Koleva-Rekalova (Department of Paleontology, Geological Institute Strashimir Dimitrov, Bulgarian Academy of Sciences) for re-examining the thin-sections, for the microphotographs reproduced in Figure 10 and Figure 11, and for her invaluable comments on the draft version of this paper, and to Elena Marrocchino and Carmela Vaccaro (both of the Department of Physics and Earth Sciences, University of Ferrara) who, as Guest Editors, invited this contribution to the Special Issue on “Geochemical, Mineralogical, and Petrographical Applications to Environment and Cultural Heritage” published in Minerals. A final note to thank Radostina Atanassova (Scientific Secretary, Geological Institute Strashimir Dimitrov, Bulgarian Academy of Sciences) and the three anonymous referees of the journal for their comments and suggestions.

Conflicts of Interest

The author declares no conflict of interest.

References

  1. Coleridge, S.T. The Notebooks of Samuel Taylor Coleridge, 2nd ed.; Coburn, K., Ed.; Routledge & Kegan Paul: London, UK, 1962; Volume 1804–1808. [Google Scholar]
  2. Blouet, B. The Story of Malta; Faber and Faber: London, UK, 1967. [Google Scholar]
  3. Palmer, R. Coleridge, material culture, and Malta. ANQ A Q. J. Short Artic. Notes Rev. 2014, 27, 5–12. [Google Scholar] [CrossRef]
  4. Quintin d’Autin, J. The Earliest Description of Malta; DeBono Enterprises: Sliema, Malta, 1980. [Google Scholar]
  5. Seddall, H. Malta: Past and Present: Being a History of Malta from the Days of the Phoenicians to the Present Time; Chapman and Hall: London, UK, 1870. [Google Scholar]
  6. Oil Exploration Directorate. Geological Map of the Maltese Islands (Sheet 1, Malta; Sheet 2, Gozo), 1:25,000; British Geological Survey: Keyworth, UK, 1993. [Google Scholar]
  7. Childe, C.V. The Dawn of European Civilisation; Kegan Paul: London, UK, 1957. [Google Scholar]
  8. Evans, J.D. The Prehistoric Antiquities of the Maltese Islands: A Survey; Athlone Press: London, UK, 1971. [Google Scholar]
  9. Renfrew, A.C. Before Civilisation; Cape: London, UK, 1973. [Google Scholar]
  10. Trump, D.H. Megalithic architecture in Malta. In Antiquity and Man: Essays in Honour of Glyn Daniel; Evans, J.D., Cunliffe, B., Renfrew, C., Eds.; Thames and Hudson: London, UK, 1981; pp. 128–140. [Google Scholar]
  11. Cherry, J.F. The first colonisation of the Mediterranean Islands: A review of recent research. J. Medit. Archaeol. 1990, 3, 145–221. [Google Scholar]
  12. UNESCO. World Heritage List: Megalithic Temples of Malta. Available online: http://whc.unesco.org/en/list/132 (accessed on 21 January 2021).
  13. Bianco, L. Architectural ruins: Geoculture of the anatomy of buildings as illustrated by Casa Ippolito, Malta. Herit. Sci. 2021, 9, 1–19. [Google Scholar] [CrossRef] [PubMed]
  14. UNESCO. World Heritage List: Ħal Saflieni Hypogeum. Available online: http://whc.unesco.org/en/list/130 (accessed on 21 January 2021).
  15. Heritage Malta. St. Paul’s Catacombs. Available online: https://heritagemalta.org/st-pauls-catacombs/ (accessed on 16 March 2021).
  16. Bonanno, A. Malta: Phoenician, Punic, and Roman; Midsea Books: Sta Venera, Malta, 2005. [Google Scholar]
  17. De Giorgio, R. A City by an Order, 2nd ed.; Progress Press Co. Ltd.: Valletta, Malta, 1985. [Google Scholar]
  18. UNESCO. World Heritage List: City of Valletta. Available online: http://whc.unesco.org/en/list/131 (accessed on 21 January 2021).
  19. Hughes, Q. The Building of Malta during the period of the Knights of St John of Jerusalem; Alec Tiranti: London, UK, 1967. [Google Scholar]
  20. Mahoney, L. 5000 years of Architecture in Malta; Valletta Publishing: Valletta, Malta, 1996. [Google Scholar]
  21. De Lucca, D. Mdina: A History of Its Urban Space and Architecture; Said International: Valletta, Malta, 1995. [Google Scholar]
  22. Hoppen, A. The Fortification of Malta by the Order of St John 1530–1798; Scottish Academic: Edinburgh, UK, 1979. [Google Scholar]
  23. Spiteri, S. Fortresses of the Cross; Print Services Ltd.: Qormi, Malta, 1994. [Google Scholar]
  24. Blouet, B.W. The Changing Landscape of Malta during the Rule of the Order of St John of Jerusalem 1530–1798. Ph.D. Thesis, University of Hull, Hull, UK, 1964. [Google Scholar]
  25. Bianco, L.; Cardona, K. Seventeenth-century building engineering in the central Mediterranean: A case study from Malta. Terra Sebus. Acta Musei Sabesiensis 2020, 12, 329–353. [Google Scholar]
  26. Fergusson, J. History of the Modern Styles of Architecture: Being a Sequel to the Handbook of Architecture; John Murray: London, UK, 1862. [Google Scholar]
  27. Bianco, L. A geohistorical retrospective analysis of cultural heritage buildings: The case of Mosta Dome, Malta. GeoJournal 2019, 84, 291–302. [Google Scholar] [CrossRef]
  28. Abela, G.F. Della Descrittione di Malta isola nel Mare Siciliano: Con le sue Antichità, ed Altre Notizie; Paolo Bonacota: Valletta, Malta, 1647. [Google Scholar]
  29. Bianco, L. Geocultural activity in seventeenth and eighteenth century Malta. GeoJournal 1999, 48, 337–340. [Google Scholar] [CrossRef]
  30. PEUPLIER. Hagar Qim. Available online: http://www.flickr.com/photos/peuplier/3019403075/ (accessed on 15 February 2021).
  31. Mamo, J. Marbles and limestones of Malta. Sands Clays Miner. 1936, 2, 83–88. [Google Scholar]
  32. Bruno, B. L’Arcipelago Maltese ill Eta’ Romana e Bizalltilla; Edipuglia: Bari, Italy, 2004. [Google Scholar]
  33. Ellul, M. Malta limestone goes to Europe: Use of Malta stone outside Malta. In 60th Anniversary of the Malta Historical Society: A Commemoration; Grima, J.F., Ed.; Veritas Press: Zabbar, Malta, 2010; pp. 371–406. [Google Scholar]
  34. Lino Bianco and Associates. Retention of the Status quo Regarding the Exportation of Maltese Stone: Study for the Ministry for Economic Services, Malta; Lino Bianco & Associates: Ħamrun, Malta, 2000; Available online: https://www.um.edu.mt/library/oar/bitstream/123456789/45116/1/Case_for_the_retention_of_the_status_quo_regarding_the_exportation_of_Maltese_stone.pdf (accessed on 22 January 2021).
  35. Bianco, L. Internal Policy on the Utilisation of Malta Stone; Ministry for Resources and Infrastructure: Beltissebħ, Malta, 2002.
  36. Cassar, J.; Torpiano, A.; Zammit, T.; Micallef, A. Proposal for the nomination of Lower Globigerina Limestone of the Maltese Islands as a “Global Heritage Stone Resource”. J. Int. Geosci. 2017, 40, 221–231. [Google Scholar] [CrossRef] [Green Version]
  37. Hodul, J.; Žižková, N.; Drochytka, R.; Borg, R.P. Influence of crystallization admixture on mechanical parameters and microstructure of polymer-cement mortars with waste limestone. Solid State Phenom. 2019, 296, 27–34. [Google Scholar] [CrossRef]
  38. Hodul, J.; Žižková, N.; Borg, R.P. The influence of crystalline admixtures on the properties and microstructure of mortar containing by-products. Buildings 2020, 10, 146. [Google Scholar] [CrossRef]
  39. University of Malta. Annual Report 2019; Marketing, Communications & Alumni Office: Msida, Malta, 2021; Available online: https://www.um.edu.mt/__data/assets/pdf_file/0004/455647/annualreport2019.pdf (accessed on 17 March 2021).
  40. Bianco, L. Some Factors Controlling the Quality of Lower Globigerina Building Stone of Malta. Master’s Thesis, University of Leicester, Leicester, UK, 1993. [Google Scholar]
  41. Bianco, L. Techniques to determine the provenance of limestone used in Neolithic architecture of Malta. Rom. J. Phys. 2017, 62, 901. [Google Scholar]
  42. Fort, R.; Alvarez de Buergo, M.; Perez-Monserrat, E.M.; Varas, M.J. Characterisation of monzogranitic batholiths as a supply source for heritage construction in the northwest of Madrid. Eng. Geol. 2010, 115, 149–157. [Google Scholar] [CrossRef]
  43. Torok, A.; Prikryl, R. Current methods and future trends in testing, durability analyses and provenance studies of natural stones used in historical monuments. Eng. Geol. 2010, 115, 139–142. [Google Scholar] [CrossRef]
  44. Bianco, L. Natural Weathering Processes Affecting Limestone. The Times [of Malta] Building and Architecture Supplement, 7 August 1994; pp. 10–11. [Google Scholar]
  45. Spratt, T.A.B. On the geology of the Maltese Islands. Proc. Geol. Soc. 1843, 4, 225–229. [Google Scholar]
  46. Spratt, T.A.B. On the Geology of Malta and Gozo; Malta Mail Office: Valletta, Malta, 1852. [Google Scholar]
  47. Adams, A.L. Outline of the geology of the Maltese Islands. Ann. Mag. Nat. Hist. 1864, 14, 1–11. [Google Scholar]
  48. Murray, J. The Maltese Islands, with special reference to their geological structure. Scott. Geogr. Mag. 1890, 6, 449–488. [Google Scholar] [CrossRef] [Green Version]
  49. Rizzo, C. Report on the Geology of the Maltese Islands: Including Chapters on Possible Ground-Water-Tables and Prospecting for Mineral-oil and Natural-Gas; Government Printing Office: Valletta, Malta, 1932. [Google Scholar]
  50. Hyde, H.P.T. Geology of the Maltese Islands; Lux Press: Valletta, Malta, 1955. [Google Scholar]
  51. Zammit-Maempel, G. An Outline of Maltese Geology; Progress Press: Birkirkara, Malta, 1977. [Google Scholar]
  52. House, M.R.; Dunham, K.C.; Wigglesworth, J.C. Geology of the Maltese Islands. In Malta: Background for Development; Bowen-Jones, H., Dewdney, J.C., Fisher, W.B., Eds.; University of Durham: Newcastle, UK, 1961; pp. 24–33. [Google Scholar]
  53. Felix, R. Oligo-Miocene Stratigraphy of Malta and Gozo; H. Veenman and B.V. Zonen: Wageningen, The Netherlands, 1973. [Google Scholar]
  54. Pedley, H.M. The Oligo-Miocene Sediments of the Maltese Islands. Ph.D. Thesis, University of Hull, Hull, UK, 1975. [Google Scholar]
  55. Pedley, H.M.; House, M.R.; Waugh, B. The geology of the Pelagian Block: The Maltese Islands. In The Ocean Basins and Margins; Nairn, A.E.M., Kanes, W.H., Stehli, F.G., Eds.; Springer: Boston, MA, USA, 1978; Volume 4B, pp. 417–433. [Google Scholar]
  56. Pedley, H.M. A new lithostratigraphical and palaeoenvironmental interpretation for the Coralline Limestone formations (Miocene) of the Maltese Islands. Inst. Geol. Sci. Overseas Geol. Min. Resour. 1978, 54, 273–291. [Google Scholar]
  57. Burollet, P.F. Petroleum geology of the Western Mediterranean Basin. In The Exploration for Petroleum in Europe and North Africa; Institute of Petroleum: London, UK, 1969; pp. 19–30. [Google Scholar]
  58. Burollet, P.F. Importance des facteurs salifères dans la tectonique tunisienne. In Livre Jubilaire de M. Solignac; Annales des Mines et de la Geéologie; Service Geéologique de Tunisie: Tunis, Tunisia, 1973; Volume 26, pp. 111–120. [Google Scholar]
  59. Burollet, P.F.; Mugniot, J.M.; Sweeney, P. The geology of the Pelagian Block: The Margins and Basins off Southern Tunisia and Tripolitania. In The Ocean Basins and Margins; Nairn, A.E.M., Kanes, W.H., Stehli, F.G., Eds.; Springer: Boston, MA, USA, 1978; pp. 331–359. [Google Scholar]
  60. Morelli, C.; Gantar, G.; Pisani, M. Bathymetry, gravity and magnetism in the Strait of Sicily and the Ionian Sea. Boll. Di. Geofis. Teor. Ed. Appl. 1975, 17, 39–58. [Google Scholar]
  61. Spratt, T.A.B. On the Bone-Caves near Crendi, Zebbug, and Melliha, in the Island of Malta. Q. J. Geol. Soc. 1867, 23, 283–297. [Google Scholar] [CrossRef]
  62. Giannelli, L.; Salvatorini, G. I foraminiferi planctonici dei sediment terziari dell’arcipelago Maltese. I. Biostratigrafia dell Globigerina Limestone. Atti Della Soc. Toscana Sci. Nat. Mem. Ser. A 1972, 79, 49–74. [Google Scholar]
  63. Pedley, H.M. Miocene sea-floor subsidence and later subaerial solution subsidence structures in the Maltese Islands. Proc. Geol. Assoc. 1974, 85, 533–547. [Google Scholar] [CrossRef]
  64. Pedley, H.M.; House, M.R.; Waugh, B. The geology of Malta and Gozo. Proc. Geol. Assoc. 1976, 87, 325–341. [Google Scholar] [CrossRef]
  65. Baldassini, N.; Di Stefano, A. Stratigraphic features of the Maltese Archipelago: A synthesis. Nat. Hazards 2017, 86, 203–231. [Google Scholar] [CrossRef]
  66. Baldassini, N.; Mazzei, R.; Foresi, L.M.; Riforgiato, F.; Salvatorini, G. Calcareous plankton bio-chronostratigraphy of the Maltese Lower Globigerina Limestone member. Acta Geol. Pol. 2013, 63, 105–135. [Google Scholar] [CrossRef]
  67. Foresi, L.M.; Mazzei, R.; Salvatorini, G.; Donia, F. Biostratigraphy and chronostratigraphy of the Maltese Lower Globigerina Limestone Member (Globigerina Limestone Formation): New preliminary data based on calcareous plankton. Boll. Della. Soc. Paleontol. Ital. 2007, 46, 175–181. [Google Scholar]
  68. Challis, G.R. Miocene echinoid biofacies of the Maltese Islands. Annales Géologiques des Pays Helléniques Tome Hors Série 1979, 1, 253–261. [Google Scholar]
  69. Bennett, S.M. Palaeoenvironmental Studies in Maltese Mid-Tertiary Carbonates. Ph.D. Thesis, University of London, London, UK, 1980. [Google Scholar]
  70. Carbone, S.; Grasso, M.; Lentini, F.; Pedley, H.M. The distribution and palaeoenvironmental of early Miocene phosphorites of southeast Sicily and their relationship with the Maltese phosphorites. Palaeogeogr. Palaeoclimatol. Palaeoecol. 1987, 58, 35–53. [Google Scholar] [CrossRef]
  71. Rose, E.P.F.; Pratt, S.K.; Bennett, S.M. Evidence for Sea-Level Changes in the Globigerina Limestone formation (Miocene) of the Maltese Islands. Paleontol. I Evol. 1992, 24–25, 265–276. [Google Scholar]
  72. Bennett, S.M. A transgressive carbonate sequence spanning the Paleogene Neogene boundary on the Maltese Islands. Ann. Géol. Pays Helléniques 1979, 1, 71–80. [Google Scholar]
  73. Continentalshelf.gov.mt. Geological Map of the Maltese Islands. Available online: https://continentalshelf.gov.mt/en/Pages/Geological-Map-of-the-Maltese-Islands.aspx (accessed on 21 April 2021).
  74. Pedley, H.M.; Bennett, S.M. Phosphorites, hardgrounds and syndepositional subsidence: A paleoenvironmental model from Miocene of the Maltese Islands. Sediment. Geol. 1985, 45, 1–34. [Google Scholar] [CrossRef]
  75. Rehfeld, U.; Janssen, A.W. Development of phosphatized hardgrounds in the Miocene Globigerina Limestone of the Maltese Archipelago, including a description of Gamopleura melitensis sp. nov. (Gastropoda, Euthecosomata). Facies 1995, 33, 91–106. [Google Scholar] [CrossRef]
  76. Colson, C.; Colson, C.H. The 160-ton hydraulic crane at Malta dockyard extension works. Minutes Proc. Inst. Civ. Eng. 1893, 114, 284–288. [Google Scholar]
  77. Adams, A.L. Notes of a Naturalist in the Nile Valley and Malta: A Narrative of Exploration and Research in Connection with the Natural History, Geology, and Archaeology of the Lower Nile and Maltese Islands; Edmonston and Douglas: Edinburgh, UK, 1870. [Google Scholar]
  78. Bianco, L. The industrial minerals of the Maltese islands: A general introduction. Hyphen 1995, 7, 111–118. [Google Scholar]
  79. Bondin, S.; Former President of the Softstone Quarry Association, Mqabba, Malta. Personal Communication, 1992.
  80. Camilleri, D.H. Globigerina limestone as a structural material. Architect 1988, 9, 17–25. [Google Scholar]
  81. Bianco, L. Mineralogy and geochemistry of blue patches occurring in the Globigerina Limestone Formation used in the architecture of the Maltese Islands. C. R. L’académie Bulg. Sci. 2017, 70, 537–544. [Google Scholar]
  82. Bianco, L. Petrological characteristics of blue lenticular patches occurring in the lower globigerina building limestone of Malta. Rom. J. Mater. 2018, 48, 115–120. [Google Scholar]
  83. Planning Authority. Available online: http://geoserver.pa.org.mt/publicgeoserver (accessed on 1 February 2021).
  84. Bianco, L. Bio-retexturing in limestone used in the built heritage of Malta. Rom. Rep. Phys. 2017, 69, 802. [Google Scholar]
  85. Bianco, L. Limestone replacement in restoration: The case of the Church of Santa Maria (Birkirkara, Malta). Int. J. Conserv. Sci. 2017, 8, 167–176. [Google Scholar]
  86. Montebello, T.; Sculptor, Restoration of Church of Santa Maria, Birkirkara, Malta. Personal Communication, 1992.
  87. Dean, W.E. Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: Comparison with other methods. J. Sediment. Petrol. 1974, 44, 242–248. [Google Scholar]
  88. Fitton, G. X-ray fluorescence spectrometry. In Modern Analytical Geochemistry: An Introduction to Quantitative Chemical Analysis for Earth, Environmental and Material Scientists; Gill, R., Ed.; Longman: Harlow, UK, 1997; pp. 97–115. [Google Scholar]
  89. Drever, J.I. The preparation of oriented clay mineral specimens for X-ray diffraction analysis by a filter-membrane peel technique. Am. Mineral. 1973, 58, 553–554. [Google Scholar]
  90. Koleva-Rekalova, E. Petrographical Descriptions of Thin Sections from the Lower Globigerina Limestone Member from Malta. Unpublished work. 2017. [Google Scholar]
  91. Dunham, R.J. Classification of carbonate rocks according to depositional texture. In Classification of Carbonate Rocks—A Symposium; Ham, W.E., Ed.; American Association of Petroleum Geologists: Tulsa, OK, USA, 1962; Memoir 1; pp. 108–121. [Google Scholar]
  92. Pye, K. Forensic examination of rocks, sediments, soils and dusts using scanning electron microscopy and X-ray chemical microanalysis. In Forensic Geoscience: Principles, Techniques and Applications; Pye, K., Croft, D.J., Eds.; Geological Society: London, UK, 2004; pp. 103–122. [Google Scholar]
  93. Reed, S.J.B. Electron Microprobe Analysis and Scanning Electron Microscopy in Geology; Cambridge University Press: Cambridge, UK, 2005. [Google Scholar]
  94. Cnudde, V.; Cwirzen, A.; Masschaele, B.; Jacobs, P.J.S. Porosity and microstructure characterization of building stones and concretes. Eng. Geol. 2009, 103, 76–83. [Google Scholar] [CrossRef]
  95. Yan, Z.; Chen, C.; Fan, P.; Wang, M.; Fang, X. Pore structure characterization of ten typical rocks in China. Electron. J. Geotech. Eng. 2015, 20, 479–494. [Google Scholar]
  96. Tucker, M.E. Sedimentary Petrology: An Introduction, 3rd ed.; Blackwell Scientific Publications: Oxford, UK, 2001. [Google Scholar]
  97. Camilleri, D.H. Structural Integrity Handbook: Building-Regulations 2000; Building Industry Consultative Council: Floriana, Malta, 2000; Available online: http://www.dhiperiti.com/wp-content/uploads/2013/10/2.Structural-Integrity-Handbook-Building-Regulations-20006.pdf (accessed on 17 March 2021).
  98. Centre Technique de Matériaux Naturels de Construction. Study on Characteristic Compressive Strength of Natural Stone Masonry Made with General Purpose Mortar; Centre Technique de Matériaux Naturels de Construction: Paris, France, 2015. [Google Scholar]
  99. Camilleri, D.H. Malta’s heritage in stone: From temple builders to Eurocodes 6/8. Mason. Int. 2019, 31, 49–67. [Google Scholar]
  100. Vasanelli, E.; Colangiuli, D.; Calia, A.; Sileo, M.; Aiello, M.A. Ultrasonic pulse velocity for the evaluation of physical and mechanical properties of a highly porous building limestone. Ultrasonics 2015, 60, 33–40. [Google Scholar] [CrossRef]
  101. Anonymous. Classification of rocks and soils for engineering geological mapping. Part I—Rock and soil materials. Bull. Int. Assoc. Eng. Geol. 1979, 19, 364–371. [Google Scholar] [CrossRef]
  102. Cox, F.C.; McC Bridge, D.; Hull, J.H. Procedure for the Assessment of Limestone Resources: Mineral Assessment Report 30; Institute of Geological Sciences: London, UK, 1977. [Google Scholar]
  103. Torrero, E.; Sanz, D.; Arroyo, M.N.; Navarro, V. The Cathedral of Santa Maria (Cuenca, Spain): Principal stone characterization and conservation status. Int. J. Conserv. Sci. 2015, 6, 625–632. [Google Scholar]
  104. Balog, A.-A.; Cobirzan, N.; Mosonyi, E. Analysis of limestones from heritage buildings as damage diagnostics. Rom. Rep. Phys. 2016, 68, 353–361. [Google Scholar]
  105. Choquette, P.W.; Pray, L.C. Geologic nomenclature and classification of porosity in sedimentary carbonates. Bull. Am. Assoc. Pet. Geol. 1970, 54, 207–250. [Google Scholar]
  106. Robertson, J.C. Report on the Possibility of Increasing the Fresh Water Supply and of Utilising the Sewage for Crop Irrigation in Malta; Government Printing Office: Valletta, Malta, 1917.
  107. Morris, T.O. The Water Supply Resources of Malta; Government of Malta: Valletta, Malta, 1952.
  108. De Boisgelin, L. Ancient and Modern Malta; Richard Philips: London, UK, 1805. [Google Scholar]
  109. Galea, R.V. Geology of the Maltese archipelago. In Malta and Gibraltar Illustrated; Macmillan, A., Ed.; W.H. and L. Collingridge: London, UK, 1915; pp. 173–182. [Google Scholar]
  110. Building Research Station. The Maltese Islands: Use of Limestone for Building; Department of Scientific and Industrial Research, Building Research Station: Watford, UK, 1958.
  111. Building Research Station. Maltese Limestones: Relation of Durability to Laboratory-Measured Properties and Efficacy of Silicone Treatments: Note No. C965; Department of Scientific and Industrial Research, Building Research Station: Watford, UK, 1963.
  112. Building Research Station. The Maltese Islands: Properties and Behaviour of Local Limestone: Internal Note 6; Department of Scientific and Industrial Research, BRS: Watford, UK, 1964.
  113. Fitzner, B.; Heinrichs, K.; Volker, M. Model for salt weathering in Maltese Globigerina limestone. In Origin, Mechanisms and Effects of Salt on Degradation of Monuments in Marine and Continental Environments; Zezza, F., Ed.; European Commission: Bari, Italy, 1997; pp. 333–344. [Google Scholar]
  114. Sandrolini, F.; Franzoni, E.; Cuppini, G. Predictive diagnostics for decayed ashlars substitution in architectural restoration in Malta. Mater. Eng. 2000, 11, 323–337. [Google Scholar]
  115. Koleva-Rekalova, E.; Bianco, L. Importance of optical microscopic investigation in provenance studies and quality characteristics of both historical and natural Lower Globigerina building limestone of Malta. Rev. Bulg. Geol. Soc. 2018, 79, 93–94. [Google Scholar]
  116. Ellul, M.; Antiquities Division, Public Works Department, Floriana, Malta; University of Malta, Msida, Malta. Personal Communication, 1992.
  117. Rossi-Manaresi, R.; Tucci, A. Pore structure and the disruptive or cementing effect of salt crystallization in various types of stone. Stud. Conserv. 1990, 36, 53–58. [Google Scholar]
  118. Leeder, M.R. Sedimentology: Process and Product, 1st ed.; Chapman & Hall: London, UK, 1982; reprinted 1994. [Google Scholar]
  119. Kozlowski, R.; Magiera, J.; Weber, J.; Haber, J. Decay and conservation of Pinczow porous limestone. Stud. Conserv. 1989, 35, 205–221. [Google Scholar] [CrossRef]
  120. Ollier, C.D. Weathering Geomorphology Texts; Oliver & Boyd: Edinburgh, UK, 1984. [Google Scholar]
  121. Bromley, R.G. Trace Fossils. Biology, Taphonomy and Applications; Chapman & Hall: London, UK, 1996. [Google Scholar]
  122. Ricken, W. The carbonate compaction law: A new tool. Sedimentology 1987, 34, 571–584. [Google Scholar] [CrossRef]
  123. Gingras, M.K.; Baniak, G.; Gordon, J.; Hovikoski, J.; Konhauser, K.O.; La Croix, A.; Lemiski, R.; Mendoza, C.; Pemberton, S.G.; Polo, C.; et al. Porosity and permeability in bioturbated sediments. In Trace Fossils as Indicators of Sedimentary Environments; Knaust, D., Bromley, R.G., Eds.; Elsevier: Amsterdam, The Netherlands, 2012; pp. 837–868. [Google Scholar]
  124. Pedley, H.M. Bio-retexturing: Early diagenetic fabric modifications in outer-ramp settings—A case study from the Oligo-Miocene of the Central Mediterranean. Sediment. Geol. 1992, 79, 173–188. [Google Scholar] [CrossRef]
  125. Cassar, J.; Vella, A.J. Methodology to identify badly weathering limestone using geochemistry: Case study on the Lower Globigerina Limestone of the Maltese Islands. Q. J. Eng. Geol. Hydrogeol. 2003, 36, 85–96. [Google Scholar] [CrossRef]
Figure 1. The Maltese Archipelago: (a) location with respect to the Mediterranean Basin; (b) the main habitable islands; the location of case-studies is also included (© Google Earth).
Figure 1. The Maltese Archipelago: (a) location with respect to the Mediterranean Basin; (b) the main habitable islands; the location of case-studies is also included (© Google Earth).
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Figure 2. Dated photo of: (a) Ħaġar Qim Neolithic Temples, Qrendi, one of the UNESCO World Heritage sites in Malta (Ħagar Qim by peuplier is licensed under CC BY 2.0) [30]; (b) the Grandmaster’s Palace, Valletta.
Figure 2. Dated photo of: (a) Ħaġar Qim Neolithic Temples, Qrendi, one of the UNESCO World Heritage sites in Malta (Ħagar Qim by peuplier is licensed under CC BY 2.0) [30]; (b) the Grandmaster’s Palace, Valletta.
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Figure 3. Geological map of the Maltese Islands [73].
Figure 3. Geological map of the Maltese Islands [73].
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Figure 4. Aerial imagery, dated 1998, of Tal-Warda quarry (outlined in red) and the Chapel of St Catherine (in red) [83]; for the location of site with respect to Malta, see Figure 1b.
Figure 4. Aerial imagery, dated 1998, of Tal-Warda quarry (outlined in red) and the Chapel of St Catherine (in red) [83]; for the location of site with respect to Malta, see Figure 1b.
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Figure 5. A lithostratigraphic column of Tal-Warda quarry with respect to the general stratigraphy of the Maltese Islands.
Figure 5. A lithostratigraphic column of Tal-Warda quarry with respect to the general stratigraphy of the Maltese Islands.
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Figure 6. Dated photo of: (a) The Church of Santa Maria, Birkirkara; (b) Detail circled in black in (a).
Figure 6. Dated photo of: (a) The Church of Santa Maria, Birkirkara; (b) Detail circled in black in (a).
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Figure 7. Variations in IR and SiO2.
Figure 7. Variations in IR and SiO2.
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Figure 8. Variations in TiO2, Al2O3, Fe2O3, K2O and P2O5.
Figure 8. Variations in TiO2, Al2O3, Fe2O3, K2O and P2O5.
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Figure 9. XRD trace patterns of 2Theta (o) (x-axis) versus intensity (a.u.) (y-axis): (a) whole rock for samples Q1_1 to Q1_16; (b) insoluble residue for samples Q1_1 to Q1_10; (c) insoluble residue for samples Q1_11 to Q1_16; and clay fraction for selected samples Q1_2 and Q1_4, (d) air-dried; (e) glycolated; (f) heated.
Figure 9. XRD trace patterns of 2Theta (o) (x-axis) versus intensity (a.u.) (y-axis): (a) whole rock for samples Q1_1 to Q1_16; (b) insoluble residue for samples Q1_1 to Q1_10; (c) insoluble residue for samples Q1_11 to Q1_16; and clay fraction for selected samples Q1_2 and Q1_4, (d) air-dried; (e) glycolated; (f) heated.
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Figure 10. Sample C1: (a) plane-polarized light: micrite is recrystallized to microspar and containing planktonic Globigerina and benthic foraminifera, in the central part the micrite is preserved and looks like as micrite intraclast; (b) cross-polarized light: same view as (a), intraparticle porosity.
Figure 10. Sample C1: (a) plane-polarized light: micrite is recrystallized to microspar and containing planktonic Globigerina and benthic foraminifera, in the central part the micrite is preserved and looks like as micrite intraclast; (b) cross-polarized light: same view as (a), intraparticle porosity.
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Figure 11. Sample C2: (a) plane-polarized light: parallel nicols, micrite matrix with planktonic Globigerina foraminifera and rare bioclasts; (b) cross-polarized light: same view as (a), inter-particle porosity; (c) plane-polarized light: micrite matrix with planktonic Globigerina foraminifera and bioclasts, in the central part an echinoid fragment with syntaxial overgrowth; (d) plane-polarized light: micrite is recrystallized to microspar and containing planktonic Globigerina foraminifers, single benthic foraminifera and bioclasts; in the central part the micrite is preserved and looks like a micrite intraclast.
Figure 11. Sample C2: (a) plane-polarized light: parallel nicols, micrite matrix with planktonic Globigerina foraminifera and rare bioclasts; (b) cross-polarized light: same view as (a), inter-particle porosity; (c) plane-polarized light: micrite matrix with planktonic Globigerina foraminifera and bioclasts, in the central part an echinoid fragment with syntaxial overgrowth; (d) plane-polarized light: micrite is recrystallized to microspar and containing planktonic Globigerina foraminifers, single benthic foraminifera and bioclasts; in the central part the micrite is preserved and looks like a micrite intraclast.
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Figure 12. SEM images illustrating the pore structure, the physico-mechanical interlocking and the fine-grained sparry calcite cement of samples from: (a) the first-quality bed within Tal-Warda quarry; (b) the Church of Santa Maria, Birkirkara; (c) the second-quality bed within Tal-Warda quarry; (d) micrite matrix in the second-quality bed within Tal-Warda quarry.
Figure 12. SEM images illustrating the pore structure, the physico-mechanical interlocking and the fine-grained sparry calcite cement of samples from: (a) the first-quality bed within Tal-Warda quarry; (b) the Church of Santa Maria, Birkirkara; (c) the second-quality bed within Tal-Warda quarry; (d) micrite matrix in the second-quality bed within Tal-Warda quarry.
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Table 1. The lithostratigraphy of the Maltese Archipelago [6].
Table 1. The lithostratigraphy of the Maltese Archipelago [6].
FormationMemberAgeThickness (t) (m)
MaltaGozo and Comino
Upper Coralline LimestoneĠebel ImbarkMessinian04 ≤ t ≤ 2504 ≤ t ≤ 20
Tal-PitkalTortonian to Messinian30 ≤ t ≤ 5001 ≤ t ≤ 30
MtarfaTortonian12 ≤ t ≤ 1602 ≤ t ≤ 16
Għajn MelelTortonian00 ≤ t ≤ 1300 ≤ t ≤ 16
Greensand Tortonian 00 ≤ t ≤ 11
Blue Clay Serravallian to Tortonian15 ≤ t ≤ 7518 ≤ t ≤ 75
Globigerina LimestoneUpper Globigerina LimestoneLanghian08 ≤ t ≤ 2602 ≤ t ≤ 15
Middle Globigerina LimestoneAquitanian to Burdigalian15 ≤ t ≤ 3800 ≤ t ≤ 15
Lower Globigerina LimestoneAquitanian00 ≤ t ≤ 8005 ≤ t ≤ 40
Lower Coralline LimestoneIl-MaraChattian00 ≤ t ≤ 2000 ≤ t ≤ 06
XlendiChattian00 ≤ t ≤ 2200 ≤ t ≤ 22
AttardChattian10 ≤ t ≤ 1510 ≤ t ≤ 15
MagħlaqChattiant > 38t unknown
Table 2. Visual description of limestone samples.
Table 2. Visual description of limestone samples.
Bed No.Thickness (m)Limestone QualitySample IDPosition *
(m)
Depth **
(m)
Description
Quarry FaceHand Specimen
11.1firstQ1_1 100.00.04Burrows and occasional nodules are present.Pale yellow in color;
Burrows: 10% of the rock sample with diameter c. 3 mm; cream in color; sample had two dark yellow “iron stained” nodules.
20.2firstQ1_2 101.11.2Burrows dark brown and fine grained.Pale yellow in color;
Burrows: light brown in color.
31.3firstQ1_3101.32.2A few burrows with “iron stained” infill.Pale to brown yellow;
Burrows: with “iron stained” infill.
40.6first 02.6
51.7secondQ1_4 103.23.8Dark, heavily infilled, burrows with occasional red-brownish stains.Cream colored, with shell fragments;
Burrows: white infill with darker cream to yellow material surrounding them.
64.4firstQ1_504.97.1Occasional burrows with infill identical to host rock.Pale to cream yellow in color.
71.2secondQ1_6 109.310.2Burrows progressively decreasing.Pale yellow in color with shell fragments;
Burrows: cream-colored infill.
81.5firstQ1_710.511.3Burrows rarely present; increase towards the top of the bed.White in color;
Burrows: diameter >18 mm, with pale brown infill.
93.0secondQ1_8 212.013.1Darker burrow infill decreasing towards the top of the bed.White in color with some shell fragments;
Burrows: brown and dark green/grey in color with occasional “iron stained” infill.
93.0secondQ1_9 112.014.2As Q1_8.White in color with even whiter burrow infill.
104.3firstQ1_1015.018.0No evidence of burrows.Pale yellow in color with occasional shell fragments.
113.2firstQ1_11 219.320.7Though not heavily burrowed; quality is deteriorating upwards.Pale yellow in color;
Burrows: dark infill.
Q1_1219.321.7
126.0firstQ1_1322.525.3Sharp contact between host rock and burrows.Pale yellow in color with shell fragments;
Burrows: compact, cream yellow to dark green/grey infill.
126.0firstQ1_14 222.528.1Density of burrows increases up in the bed.Pale yellow in color;
Burrows: compact, cream yellow infill rich in shell fragments.
133.3firstQ1_1528.530.8As per bed 12.Cream colored with shell fragments;
Burrows: two (10 and 12 mm in diameter) with cream-colored infill.
133.3firstQ1_1628.531.7As per bed 12.Cream-colored, weathered specimen rich in shell fragments;
Burrows are present.
* Position of the upper side of the bed from quarry top; ** depth of extraction of sample; 1 reproduced in [41]; 2 reproduced in [84]; n.d.: not determined.
Table 3. LOI and chemical composition (%).
Table 3. LOI and chemical composition (%).
Sample IDLOIX-ray Fluorescence Analysis
CaOSiO2Al2O3MgOFe2O3K2OP2O5TiO2Na2OMnO
Q1_1 141.3749.38907.95401.28801.17600.73900.42500.22000.18400.03700.032
Q1_2 141.7249.66806.76701.06201.14200.59500.35600.21100.15100.04200.033
Q1_3 142.2450.04405.51600.80601.11300.49800.27800.20400.12200.02500.035
Q1_4 140.4348.16509.69501.38801.14001.03800.51200.30000.22100.05700.036
Q1_541.4949.24507.02100.85600.98800.65200.36000.24300.14800.06200.034
Q1_6 139.4346.65111.98501.61501.15700.97800.63400.25800.25800.09000.033
Q1_741.4448.77807.21800.91901.18900.74200.37000.28500.12800.06600.034
Q1_8 241.7549.57706.50600.83801.17300.82200.36900.70700.11700.05500.035
Q1_9 142.1849.67106.32900.85601.13700.64900.33200.31000.10700.05800.034
Q1_1042.6050.08804.84700.58400.98900.45000.22400.21800.08300.06500.032
Q1_11 242.8150.22204.27200.43400.71900.31700.18500.18600.07000.07100.036
Q1_1242.8349.61904.05400.39800.99400.42000.17100.54600.06700.08400.035
Q1_1342.5749.77204.45200.43401.11600.36700.19400.57600.07500.07300.034
Q1_14 242.0148.78905.09400.51301.17200.63300.25101.08000.08400.14100.036
Q1_1541.2748.54506.80100.72601.18000.78100.30401.12100.12400.11400.037
Q1_1641.8749.43506.10900.65101.13300.65600.28100.95200.09700.03700.036
1 Reproduced in [41]; 2 reproduced in [84].
Table 4. IR and X-ray diffraction analysis: summary of identified minerals.
Table 4. IR and X-ray diffraction analysis: summary of identified minerals.
Sample
ID
IR
(%)
X-ray Diffraction Analysis 1
Whole RockIRClay Fraction
calqtzqtzKfsmsklnillsmegpalrtklnillsmeqtzKfs
Q1_1 106.1xxxxxx n.d.n.d.n.d.n.d.n.d.
Q1_2 104.8x xx xx xxxxx
Q1_3 103.6xxxxxx n.d.n.d.n.d.n.d.n.d.
Q1_4 108.2xxxxx xxxxx
Q1_505.9xxxxx n.d.n.d.n.d.n.d.n.d.
Q1_6 111.3xxxxx n.d.n.d.n.d.n.d.n.d.
Q1_705.7xxxxx n.d.n.d.n.d.n.d.n.d.
Q1_8 204.6xxxxxx n.d.n.d.n.d.n.d.n.d.
Q1_9 104.7xxxxx n.d.n.d.n.d.n.d.n.d.
Q1_1003.3xxxxxx n.d.n.d.n.d.n.d.n.d.
Q1_11 202.8x xx x x xxxxx
Q1_1202.6x xx xxx n.d.n.d.n.d.n.d.n.d.
Q1_1303.1xxxxx x xxxxx
Q1_14 203.8xxxxx xn.d.n.d.n.d.n.d.n.d.
Q1_1505.4xxxxx x n.d.n.d.n.d.n.d.n.d.
Q1_1604.3xxxxx xxxxx
1 Reproduced in [41]; 2 reproduced in [84]; x: mineral is present; n.d.: not determined.
Table 5. Petrographical characteristics (%).
Table 5. Petrographical characteristics (%).
CharacteristicsSample ID 1
Old Church, Birkirkara Mqabba Quarry
C1C2Q1Q2Q3
Matrix8075807065
Allochems2025203035 2
Terrigenous componentssingle; sizes < 0.1 mmsparodic; sizes < 0.08 mmrarerarerare
Classificationwackestonewackestonewackestonewackestonewackestone
1 The analyses of these samples are contained in [85]; 2 there is a typo error in [85].
Table 6. Interpolated mercury intrusion pore radius.
Table 6. Interpolated mercury intrusion pore radius.
Sample IDTotal VolumeVolume in Pores (cm3/g)
Pore Radius
>40,000Å
Pore Radius
40–40,000Å
Pore Radius
<40Å
Q1_2 10.10970.00180.10710.0008
Q1_4 10.09230.00010.08980.0024
Q1_6 10.08930.00680.08210.0004
Q1_9 10.10920.00110.10720.0009
Q1_110.06000.00010.05930.0006
Q1_130.09170.01120.08030.0002
Q1_140.10840.01120.09600.0012
1 Reproduced in [41].
Table 7. Petrophysical analysis.
Table 7. Petrophysical analysis.
Sample
ID
Apparent Density (kg/m3)CS (N/mm2)UPV (km/s)
Oven DriedSaturatedOven DriedSaturatedPerpendicularParallel
Q1_2 11778201630.1815.423.062.99
Q1_4 11844206828.8312.892.912.97
Q1_6 11844206725.3015.262.982.99
Q1_9 11719195421.8215.092.912.86
Q1_111717194915.5808.272.692.71
Q1_131784197527.8817.433.223.11
Q1_141787199922.5416.953.072.94
1 Reproduced in [41].
Table 8. Color measurements.
Table 8. Color measurements.
Sample
ID
Wavelength
(nm)
Purity
(%)
Brightness (%)Filter ValueL *a **b ***Eab #Eab *##
Red (x)Green (y) Blue (z)
Q1_15817570.572.575.361.289.54−5.7012.14137.853.57
Q1_25817571.573.976.563.390.10−5.2411.18138.063.36
Q1_35827571.774.376.863.890.24−5.0210.97138.023.40
Q1_45807569.170.473.857.088.84−7.0414.90137.204.22
Q1_55807572.775.077.763.390.65−5.3812.13137.533.89
Q1_65807569.871.674.659.589.22−6.1613.16137.573.85
Q1_75827571.373.676.462.190.05−5.6512.19137.733.69
Q1_85827571.473.676.562.290.10−5.8512.18137.893.53
Q1_95827573.275.678.265.190.88−5.1610.91138.123.30
Q1_105827375.378.980.669.191.96−3.2909.30137.703.72
Q1_115827377.681.183.072.093.02−3.6108.69138.283.14
Q1_125827574.477.279.667.191.51−4.7010.25138.163.26
Q1_135827574.277.479.367.891.38−3.7209.41137.983.44
Q1_145827570.472.475.660.689.68−6.5112.94137.953.47
Q1_155807569.371.274.258.889.03−6.1813.50137.394.03
Q1_165807571.273.475.861.089.78−4.8612.73136.814.61
* L = 116[(y/100)1/3] − 16; ** a = 500[(x/100)1/3 − (y/100)1/3]; *** b = 200[(y/100)1/3 − (z/100)1/3]; # Eab = [(L − 100)2 + (a − 100)2 + (b − 100)2]1/2; ## Eab* = (141.42 − Eab).
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Bianco, L. Geochemistry, Mineralogy and Textural Properties of the Lower Globigerina Limestone Used in the Built Heritage. Minerals 2021, 11, 740. https://doi.org/10.3390/min11070740

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Bianco L. Geochemistry, Mineralogy and Textural Properties of the Lower Globigerina Limestone Used in the Built Heritage. Minerals. 2021; 11(7):740. https://doi.org/10.3390/min11070740

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Bianco, Lino. 2021. "Geochemistry, Mineralogy and Textural Properties of the Lower Globigerina Limestone Used in the Built Heritage" Minerals 11, no. 7: 740. https://doi.org/10.3390/min11070740

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