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

: 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available a corpus of ﬁndings relating to the geochemistry, mineralogy and textural properties of this limestone—mostly unpublished and undertaken nearly three decades ago—which provide a useful source to understand its behavior.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 Fe 2 O 3 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.


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][38][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 km 2 , 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.

Utilization of Lower Globigerina Limestone in Buildings
LGL has been quarried since the Neolithic period (3600-2500 BC), and is still the p cipal source of limestone for buildings on the Maltese Islands-whether for the const tion of contemporary architectural and infrastructural works or for the restoration of heritage. It was utilized by builders flourishing during one of the oldest Neolithic c zations of the Mediterranean Basin [7][8][9][10][11]. Archeological structures dating from this riod illustrate that those who built them differentiated between the softer, less dur LGL and the harder, denser, stronger and more durable varieties of Coralline Limes (CL) used for external walls, which were uncarved. In contemporary minerals plan practice in Malta, LGL and CL are referred to softstone and hardstone, respectively.
Malta's numerous free-standing megalithic structures dating from the Neolithic riod, a number of which are listed as UNESCO World Heritage sites [12] (Figure 2a examples of anthropogenic geodiversity par excellence [13]. The softness of LGL imp that it is easily hewed; as a result, there are many hypogea and catacombs carved outcrops throughout the islands, most notably the Ħal Saflieni Hypogeum (c. 2500 [14] and St Paul's Catacombs (early centuries of Christianity and used until sevent eighth century AD) [15].
LGL was used in Mdina, the old capital city which originally formed part of the cient of city of Melite (Greek: Μελίτη; Maleth under the Phoenicians [16]), and in Vall the Late Renaissance capital, which was founded in 1566. Laid out as a Hippodamian on a peninsula between two natural harbors [17], Valletta boasts an ensemble of 320 m uments within a superficial area of 55 ha (0.56 km 2 ) (Figure 2b). This entire area is re tered as a UNESCO World Heritage Site [18]. LGL was used in Malta not only in build and monuments (see, for example, [19,20]), but also in the erection of fortifications rounding Mdina [21], Valletta [22,23] and Cottonera [23]. The Hospitaller Order of St (1530-1798) made extensive use of Globigerina limestone from local quarries throug the duration of their stay [24,25].
The structural engineering properties of LGL were tested to the limit with the e tion of the Neoclassical Rotunda of Mosta, one of the largest unsupported masonry do in the history of building construction [26,27]. During the early years of the Order's p

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

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 km 2 ) (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 al 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: Mελίτη; 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 km 2 ) (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]. n of Lower Globigerina Limestone in Buildings been quarried since the Neolithic period (3600-2500 BC), and is still the prinof limestone for buildings on the Maltese Islands-whether for the construcporary architectural and infrastructural works or for the restoration of built as utilized by builders flourishing during one of the oldest Neolithic civilie Mediterranean Basin [7][8][9][10][11]. Archeological structures dating from this pee that those who built them differentiated between the softer, less durable harder, denser, stronger and more durable varieties of Coralline Limestone r external walls, which were uncarved. In contemporary minerals planning alta, LGL and CL are referred to softstone and hardstone, respectively. numerous free-standing megalithic structures dating from the Neolithic peer of which are listed as UNESCO World Heritage sites [12] ( Figure 2a) are anthropogenic geodiversity par excellence [13]. The softness of LGL implies ily hewed; as a result, there are many hypogea and catacombs carved into oughout the islands, most notably the Ħal Saflieni Hypogeum (c. 2500 BC) aul's Catacombs (early centuries of Christianity and used until seventh or ry AD) [15]. s used in Mdina, the old capital city which originally formed part of the anf Melite (Greek: Μελίτη; Maleth under the Phoenicians [16]), and in Valletta, aissance capital, which was founded in 1566. Laid out as a Hippodamian grid la between two natural harbors [17], Valletta boasts an ensemble of 320 monin a superficial area of 55 ha (0.56 km 2 ) ( Figure 2b). This entire area is regis-ESCO World Heritage Site [18].
LGL was used in Malta not only in buildings ents (see, for example, [19,20]), but also in the erection of fortifications surina [21], Valletta [22,23] and Cottonera [23]. The Hospitaller Order of St John ade extensive use of Globigerina limestone from local quarries throughout of their stay [24,25]. ctural engineering properties of LGL were tested to the limit with the ereceoclassical Rotunda of Mosta, one of the largest unsupported masonry domes of building construction [26,27]. During the early years of the Order's pres-, CL was used in the production of small works such as mill stones, with one operating in Birgu [28]. By the late seventeenth century, it was used at sites ea-spray. For example, the exterior walls of the bastions facing Marsamxett stone in Buildings eolithic period (3600-2500 BC), and is still the prinon the Maltese Islands-whether for the construcinfrastructural works or for the restoration of built urishing during one of the oldest Neolithic civili -11]. Archeological structures dating from this pem differentiated between the softer, less durable nd more durable varieties of Coralline Limestone ere uncarved. In contemporary minerals planning red to softstone and hardstone, respectively. egalithic structures dating from the Neolithic pe-NESCO World Heritage sites [12] (Figure 2a) are ty par excellence [13]. The softness of LGL implies re are many hypogea and catacombs carved into notably the Ħal Saflieni Hypogeum (c. 2500 BC) nturies of Christianity and used until seventh or apital city which originally formed part of the analeth under the Phoenicians [16]), and in Valletta, s founded in 1566. Laid out as a Hippodamian grid rbors [17], Valletta boasts an ensemble of 320 monha (0.56 km 2 ) ( Figure 2b). This entire area is regise [18]. LGL was used in Malta not only in buildings ,20]), but also in the erection of fortifications surd Cottonera [23]. The Hospitaller Order of St John bigerina limestone from local quarries throughout ties of LGL were tested to the limit with the erecta, one of the largest unsupported masonry domes [26,27]. During the early years of the Order's presuction of small works such as mill stones, with one the late seventeenth century, it was used at sites exterior walls of the bastions facing Marsamxett agar Qim by peuplier is licensed under CC BY 2.0) [30]; (b) the Grandmaster's Palace, Valletta.
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].

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.
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].

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 Minerals 2021, 11, 740 6 of 27 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].

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 (1864Colson ( -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 CaCO 3 content between 63.20% and 94.73%; the upper beds had 30% to 40%. In most beds, MgCO 3 was present. All contained traces of Ca 3 (PO 4 ) 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.

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 "hafna frak tal-hġieġ" (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 ahmar" (red sol) and "sol ikhal" (blue sol); the latter is also known as "ġebla l-kahla" or "hadra", which translates as "the blue stone" or "green".
LGL has a specific blue-colored bed of sol ikhal, 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 ikhal; their SiO 2 content is >10% whilst that of sol ikhal <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 ikhal, they differ from sol ahmar, 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 ahmar.

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.  Cream colored with shell fragments; Burrows: two (10 and 12 mm in diameter) with cream-colored infill. 13 3.3 first Q1_16 28.5 31.7 As 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. 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.

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 CO 2 , 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 planepolarized 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.

Geochemistry
The XRF analyses for major oxides and the LOI are given in Table 3. LOI was <44%, the theoretical value for pure CaCO 3 . Samples Q1_4 and Q1_6, identified in the field as second-quality LGL (Table 2), have low CaCO 3 , whilst samples Q1_8 and Q1_9 have a CaCO 3 content comparable to first-quality lithotype. A lower CaCO 3 content is present in Q1_15. Burrow infills at the stratigraphic level from which Q1_8 was extracted had a similar CaCO 3 content to first quality, while similar infills at the lithostratigraphic beds from which Q1_11 and Q1_14 were extracted had a similar CaCO 3 content to second-quality lithotype [84], which indicates that burrowing activity introduced material different from the host rock. The main non-carbonate fraction is SiO 2 ( Figure 7); its presence is inversely proportional to the CaCO 3 content. The variation in Al 2 O 3 , Fe 2 O 3 , K 2 O and TiO 2 , although less pronounced, is similar to SiO 2 ( 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]. Na 2 O 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. CaCO 3 content decreases progressively as the quality of the LGL decreases [85]. The principal non-carbonate oxides-SiO 2 , Al 2 O 3 , Fe 2 O 3 and K 2 O-are minimally present in the first-quality LGL.
Sol ikhal has a lower CaCO 3 content than sol ahmar which, in turn, has a lower CaCO 3 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 SiO 2 comes from quartz, kaolinite, etc.

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.

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.  1 The analyses of these samples are contained in [85]; 2 there is a typo error in [85].
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.

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.

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/mm 2 , the CS of 100 × 100 × 100 mm samples varies between 15.6-30.2 N/mm 2 for dry states and 8.3-17.4 N/mm 2 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 (f k ) and lower porosity. According to the Centre Technique de Matériaux Naturels de Construction [98], LGL is definitely not a hard stone (f k > 40 N/mm 2 ) [99]; some lithostratigraphic beds qualify as a compact (10 ≤ f k ≤ 40 N/mm 2 ) rather than as a soft stone (f k < 10 N/mm 2 ). 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.

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 CaCO 3 content [102], the LGL is of medium (93.5% < CaCO 3 < 97.0%) to high (97.0% < CaCO 3 < 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.

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 intraparticle 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.

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 postdepositional 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.

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 interlayer 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/mm 2 [120].

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 Fe 2 O 3 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.

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.

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]: 1.
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 firstquality 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 Fe 2 O 3 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 reactionsboth 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.