Diatomaceous Silica in Environmental Applications: A Case Study from the Lacustrine Deposit of Limnos Island, Aegean Sea, Greece

: Diatomitic and clastic-volcanoclastic marly samples from the Paranisia area of Limnos Island, were studied mineralogically by X-ray diffraction (XRD), chemically by X-ray ﬂuorescence spectroscopy (XRF) and morphologically by scanning electron microscopy (SEM-EDS), together with some physical properties such as the insulation block density, speciﬁc surface area and porosity. The diatomaceous samples were classiﬁed as porcelaneous or clayey (moler type) diatomite. Opal-CT forms microplates disseminated in the groundmass and are diagenetically formed in expense of opal-A dissolution. The purest diatomaceous beds have been transformed into opaline beds (opal-CT-rich rocks), whereas the clayey beds were not inﬂuenced by diagenetic transformations. The studied diatomites from Limnos Island are suitable materials for environmental uses, as an absorbent, for the production of insulation bricks or as lightweight aggregates.


Introduction
Diatomite is a friable, earthy, very fine-grained, siliceous sedimentary rock resulting from the accumulation of fossil frustules of diatoms. They consist of porous siliceous algae skeletons with Si-O tetrahedral interconnection nets. [1,2]. Pure diatomites are usually light in colour, with low thermal conductivity and a high fusion point. Besides the silica polymorphs, diatomitic rocks may also contain clay minerals, carbonates, feldspars, volcanic glass, silica sand, iron oxides and organic matter [3,4].
Diatomite formations exist in many countries all over the world: USA (especially California), Canada, Belgium, Denmark, Germany, USSR, Canada, South Korea, Romania, Japan, South Africa, Ireland, etc. They also appear in Mediterranean countries such as Greece, Italy, Spain, France, Morocco, Algeria and Cyprus [5].
Worldwide diatomite production has increased by 4% from previous years, more specifically, 2.6 million tones in 2019. The USA is the dominant country, with 38% of global production. China follows with 16%, Turkey 9%, Denmark, Peru, Mexico, France and Argentina, 4% each. Denmark produces mostly moler-type diatomite (a diatomite with high percentage of clays). More specifically, in 2019 the USA produced 980 thousand tons of diatomite, China 420 thousand tons, Turkey 243 thousand tons and Denmark 108 thousand tons. Total world production amounts to 2.6 million tons (BGS, World Mineral Production 2019).
There are several locations in Greece where diatomaceous formations can be found. Greek diatomite is related to either marine or tertiary lake sedimentary basins. The sedimentary basins of Kleidi in the Florina region, Komninon in the Ptolemaida region, Eani in the Kozani region, Elasona, Megalopoli and the basin of Mitilinion in Samos Island are the locations with the most significant resources. Other locations are the Islands of Aigina, Milos, Gavdos, Zakinthos and the basins of Kastelli kai Heraclium in Crete [6][7][8][9].
In the present study, a new deposit of diatomaceous-rich sediments from Limnos Island, Greece, were studied mineralogically and geochemically in order to classify them, while some physical parameters, such as insulation block density, porosity and specific surface area were measured in order to ascertain their possible environmental uses.

Materials and Methods
Limnos Island is located in the NE Aegean Sea and is part of the broader tertiary sedimentary basin located on the Rhodope metamorphic unit. The basin's sedimentary rocks can be delineated into two discrete units: the upper unit that consists of Oligocene marine and continental sedimentary rocks and tuffs and the lower sedimentary sequence that is comprised of Eocene-Oligocene sandstones, conglomerates and flysch.
The study area ( Figure 1) belongs to the Therma unit (Mi.m), which is part of the lower sedimentary sequence and consists of thinly bedded, whitish-yellow marls with diatomaceous intercalations and abundant plant remains, conglomerate bodies with oxidized pebble and volcanic clasts near the base. The diatomitic beds were found immediately below the volcanic tuffs of the upper unit, with thicknesses varying from 0.5 to 30 m. The average depth of the drilling procedure in the Paranisia area was about 25 m and the main rock types drilled were diatomite, clayey diatomite and tuffs. Most of the samples were alerted, except samples from drill A9, where the whole stratigraphic column exists. A total of eight samples from drill A9 were collected and studied.  [6][7][8][9].
In the present study, a new deposit of diatomaceous-rich sediments from Lim Island, Greece, were studied mineralogically and geochemically in order to classify th while some physical parameters, such as insulation block density, porosity and spe surface area were measured in order to ascertain their possible environmental uses.

Materials and Methods
Limnos Island is located in the NE Aegean Sea and is part of the broader tert sedimentary basin located on the Rhodope metamorphic unit. The basin's sedimen rocks can be delineated into two discrete units: the upper unit that consists of Oligo marine and continental sedimentary rocks and tuffs and the lower sedimentary sequ that is comprised of Eocene-Oligocene sandstones, conglomerates and flysch.
The study area ( Figure 1) belongs to the Therma unit (Mi.m), which is part of lower sedimentary sequence and consists of thinly bedded, whitish-yellow marls with atomaceous intercalations and abundant plant remains, conglomerate bodies with dized pebble and volcanic clasts near the base. The diatomitic beds were found imm ately below the volcanic tuffs of the upper unit, with thicknesses varying from 0.5 t m. The average depth of the drilling procedure in the Paranisia area was about 25 m the main rock types drilled were diatomite, clayey diatomite and tuffs. Most of the s ples were alerted, except samples from drill A9, where the whole stratigraphic colu exists. A total of eight samples from drill A9 were collected and studied.   The study area ( Figure 1) belongs to the Therma unit (Mi.m) which is part of the lower sedimentary sequence and consists of thinly bedded, whitish-yellow marls with diatomaceous intercalations and abundant plant remains, conglomerate bodies with oxidized pebble and volcanic clasts near the base. The diatomitic beds were found immediately below the volcanic tuffs of the upper unit, with thicknesses varying from 0.5 to 30 m. The average depth of the drilling procedure in Paranisia area was about 25 m and the main rock types drilled were diatomite, clayey diatomite and tuffs. Most of the samples were alerted, except samples from drill A9, where the whole stratigraphic column exists. A total of eight samples from drill A9 were collected and studied.

X-ray Diffraction Analysis (XRD)
A representative amount of the collected diatomite samples were powdered in an agate mortar and analyzed by X-ray diffraction to determine their mineralogical composition. XRD analysis was performed using a Phillips (PW1820/00) diffractometer with Ni-filtered CuKα radiation (PHILLIPS Analytical, Eindhoven, The Netherlands). The counting statistics of the XRD study were, step size: 0.01 • 2θ, start angle: 3 • , end angle: 63 • and scan speed: 0.02 • 2θ/s. Quantitative estimates of the abundance of the mineral phases were derived from the XRD data, using the intensity of certain reflections and external standard mixtures of minerals [27,28]. Corrections were made using the MAUD-Material Analysis Using Diffraction software (v. 2.992, Lutterotti L., University of Trento, Trento, Italy) with RIETVELD refinement [29]. Clay mineralogy was identified from air-dried, glycolated and heat-treated oriented samples scanned from 3 to 23 • 2θ at a scanning speed of 1.2 • /min. From the morphological examination of the background of the XRD patterns, a very broad background hump between 10-30 • 2θ clearly indicates the presence of amorphous material. The content of the amorphous phase was calculated according to the methodology proposed by Kantiranis et al. [27]. The detection limit for crystalline and amorphous phases was ±2% w/w.

Scanning Electron Microscopy (SEM-EDS)
The morphological study and microanalysis of representative diatomaceous samples was performed using scanning electron microscopy on a Jeol JSM-840: a scanning electron microprobe equipped with a LINK 10,000 energy dispersion analyzer (JEOL, Tokyo, Japan). The instrument operating conditions were voltage 15 kV, electron beam intensity < 3 nA and 1 mm diameter, while the measurement time was 60 s. Corrections were made using the ZAF-4/FLS software provided by LINK. Fragments of the studied samples were sprinkled onto double-sided aluminum tape mounted on a SEM stub, carbon-coated and viewed through randomly selected fields of view.

X-ray Fluorescence (XRF)
Bulk analyses of the studied samples were determined by a Bruker S4-Pioneer X-Ray Fluorescence wavelength dispersive spectrometer (Brucker AXS GmbH, Karlsruhe, Germany). The spectrometer was fitted with a Rh tube, six analyzing crystals, namely: LIF200, LIF220, LIF420, PE, TLAP and PX1 and the detectors were a gas-flow proportional counter, a scintillation detector or a combination of the two. The gas-flow proportional counter uses P10 gas, which is a mixture of 90% argon and 10% methane. Major elements were analyzed on a fused glass bead at 60 kV and 45 mA tube-operating conditions. Matrix effects in the samples were corrected for by applying theoretical alpha factors and measured line overlap factors to the raw intensities measured. The standards that were used in the calibration procedures for major element analyses were: AGV-1 (Andesite) (USGS), JG-1 (Granodiorite) (GSJ), JB-1 (Granodiorite) (GSJ), NIM-G (Granite) (MINTEK), GA (Granite) (CRPG) and GH (Granite) (CRPG). According to ASTM D4404-84 (2004), the dry unit weight is defined as the quotient of the weight of a unit to the volume of the unit: d = w/v (g/cm 3 ). Where W is the total weight of the unit including the pores with their content (air) and V is the total volume of the unit. When a material is to be used as an insulation agent, the measured dry unit weight represents the insulation block density of the material. According to their insulation density, the studied diatomite rocks were evaluated as possible materials in the cement and insulation bricks industry.

Specific Surface Area
Specific area measurement is used in various science and technology fields. The most important method, among others, is the absorption of low thermal nitrogen of Brunauer, Emmett and Teller [29,30]. For the measurement, two glass bottles with the same volume are required. The first bottle is filled with the examined sample and the other is empty. In room temperature conditions, the two bottles are filled with nitrogen gas (pressure 1 atm). After that, the bottles are frozen by immersing them in liquid nitrogen (circa −190 • C). The absorbed nitrogen gas by the sample leads to a difference between the two bottles. The equilibrium pressure in the bottle with the material is stabilized by itself, in the field of the B.E.T. method, under low temperature conditions and can be determined without additional pressure measurements. The specific surface area of a sample (Sg) is calculated by the following equation [30]: Sg = A*∆h/m (m 2 /g), where A is a coefficient obtained from a special nomogram using the pressure difference of the manometer (∆h) in mm and the atmospheric pressure in mmHg, while m is the mass in g of the material used.

Porosity
Lightweight materials are characterized by the presence of pores in their structure. Porosity is described as the quotient of the pore volume of a sample to the total volume of the sample. The propagation speed of elastic waves through a sample is defined by the porosity. According to Wyllie et al. (1958) [31], the velocity of the P-waves (Vp) is related to the porosity (ϕ) by the following equation: 1/Vp = (ϕ/Vpf) + (1 − ϕ/Vpm), where Vpf is the velocity of P-waves through the pores of a rock sample filled with fluid (liquid or gas). The velocity of P-waves in the water is 1450 m/s, while in the air it is 330 m/s [32]. Vpm is the P-waves velocity through a rock sample without pores. From the previous equation it can be concluded that samples with high porosity values lead to a decrease P-wave velocity.
A special portable ultrasound speed measuring device Pundit Lab+ (Proceq, Switzerland) was used for the porosity measurement of the studied bulk samples. Since the dimension of the specimens are known, the accuracy of the method depends on the precise measure of the P-wave propagation time. In this device, the recording range of the propagation time of the ultrasound ranges from 0.1 to 9999 µs, while the accuracy of the measurement is 0.1 µs. For the precise operation of the instrument, special 54 kHz cylindrical transducers were used, which are fitted diametrically to the ends of the studied samples. To improve the acoustic contact between the samples and the transducers, plasticine was used as the contact medium [33,34], while a cylindrical standard of synthetic material was used to adjust the accuracy of the instrument.

Mineralogical Composition
The mineralogical composition of the studied samples is presented in Table 1, while their XRD patterns are shown in Figure 2. Opal-A and opal-CT were found in five samples and its total percentage varied from 22 (sample A9h) to 85 wt% (sample A9g). According to glycolated samples, the only identified clay mineral is smectite and in particular montmorillonite (very good matching of the montmorillonte ICDD file 13-0135). Clay minerals were found in all samples, in amounts ranging between 4 (sample A9g) and 45 wt% (sample A9f).

Morphology of the Studied Diatomites
Results from the SEM-EDS study showed that the diatom frustules were mostly cylindrical, resembling the diatom species that have been recorded previously in the lacustrine basins of central and northern Greece [3,41]. The frustules were either well preserved (clayey diatomite beds, Figure 3) or highly dissolved from diagenetic transformations   The samples with significant amounts of clay minerals and micas can be classified as moler-type diatomites (clayey diatomite, lack of opal-A or opal-CT), while the diatomaceous samples with a predominance of opal-A and opal-CT can be classified as porcelaneous diatomites.
In general, diatomaceous formations are divided into two categories. The distinction between them is dependent on the mineralogy composition and their diagenetic conditions. They are divided into: (a) porcelanites, (b) clayey diatomites, moler type. Porcelanite diatomites is characterized by the high content in opal-A and opal-CT [35]. According to Minoura et al. [36], porcelanites are related to the diagenetic environment of diatomite formation. Specifically, the conversion of diatomite deposits into porcelanite formations is attributed to the dissolution of opal of the former (opal-A) and its conversion initially to cristobalite-tridymite and finally to quartz [37], with increasing depth. Clay diatomites or diatomites of the moler type or moler earth type, according to Harben [38] and Stamatakis et al. [35], refer to all the remaining diatomite types. Diatomite types, whose content of clay minerals are superior to the other mineral components, was first located in northwestern Denmark, mainly on the islands of Fur and Mors. They are one of the most important centers for the production of clay diatomaceous earth in the world.
Detrital and/or volcanogenic minerals, such as feldspars, quartz and muscovite were found in most of the tuffaceous samples, sometimes exceeding 80 wt% (e.g., sample A9c). Evaporitic minerals such as gypsum were found in the deepest horizons (samples A9e and A9h) with percentages varying from 11 to 16 wt%. The presence of gypsum in the studied samples is particularly important as it indicates an evaporative deposition environment [39]. Carbonate minerals are absent, and this is beneficial for certain industrial applications [40].

Morphology of the Studied Diatomites
Results from the SEM-EDS study showed that the diatom frustules were mostly cylindrical, resembling the diatom species that have been recorded previously in the lacustrine basins of central and northern Greece [3,41]. The frustules were either well preserved (clayey diatomite beds, Figure 3) or highly dissolved from diagenetic transformations (opaline beds, Figure 4). Lepispheres constitute the intermediate stage of the gradual diagenetic transformation of the amorphous silicon of diatom shells to more crystalline phases such as opal-CT.
(opaline beds, Figure 4). Lepispheres constitute the intermediate stage of the gradual diagenetic transformation of the amorphous silicon of diatom shells to more crystalline phases such as opal-CT.  Similar diagenetic changes have been recorded in the volcanic islands of Milos and Aegina, where the opal-A has been diagenetically altered due to the high heat flow rates, related to contemporary volcanism in those islands [35,41]. (opaline beds, Figure 4). Lepispheres constitute the intermediate stage of the gradual diagenetic transformation of the amorphous silicon of diatom shells to more crystalline phases such as opal-CT.  Similar diagenetic changes have been recorded in the volcanic islands of Milos and Aegina, where the opal-A has been diagenetically altered due to the high heat flow rates, related to contemporary volcanism in those islands [35,41]. Similar diagenetic changes have been recorded in the volcanic islands of Milos and Aegina, where the opal-A has been diagenetically altered due to the high heat flow rates, related to contemporary volcanism in those islands [35,41].
All the other major oxides measured were present in minor amounts. Chemical analysis showed that the results from the XRF analysis are in good agreement with those of the mineralogical analysis. It is noteworthy that samples containing high percentages of SiO 2 also have high percentages of silica minerals (opal A + CT).

Physical Properties
In Table 3, the physical properties measured for the studied samples are presented. Table 3. Insulation block density (g/cm 3 ), porosity (% v/v) and specific surface area (m 2 /g) measurements for the clay-diatomite sample (A) and the silica polymorphs-rich sample (B).  [47], forming a stable phase retaining the initial clay structure up to about 850 • C. As temperature increases, cristobalite and probably mullite crystallize at temperatures up to 1250 • C, resulting to crystals with high length [47][48][49]. According to Yang et al. [50], the crystal length is dependent on the chemical composition of the initial material melting in high temperatures. Materials containing high amounts of K 2 O and Na 2 O favor the development of long mullite crystals and a more porous structure [49].

Sample
Moreover, dried at 105 • C, diatomite samples of porcelanite-type showed higher specific surface area (132 m 2 /g) compared to diatomite samples of molar-type (77 m 2 /g). This difference can be attributed to the higher participation of diatoms in the porcelanitetype sample as shown by the opal concentration (A + CT), due to the complex microporous structure that the diatoms present and also to the contribution of the microporosity of clay minerals present in significant amounts of the studied porcelanite-type samples. However, both samples had almost the same insulation block density at 1100 • C (Table 3). Chen [42] found that clayey diatomite (molar-type) showed lower specific surface area values (92.51 m 2 /g) compared to treated diatomite (111.53 m 2 /g).

Porosity
According to Kantiranis et al. [33,34], the measurement of the P waves velocity is a reliable method to determinate a rock sample's porosity. Vogiatzis [51] has proved that the P wave velocity is dependent on the grain size and the porosity of the mortar materials. Porosity of the initial materials A and B, dried at 105 • C, measured 26.1 and 28.6% v/v, respectively (Table 3). Differences between the studied samples can be attributed to the complex microporous structure of the diatoms contained in higher amounts in sample B. After the samples were calcined, at 1100 • C porosity decreased to 15.7% v/v for sample A and 16.0% v/v for sample B. According to Pimraksa and Chindaprasit [52], the percentage of clay minerals in a sample decreased while the temperature rises, resulting in a decrease in the porosity. On the other hand, the decrease in the porosity of sample B (porcelanite-type material) is due to the progressive transformation of diatoms opal-A at elevated temperature (1100 • C) in a more crystalline and dense structure (opal-CT) losing a significant part of their microporous structure.
Considering mineralogical and chemical analyses and measurements of selected physical properties, both samples could be used as an absorbent or for the production of insulation bricks. In particular, the "moler type" diatomite is favorable as a potential insulation material due to its high percentages of aluminum oxide. Another potential use, based on the specific properties of diatomite, is as lightweight aggregates with various cements in mortar and concrete for insulating properties too. This insulation technique, among other advantages, leads to avoiding heavy construction elements, while respecting the requirements of thermal regulation standards [53][54][55][56][57][58][59][60].

Conclusions
In this study, analysis of the diatomaceous samples obtained from the area of Paranisia in Limnos Island showed that they can be classified as porcelaneous diatomite (with a predominance of opal-A and opal-CT) and clayey diatomite (moler-type, with a predominance of clay minerals, micas and opal-A). Opal-CT forms micro-plates disseminated in the groundmass and is diagenetically formed in expense of opal-A dissolution. The purest diatomaceous beds have been transformed into opaline beds (opal-CT-rich rocks), whereas the clayey beds were not influenced by diagenetic transformations. The presence of gypsum in the studied samples is particularly important as it indicates an evaporative deposition environment, while the absence of carbonate minerals is beneficial for certain industrial applications.
Analysis of two samples, one rich in clay, volcanic and detrital minerals (molar-type), and the other rich in silica polymorphs (opal-A and opal-CT), showed that the latest polcelanite-type sample has a higher specific surface area after drying at 105 • C. Both samples have almost the same insulation block density and significant porosity at 1100 • C, thus indicating that the diatomite could be used as an absorbent or for the production of insulation bricks. The "moler type" diatomite is favorable as potential insulation material due to its high percentages of aluminum oxide, while both diatomitic materials can be used as lightweight aggregates to prepare mortars and concretes with thermal insulating properties and avoid heavy constructions.