Mineralogy and geochemistry of diatomite associated with lignite seams in the Komnina Lignite Basin, Ptolemais, Northern Greece

International Journal of Coal Geology 71 (2007) 276 – 286 www.elsevier.com/locate/ijcoalgeo

Mineralogy and geochemistry of diatomite associated with lignite seams in the Komnina Lignite Basin, Ptolemais, Northern Greece Nikolaos Koukouzas ⁎ Centre for Research and Technology Hellas/Institute for Solid Fuels Technology and Applications, Attica Technology Park, 15310 Agia Paraskevi, Athens, Greece Received 8 June 2006; received in revised form 1 September 2006; accepted 5 September 2006 Available online 20 October 2006

Abstract Diatomite with variable chemical and mineralogical composition occurs in the Komnina Lignite Basin. The diatomite layers, which overly lignite seams of Upper Miocene age, are rich in silica or calcium. These layers correspond respectively to quartz-rich and calcite-rich layers. The diatomite mainly consists of opal-A, while other minerals are quartz, feldspars, calcite, smectite, illite, kaolinite, chlorite, cristobalite, and muscovite. The well-preserved nature of the diatom species indicates a weak silica diagenesis. This is also indicated by the mineralogical composition of diatomite, especially the absence of opal-CT. The organic material present in the lake acted as coating and prevented diatom dissolution. The shallow lake, where the diatoms were deposited, did not allow diagenesis of diatomite. Diatoms were deposited in the Komnina Lake as a result of the acidic pH, the warm-humid conditions, and the silica-rich environment that occurred due to volcanic activity in the nearby area (Aridea). Remains of volcanic ash have been identified in the lignite deposits of the wider Ptolemais area. The association of diatomite with lignite was brought about by tectonic movements that occurred in the wider area of Monastir–Florina–Ptolemais–Kozani–Elassona–Sarantaporo in the Upper Miocene, resulting to the development of individual small shallow lakes. The paleoenvironment changed to acidic from the previous alkaline pH, the vegetation died off, the silica produced from the volcanic activity and the calm water conditions allowed the deposition of diatoms. © 2006 Elsevier B.V. All rights reserved. Keywords: Diatomite; Lignite; Komnina; Ptolemais; Mineralogy; Geochemistry

1. Introduction Diatoms are single-celled organisms, related to algae, with a soft body enclosed by an opaline exoskeleton. The exoskeleton or frustule is composed of two halves, the smaller half fitting inside the larger half. Frustules are either circular (centric) or elliptical (pennate) in form, and are ornamented with sieve-like perforations (punc⁎ Tel.: +30 210 6546637; fax: +30 210 6527539. E-mail address: [email protected]. 0166-5162/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2006.09.002

tae) and intricate rib structures (costae). Diatoms create their food by combining carbon, obtained from photosynthesis of carbon dioxide, with nutrients extracted from seawater. Diatoms are adapted to a wide range of aquatic environments, including marine, brackish, and fresh waters. The organisms require suitable environmental conditions if they are to flourish, including appropriate temperature and photic conditions, a narrow salinity and acidity range, and a stable supply of nutrients including silica, nitrogen, phosphorous, iron, oxygen, and carbon dioxide. Diatoms inhabit the photic zone at

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depths down to 200 m, and thrive in the cold waters of sub-polar and temperate regions. The presence of diatoms in sedimentary rocks can provide valuable information about the environment of deposition of those rocks. Benthic diatoms may indicate inner continental shelf, coastal and estuarine environments, whereas plank-

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tonic forms are found in shelf and deeper water deposits (Sancetta, 1983). Diatoms first appeared in the geological record about 100 Ma during the Late Cretaceous, but most economic deposits are of Miocene–Pleistocene age. Diatomite deposits are frequently associated with volcanic activity,

Fig. 1. Location of Komnina Lignite Basin (Ko = Komnina) and the lignite deposits of Ptolemais and Florina sedimentary basins (Pt = Ptolemais, Pr = Proastio, Ar = Ardassa, An = Anatoliko, Pel = Pelargos, Pe = Perdikas, Am = Amynteo, Va = Valtonera, Ve = Vegora, Pet = Petres, Vev = Vevi, Lo = Lofi, Ac = Achlada).

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with air-fall ash and run-off waters providing a source of dissolved silica to replenish that extracted by the diatoms.

In Greece, two types of diatomites have been identified, based on their origin: diatomites of marine origin, found in the islands of Samos, Aegina, Zakynthos and

Fig. 2. Stratigraphic column of Komnina and Ptolemais Lignite Basins.

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Kephallonia and those of lacustrine and fresh waters, found in Vegora, Florina, Komnina, Kozani, and Elassona. (Velitzelos and Schneider, 1977; Dermitzakis, 1978;

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Gersonde and Velitzelos, 1978; Gersonde, 1979; Van de Weerd, 1979, Heimann et al., 1979; Kiritopoulos, 1984; Knobloch and Velitzelos, 1986; Stamatakis and

Fig. 3. Komnina and Ptolemais Lignite Basin (1) during the deposition of lignite and diatomite (Upper Miocene–Lower Pliocene), (2) during the deposition of lignite (Upper Pliocene), and, (3) as today.

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Magganas, 1988; Stamatakis et al., 1989; Koukouzas, 1991; Stamatakis, 1994; Stamatakis and Koukouzas, 2001). The Komnina Lignite Basin, which hosts the diatomite layers, is located in the northeastern part of Greece (West Macedonia), southeast of Amynteo city and northeast of Ptolemais city (Fig. 1). Detailed exploration has been carried out in the Komnina Lignite Basin to determine the lignite reserves of the area as feed for the lignite-fired power plants of the Ptolemais– Amynteo region (Koukouzas et al., 1984). Diatomite forms the overburden of the lignite layers, and has to be extracted before the exploitation of the lignite itself. This paper aims to identify the chemical and mineralogical composition of the diatomite, and to shed light on the origin and occurrence of diatoms in the lignitebearing Komnina sedimentary basin. 2. Geological setting 2.1. Komnina Lignite Basin The Komnina Lignite Basin, which is located close to the village of Komnina, is situated at the northeastern part of the Ptolemais sedimentary basin and in the middle of the greater Florina–Ptolemais Basin (Fig. 1). The Komnina Lignite Basin is part of a huge graben which extends southwards from the Former Yugoslavia Republic of Macedonia (FYROM) —near the town of Monastir, to the town of Kozani. The Monastir–Florina– Ptolemais–Kozani graben, with a length exceeding 150 km and an axis trending in a N.W.–S.E. direction, was developed during the Tertiary period by big faults striking N.W.–S.E. to N.N.W.–S.S.E. The largest coal deposits of Greece were formed in this graben due to excellent geological and paleogeographical conditions. The lignite deposits are classified into two types; an “earthy” lignite or lignite of the Ptolemais type, and the xylite or Komnina type lignite that is the subject of this paper (Brunn, 1982; Koukouzas et al., 1984). Fig. 2. The Komnina Lignite, which is of the xylitic type, is of Early Neogene age, along with the lignite of Anatoliko, Pelargos (Ptolemais sedimentary basin), Vegora, Petres, Vevi, Lofi, and Achlada (Florina sedimentary basin) (Fig. 1). The Komnina Lignite is older than the lignite of the Ptolemais type, which is of Upper Neogene age and is found in Ptolemais, Proastio, Perdikas, and Amynteo. Quaternary lignite is found in the Ardassa and Valtonera areas. The diatomite layers have been identified in the sedimentary succession of the Komnina Lignite Basin, overlying a basement that consists of limestones and

dolomites of Triassic–Jurassic age (Brunn, 1982). The sediments consist of unconsolidated sand, silt, claystone, siltstone, sandstone and marl of Upper Miocene–Lower Pliocene age. Lignites of the xylitic type have been identified in this stratigraphic succession. The diatomite layers occur as overburden and interbedded sediments to the lignite seams. Fig. 3. Komnina Lignite has an upper heating value of 9.8 MJ/kg and a lower heating value of 8.2 MJ/kg. This lignite has a moisture of 42% and ash content of around 17% (on dry basis). The lignite seam varies from several meters to 30 m in thickness, but has an average thickness of 11.40 m. The proven reserves of lignite are estimated at 250 Mt (Koukouzas et al., 1984). 2.2. Diatomite section The average thickness of the diatomite deposit is 44 m, varying from 1.80 to 98.90 m. The greatest thickness is present in the middle of the Komnina Lignite Basin and in the southwest part of the deposit (Koukouzas, 1991). Samples of diatomite collected from boreholes in the Komnina deposit have a green-grey and occasionally light brown color. This depends on the proportion of clays (green-grey) or calcite (light brown) included in the diatomite layers. When compared to the normal clays of the area, the Komnina diatomite is much lighter in weight. Sometimes organic material, rootlets and fossils are present in the collected samples. Marl occurs in the same stratigraphic succession and between the diatomite and the lignite beds. The marl has an average thickness of 5 m, and includes several fossils such as unio and ostracods. The diatom assemblage from the Komnina deposit is composed of the following genera: Actinocyclus, Cestodiscus, Coscinodiscus, Cyclotella, Nitzschia, Thalassiosira, Achnanthes, Cocconeis, Diploneis, Thalassionema and Melosira (Vassiliou, 1982), which are similar to the diatom genera found in Vegora Lignite Basin (Coscinodiscus, Fragilaria, Cyclotella and Spongilla) (Gersonde and Velitzelos, 1978). 3. Methodology Representative samples of diatomite were collected from the Komnina Lignite Basin. Twenty-six samples from four boreholes were collected and analyzed by means of X-ray diffraction, differential thermal and thermogravimetric analysis, scanning electron microscopy and X-ray fluorescence spectrometry in the Department of Geology, University of Leicester, United Kingdom.

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The instrument used for the X-ray diffraction analysis was a Philips X-ray diffractometer. The diatomite samples were analyzed as raw material, and treated to determine the clay minerals under different experimental conditions (air dried, glycolated, heated to 335 °C and 575 °C, and after being calcined at 960 °C). The instrument used for the differential thermal and thermogravimetric analysis was a Stanton Thermal Analyzer, model 1500. The Scanning Electron Microscope (SEM) was a Hitachi SEM, model S-520 with an attached “Link” energy dispersive X-ray detector. The X-ray fluorescence (XRF) apparatus was an “A.R.L.” XRF spectrometer, model 8400 plus, with a rhodium anode tube. 4. Results and discussion Two types of diatomite have been identified in Komnina, one rich in silica and one rich in calcium. Although both types can be found throughout the stratigraphic succession, those rich in calcium are on the top and those rich in quartz in the middle and at the base of the deposit. The diatomite rich in silica shows high proportions of quartz, chlorite and muscovite (Table 1), while the calcium-rich material has high proportions of calcite. The major component of all samples, except the calcium-rich diatomites, is opal-A (Table 1). Opal-A is X-ray amorphous biogenic silica (Jones and Segnit, 1971). A characteristic hump is produced on the XRD diffractograms by this material. No transformation of opal-A to opal-CT or other SiO2 polymorphs has occurred, which indicates that the rocks have not been subjected to burial diagenesis, or exposed in sufficient saline–alkaline conditions for the opaline silica to be dissolved (Stamatakis et al., 1989). Table 1 Representative mineralogical analyses of Komnina diatomite

KOMN1 KOMN2 KOMN3 KOMN4 KOMN5 KOMN6 KOMN7 KOMN8

O-A Cc Qtz Cl

Ilt

Kao Sm

Fld

Cr

Musc

MJ MJ MJ MD MJ MJ MD MD

MD MD MD MD MD MD MD MD

MD MD MD MD MD MD MD MD

MD MD MD MD MD MD MD MD

TR TR TR TR TR TR TR TR

MD MD MD TR MD MD TR TR

TR TR TR MJ TR TR MJ MJ

MJ MJ MJ TR MJ MJ TR TR

MJ MJ MJ MD MJ MJ MD MD

MD MD MD MD MD MD MD MD

O-A: Opal-A, Cc: Calcite, Qtz: Quartz, Cl: Chlorite, Ilt: Illite, Kao: Kaolinite, Sm: Smectite, Fld: Feldspar, Cr: Cristobalite, Musc: Muscovite, MJ: Major, MD: Moderate, TR: Trace.

Fig. 4. X-ray diffraction patterns showing clay minerals found in the diatomite.

Quartz varies from trace to major amounts, and is a detrital rather than diagenetic component in the diatomite samples. Cristobalite is present at trace concentrations. Small amounts of cristobalite remain after calcination at 960 °C, temperatures at which calcite and illite disappear. This is a clear indication that the diatomite is not a pure type of diatomite with a high proportion of cristobalite. Calcite is present in major proportions in the calcium-rich diatomite, but only occurs in trace amounts in the silica-rich diatomites. Chlorite is the main constituent of the clay minerals found in the diatomite samples, varying from 35.81 to 44.36%. Illite varies from 16.55 to 28.84%, kaolinite from 12.98 to 20.55% and smectite from 15.07 to 25.97%, as indicated by treatment of the clay minerals under different experimental conditions (air dried, glycolated, and heated to 335 °C and 575 °C) (Fig. 4). Feldspars are present in moderate amounts, while muscovite varies from trace to moderate amounts. Differential thermal and thermogravimetric analysis (DTA–TGA) indicate that dewatering of illite and kaolinite occurs at 40 °C (weight loss of 1.5%). Burning of the organic material takes place at 360 °C (3.25%), dehydration of chlorite at 520 °C (2.5%), the α–β quartz transformation at 580 °C (2.5%), the loss of CO2 from calcite at 720 °C (1%), and the phase changes in the clay minerals at 850–930 °C (0.75%) (Fig. 5a). These weight losses appear in the diatomite

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Fig. 5. Differential thermal and thermal gravimetric analyses of Komnina diatomite.

rich in silica. Similar weight losses occur in the diatomite rich in calcium, but there are larger weight losses associated with calcite break down to CO2 and CaO at 720 °C (27.7%) compared to the other DTA– TGA curves (0.5% at 40 °C, 4.03% at 360 °C, 0.5% at 850–930 °C) (Fig. 5b). The chemical composition of the diatomite varies consequently from Si-rich to Ca-rich samples (Table 2). The Fe content of the diatomite is also relatively high. The high proportion of Mg is due to the presence of ultra basic rocks surrounding the Komnina Lignite Basin. When diatomite layers rich in Ca are present the Fe and Mg contents are low, as a result of the increased penetration of limestone conglomerates in the basin and consequently the decreased contribution of the ultra basic rocks. This is also associated with changes in the trace element concentrations, as the high proportions of nickel are also derived from the ultra basic rocks. It is worthwhile to stress the high proportion of phosphorous in some diatomite samples is a result of the appearance of phosphate minerals in the sedimentary sequence, especially in association with the diatomite layers.

4.1. Phosphorous and iron The precipitation of phosphate minerals in the Komnina Basin is mainly due to the deposition of organic matter, the subsequent moderate enrichment of the sediment by groundwater, and the highly reducing environment at depth in both the lake water and the bottom sediment. The appearance of phosphate minerals, such as anapaite, vivianite, mitridatite, and hydroxylapatite, in clayey diatomite forming lignite overburden has been reported previously in the literature (Stamatakis and Koukouzas, 2001). The diatomite beds of the Komnina Basin are of similar chemical and mineralogical composition with those of the Sarantaporo–Elassona Basin which occurs within the same graben structure. In the Sarantaporo–Elassona lacustrine basin massive, irregular phosphate-rich concretions and veins cut the diatomite, which overlies lignite seams of Upper Miocene age (Stamatakis and Koukouzas, 2001). The accumulation of Fe in the Komnina Basin, derived from leaching of the ultrabasic basement rocks, indicates the presence of an eutrophic lake environment.

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Table 2 Representative chemical analyses of Komnina diatomite KOMN1

KOMN2

KOMN3

KOMN4

KOMN5

KOMN6

KOMN7

KOMN8

Major elements (%) 72.60 SiO2 TiO2 0.42 Al2O3 7.43 Fe2O3 4.03 MnO 3.70 MgO 2.61 CaO 0.65 Na2O 0.47 1.07 K2O P2O5 0.48 LOI 7.50 Total 100.96

55.67 0.77 14.12 6.71 6.48 5.43 1.39 0.89 1.82 0.10 7.40 100.78

50.74 0.71 13.18 6.88 8.88 4.48 1.61 1.11 1.97 0.13 11.20 100.89

44.22 0.73 10.44 6.37 5.00 3.40 11.98 0.50 0.96 0.12 16.71 100.43

42.95 0.22 5.52 4.81 8.15 2.13 3.37 0.35 0.49 0.16 32.80 100.95

58.85 0.75 13.66 7.82 0.12 4.62 1.62 1.87 1.89 0.16 8.80 100.16

41.05 0.51 10.64 8.34 0.17 3.69 14.74 0.67 1.29 0.25 19.60 100.95

32.20 0.38 8.79 7.10 0.16 3.07 21.36 0.43 0.88 0.41 25.40 100.18

Trace elements (ppm) Nb 14 Zr 197 Y 33 Sr 122 Rb 92 Th 11 Ga 15 Zn 102 Ni 341 Pb 19

14 155 32 84 101 11 16 121 468 22

16 144 30 91 105 15 17 122 407 23

18 156 32 107 113 12 19 129 311 25

9 74 16 87 55 3 11 56 227 12

12 138 33 96 102 14 16 117 393 23

12 98 26 86 72 11 12 82 355 15

8 78 19 78 52 4 11 65 264 12

4.2. Depositional environment A deep lake paleoenvironment, with highly stagnant waters and high biological productivity, promoted by warm-humid conditions, is evident from the absence of coarse sediments, turbidite layers or cross bedding features in the stratigraphic succession. The appearance of calcite in the diatomite reflects the high calcium content of the lake water during sedimentation. The water that was rich in calcium did not allow precipitation of large amounts of phosphate minerals, as happened in the Sarantaporo–Elassona Basin (Stamatakis and Koukouzas, 2001). The presence of smectite and vermiculite also promoted the adsorption of phosphate ions in the sediments of the basin. The diatom species found in the Komnina Lignite Basin indicate a clear-water lake environment (Krumm, 1980). The presence of Melosira is particularly characteristic of a deep-water lake environment (Inglethorpe and Morgan, 1992). The well-preserved diatom frustules, which retained their original shapes, indicate a weak silica diagenesis. The absence of opal-CT, diagenetically derived from opal-A, is evidence of a not highly alkaline diagenetic environment and/or relatively shallow burial conditions for diatomite sedimentation.

Similar diatomite formations have been found in the Monastir basin in FYROM, which is a northern extension of the Ptolemais–Florina sedimentary basin. The diatomite layers have been identified in “Veshie”, region of Negotino, stratigraphically between andesitic tuffs and conglomerates, and are of 1.0 to 1.2 m thickness. The diatomite formation, which is named “Trepel”, forms the overburden of the lignite seams, and has a total average thickness of 100–150 m. In the same formation, and in particular parts of the diatomite deposit, sand and clays are also present as interbedded seams (Bundovsky, 1987). The genesis of the diatomite deposit results from the conditions that dominated during the Pleistocene in the area. The volcanic material which consisted of ash and tuffs, was deposited in a lake that was poor in calcium, and silica. The addition of silica-rich material produced diatoms such as Cyclotella, Navicula, Symbella, Pinnularia, Fagilaria, and Melosira. As a result of deposition of the diatoms, the lake became narrow and of shallow depth, resulting in erosion of the surrounding rocks and the deposition of conglomerates. The same conditions occurred in the Komnina Lignite Basin, and produced the deposition of diatoms in a lake environment. Diatomite deposits that are not directly associated with lignite seams are noted in several other places of FYROM, such as Pulik, Prenka, Zovic and Veshie,

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Plate 1. Well-preserved diatom species of circular and elliptical shape.

which are located in the area between Monastir and Beshishte (Yenko and Giuzelkovsky, 1956). The diatomite of Monastir–Beshishte, which stratigraphically underlies travertine and overlies andesitic tuffs and gneisses, includes mainly Cyclotella macedonica (over 97% of the diatom species) and is of Pliocene age. This formation is made up of pure diatomite, including 92% silica (Yenko and Giuzelkovsky, 1956). The genesis of the diatomite is associated with the volcanic activity in the area, and the increase of silica within the water, provided the conditions required for the development of the diatoms. The water of the lakes, where the diatoms were deposited, also had high pH values. The possible association of diatom deposition in the Komnina Lignite Basin with volcanic activity in the wider Florina–Ptolemais–Komnina–Almopia area is further indicated by the results from the examination of the volcanic rocks in the Aridea area. Volcanic ash was erupted at an early stage of the volcanic activity in the Aridea area and was spread out for several hundred kilometers (Vougioukalakis, 1994). Remains of this volcanic ash have been identified in layers forming the overburden of the lignite seams in the wider area of Ptolemais–Florina Lignite Basin (Anastopoulos and Koukouzas, 1972). Taking into account that similar volcanic activity is associated with the deposition of diatoms in the lignite basins of FYROM (Yenko and Giuzelkovsky, 1956; Bundovsky, 1987), the presence of a silica-rich environment in the Komnina Basin could possibly be associated with the volcanic activity that occurred in the Almopia area. This resulted in a significant change to the paleonvironment of the Komnina Basin. The pH became acidic

instead of alkaline and tectonic movements produced shallow lakes instead of narrow lakes when the growth of trees became difficult. Consequently the peat accumulation ceased and diatoms were produced and deposited in the lake environments. The deposition of diatoms occurred in lakes that had previously produced peat and consequently lignite of the same type (xylitic type) as in the Komnina Basin, namely at Vegora (Florina), Lava, Prosilio, and Elassona– Sarantaporo. This is an indication that a lake with an approximate size of 150 km2, starting from Monastir, extended via Florina, Ptolemais, Lava and Prosilio and ended near Elassona–Sarantaporo. This was a shallow lake in the Late Miocene, with no trees (as had been present before during the deposition of the xylite), which allowed extensive deposition of diatoms. At a later stage (Early Pliocene) tectonic movements occurred in the area and several horsts developed (Lava, Prosilio, and Petres) (Anastopoulos and Koukouzas, 1972). The volcanic activity continued during the Late Pliocene, when the lignite of Ptolemais was deposited, but no diatoms were deposited in the overburden sediments of the Pliocene lignite, due to the lack of conditions similar to these of Miocene times. The presence of lignites in the same sedimentary succession as the diatoms can give an indication of the temperature for the diatomite's development. The maximum burial temperature of the lignite is estimated at around 40 °C, and thus the same temperature can be given for the diatomite (Stamatakis and Koukouzas, 2001). The diatoms in the Komnina Lignite Basin are wellpreserved due to the fact that numerous organic and

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inorganic coatings are present on the diatoms and therefore the rate of their dissolution was reduced. The absence of opal-CT in the Komnina diatomite is thought to be because silica diagenesis had not been advanced enough to produce either opal-CT or quartz. The quartz that occurs in the diatomite samples is detrital in origin. The high preservation of the diatoms, as shown by SEM studies (Plate 1), indicates that no or only partial corrosion occurred following deposition and burial of the diatoms. In addition, the shallow nature of the lakes did not allow dissolution of the diatom frustules. Compaction of diatomite usually gives rise to two stages of diagenesis: opal-A → opal-CT and opal-CT → quartz. The almost complete absence of clastic sediments in the Komnina Basin is of major importance for the development of diatoms in the lake. This happened because, during the Miocene, clastic sediments came from the western part of the lake, the Mountain Siniatsiko which consists of schist, and not from the eastern part, the Mountain Vermio, which consists of limestone and is located close to the Komnina Basin. Thus the lake of the Komnina Basin remained stable and not infilled by clastic sediments, so that high concentrations of diatoms were able to be deposited. 5. Conclusions The diatomite of the Komnina Lignite Basin occurs stratigraphically above lignite seams of Late Miocene age. This diatomite has a variable chemical composition, with silica content ranging from 20 to 75%, aluminium 7–14%, and calcium 0.5–21%. Two types are distinguished, one rich in silica and the other rich in calcium, in the diatomite bed. This is represented in the mineralogical compositions of the diatomites, which are rich in quartz and calcite respectively. The diatomite mainly consists of opal-A, which is the amorphous phase making up the original diatoms, while other minerals are quartz, feldspars, calcite, smectite, illite, kaolinite, chlorite, cristobalite, and muscovite. The well-preserved diatom species found in the Komnina diatomite indicate a weak silica diagenesis, as is also shown from the mineralogical characteristics of the diatomite and the absence of opal-CT. Organic material present in the lake acted as coating and reduced the rate of diatom dissolution. The shallow lake in which the diatoms were deposited did not allow significant diagenesis of the diatomite. Diatoms were deposited in the Komnina Lake as a result of the acidic pH and the warm-humid conditions, as well as the silica-rich environment that occurred due

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to volcanic activity in the nearby area (Aridea). Remains of volcanic ash have been identified in the lignite deposits of the wider Ptolemais area. The association of diatomite with lignite reflects tectonic movements that occurred in the wider Monastir–Florina–Ptolemais–Kozani–Elassona Sarantaporo area during the Late Miocene, resulting in the development of individual small shallow lakes. The paleoenvironment changed to acidic instead of alkaline, vegetation growth ceased, the silica produced from the volcanic activity and the calm water conditions allowed the deposition of diatoms to occur. These conditions were not reached after the latest deposition of lignite, in the Late Pliocene, where conglomerates and river sediments dominated after the deposition of the lignite beds. It is worthwhile to mention that the lakes that produced lignite deposits in the Pliocene were mainly located in the western part of the wider Ptolemais–Florina area, closer to the Mountain Siniatsiko where the erosion of schists and ultrabasic rocks resulted in an increased number of conglomerates. Acknowledgment The author wishes to thank Professor Colin Ward from the School of Biological, Earth and Environmental Sciences, University of New South Wales, for his careful review of the manuscript and helpful comments and suggestions. References Anastopoulos, I.X., Koukouzas, C.N., 1972. Geological study of the southern part of Ptolemais Lignite Basin. IGME, Geol. Geophys. Stud. 1, XVI, 117–121. Brunn, J.H., 1982. Geological Map of Greece. Pyrgi Sheet. Scale 1:50 000. Institute of Geology and Mineral Exploration, Athens. Bundovsky, N., 1987. Research activities on diatomaceous earth in “Veshie”, Negotino region. Geological Institute of Skopje 9–26. Dermitzakis, M., 1978. Stratigraphy and sedimentary history of the Miocene of Zakynthos (Ionian islands, Greece). Annales Geologiques des Pays Helleniques. 29/1, 147–186. Gersonde, R., 1979. Diatoms in the Mediterranean Neogene, a short review. Annales Geologiques des Pays Helleniques. Hors Serie III, 13481362. Gersonde, R., Velitzelos, E., 1978. Diatomeenpalaeookologie im NeogenBecken von Vegora N–W Mazedonien (Vorlaeufige Mitteilung). Annales Geologiques des Pays Helleniques 29, 373–382. Heimann, K.O., Just, J., Muller, C., 1979. Sedimentological and paleontological aspects of Messinian strata on the Ionian islands, Greece. Annales Geologiques des Pays Helleniques. Hors Serie 1, 501–514. Inglethorpe, S.D.J., Morgan, D.J., 1992. The Laboratory Assessment of Diatomite. National Conference on Geological Resources of Thailand: Potential for Future Development. Department of Mineral Resources, Bangkok, pp. 210–221.

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