Alkaline and Alkaline Earth Major Elements in Lignite from the Sofia Coal Province, Bulgaria

СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 80, кн. 2, 2019, с. 3–12 REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 80, part 2, 2019, p. 3–12

Alkaline and alkaline earth major elements in lignite from the Sofia coal province, Bulgaria

Jordan Kortenski, Alexander Zdravkov

Department of Geology and Exploration of Mineral Deposits, University of Mining and Geology “St. Ivan Rilski”, 1700 Sofia, Bulgaria; E-mails: [email protected]; [email protected]

Алкални и алкалоземни пепелообразуващи елементи в лигнитите от Софийската въглищна провинция, България

Йордан Кортенски, Александър Здравков

Катедра „Геология и проучване на полезни изкопаеми“, Минно-геоложки университет „Св. Ив. Рилски“, София 1700, България

Резюме. Лигнити и въглищни глини от седем въглищни басейна от Софийската въглищна провинция са изследвани за опре- деляне на съдържанията и привързаността към органичното вещество на алкалните и алкалоземните пепелообразуващи елементи (Na, K, Mg, Ca). За целта на настоящото изследване 420 лигнитни проби и 75 проби от въглищните глини са опепелени при температура от 815 °С. Концентрациите на всички изследвани елементи надвишават средните за света стойности. Получените резултати свидетелстват за много ниски съдържания на Na и K в пепелта и лигнитите от Станянския басейн. Високи съдържания на магнезий и калций се установяват във всички изследвани лигнитни пластове. Докато концентрацията на Mg е сравнително постоянна в цялата въглищна провинция, тази на Ca варира значително, като най-високи стойности са установени в басейните от западната част на провинцията. Последното се възприема като резултат от наличието на карбонатни скали в подхранващата про- винция. Обратно, съдържанието на K във въглищните глини е близко до кларковите стойности за глини, а това на Na във всички басейни, с изключение на Софийския, е по-ниско от кларка. Във всички лигнитни пластове Са, Na (с изключение на Софийския басейн) и Mg показват преобладаващо органичен афинитет, докато калият е основно с неорганичен такъв. Основните фактори, контролиращи присъствието и разпределението на изследваните елементи, са тяхната концентрация в растенията-въглеобразу- ватели и подхранващата провинция, както и условията на торфообразуващата среда.

Ключови думи: Софийска въглищна провинция, лигнити, алкални и алкалоземни пепелообразуващи елементи, привързаност към органично вещество.

Abstract. Lignite seams and carbonaceous shales from seven basins from the Sofia coal province were sampled. For the purpose of the investigation 420 lignite and 75 shale samples were ashed at 815 °С and the concentrations of Na, K, Mg and Ca were determined. All elements are in contents higher than the world average. The results indicate very low K and Na contents in Staniantsi lignite and its ash. Magnesium and especially Ca concentrations are high in all studied lignite seams. While Mg contents are relatively constant throughout the coal province, the Ca concentrations show variations and are highest in the western part of the province. The latter is presumed to be a result of the presence of carbonate rocks within the catchment areas of the Sofia coal province. For the same reason, on contrary, the K contents in the carbonaceous shale were found to oscillate around the Clarke values, whereas the Na contents are everywhere (except in the Sofia Basin) lower than the Clarke values. All lignite seams are characterized by predominantly organic affinity of Ca, Na (except in Staniantsi Basin), and Mg, whereas potassium seems to have mainly inorganic affinity. The main factors, controlling the presence and concentration of the studied elements in lignite, are their contents in the peat-forming plants and the rocks from the catchment areas, as well as the environmental settings during the peat formation.

Keywords: Sofia coal province, lignite, alkaline and alkaline earth major elements, organic and inorganic affinity.

Introduction SE oriented grabens, which were formed along sub- parallel faults as a result of crustal extension and The Sofia coal province (Minčev, 1960; Šiškov, collapse of crustal blocks (Tzankov et al., 1996; 1997) comprises eight basins within the western Nakov et al., 2001) during the Late Alpine develop- part of the Srednogorie Zone (Fig. 1). The Neogene ment of the Balkan Orogen (Boyanov et al., 1989; lignite-bearing sediments infill predominantly NW- Dabovski et al., 2002; Zagorchev, 2005).

3 4 Fig. 1. a) Schematic tectonic map of Bulgaria (modified after Dabovski et al., 2002); b) Simplified geological map of the western part of the Srednogorie Zone (modified after Cheshitev et al., 1989), showing the distribution of the lignite-bearing basins in Sofia coal province SoB, Sofia Basin; AB, Aldomirovtsi Basin; BB, Beli Breg Basin; StB, Staniantsi Basin; KB, Karlovo Basin; SkB, Samokov Basin; CB, Chukurovo Basin; GD, Gabrovitsa Deposit. Фиг. 1. a) Схематична тектонска карта на България (по Dabovski et al., 2002, с изменения); b) Схематична геоложка карта на Западното Средногорие (по Cheshitev et al., 1989, с изменения) с местоположението на лигнитните басейни от Софийската въглищна провинция SoB – Софийски басейн, AB – Алдомировски басейн, BB – Белобрежки басейн, StB – Станянски басейн, KB – Карловски басейн, SkB – Самоковски басейн, CB – Чукуровски басейн, GD – находище Габровица.

The purpose of the present work is to investigate presented underneath the Balsha Member. The fol- the behavior of the alkaline and alkaline earth ma- lowing upwards middle Pontian to early Dacian jor elements in the lignite of Sofia coal province. Novi Iskar Formation is composed of monotonous According to Yudovich (1978), all elements in con- (100–400 m thick) greyish finely laminated clays, centration greater than 0.5% in coal ash are defined with a thin interbed of whitish tuffs. The rocks are as major elements. Alkaline and alkaline earth ele- overlain by the fluvial sediments of the Dacian–Ro- ments include Na, K, Mg and Ca. The distribution manian Lozenetz Formation (150–200 m thick), of K and Na in coal from other Bulgarian basins which towards the eastern part of the Sofia Basin was previously investigated by Eskenazy and Ivchi- contains another lignite bed, i.e. the Novi Han bed. nova (1987). In the present work, the contents of The latter is composed of carbonaceous shales and Na, K, Mg and Ca, as well as their organic affin- up to 10 thin lignite seams with limited distribution ity and the factors determining their presence in the and very high ash yields (Kamenov, Kojumdzhieva, lignite seams and carbonaceous shales of the Sofia 1983). The Neogene lignite-bearing succession is coal province, are studied. covered by Pleistocene and Holocene alluvial and talus sediments. Chukurovo Basin. The basin represents a NW- Geological setting SE oriented graben structure, situated to the south- east of Sofia Basin (Fig. 1b). The basement and Sofia Basin. The basement and the provenance of provenance are composed of Proterozoic diabase the basin (Fig. 1b) are composed of Permian and and amphibole schists, Ordovician phyllites and Triassic fluvial red beds, Jurassic and Lower Cre- schists, Permian to Middle Triassic siliciclastic red taceous carbonate, siliciclastic and argillaceous beds and tidal calcareous and dolomitic sandstones, sediments, and Upper Cretaceous siliciclastic, car- Middle Jurassic limestones and Upper Cretaceous bonate, volcano-sedimentary and volcanic rocks andesitic tuffs, argillaceous limestones and marl- (Yanev et al., 1995). The Neogene sedimentation stones (Katskov, Iliev, 1993). The Neogene sedi- commenced within the NW-SE oriented Sofia mentation commenced towards the eastern part of graben with the deposition of the Meotian clays, the graben with the deposition of up to 50 m thick sands and sandstones of the informal variegated fining upwards siliciclastic sediments (weakly lithi- terrigenous formation (Kamenov, Kojumdzhieva, fied conglomerates, indurated sandstones, clays and 1983). These are overlain by the sediments of the carbonaceous clays) with 1 to 10 m thick lignite Sofia Group, consisting of Gnilyane, Novi Iskar and seam (i.e. Gabra seam) on top. Because of active Lozenetz Formations (Kamenov, Kojumdzhieva, movements along thrusted faults from the eastern 1983). The lowermost Gnilyane Formation is up periphery of the graben, the beds are locally inclined to 100–150 m thick and is composed of fining up- or even overturned. Subsequently, sedimentation wards Pontian terrigenous rocks (pebble to gravel commenced with the deposition of Badenian–Vol- breccia and conglomerates, sands and sandstones, hynian (Palamarev, 1964) argillaceous sandstones siltstones and clays), covered by the Balsha Mem- with thin conglomerate and sandy claystone inter- ber. The latter represents thick lignite seam (avg. beds and the Chukurovo lignite seam. The latter is thickness 30 m), which due to tectonic influences, up to 40 m thick within the southern part of the ba- is locally thickened to 70–95 m. At places, another sin, but splits into 12–18 thinner seams towards its high ash lignite seam (i.e. Kremikovtsi seam) is northern part (Katskov, Iliev, 1993).

5 Staniantsi Basin. The basin represents small sand-conglomerate formation), followed upwards E-W oriented graben, formed along the southern by Pontian–Dacian clays, sandstones, and up to margin of the Balkan Zone (Fig. 1). The basement 10 m thick lignite seam (Alino Formation), and up- and provenance are composed of Lower Triassic per Pliocene coarse-grained siliciclastic rocks and siliciclastic fluvial red beds, Middle Triassic lime- clays (Relyovo Formation). The lignite-bearing and dolostones, Jurassic and Upper Cretaceous succession is overlain by fluvial and talus Quater- limestones, locally with sandstone and marlstone nary deposits (Zagorčev et al., 1994). interbeds (Haydutov et al., 1995). The basin infill Gabrovitsa deposit. It is located within the overlies denudated pre-Neogene rocks and is sub- central parts of the NW-SE oriented Kostenets divided into four formal lithostratigraphic units, the graben. The latter is superimposed over Protero- former three (i.e. Dvechke, Belozem and Zainitsa zoic mica and amphibole gneisses, mica schists, Formations) united in the Mazgosh-Staniantsi amphibolites and marbles, and Paleogene silici- Group (Vatsev, 1999). Sedimentation commenced clastic rocks (Fig. 1b). The Pontian–Dacian lig- with the deposition of the 15–50 m thick sandy nite-bearing formation is composed of weakly claystones and clayey sandstones of the Dvechke lithified clayey sandstones, variegated clays and Formation (Khersonian), followed upwards by up to two lignite seams with thickness of up to 1.5–2 m 60 m thick carbonaceous claystones and marlstones each (Dimitrova, Katzkov, 1990). The sediments of the Belozem Formation (Khersonian–Pontian) are overlaid by Romanian breccia-conglomerates, with lignite seam (up to 20 m thick) at the base. sandstones and siltstones with total thickness of The rocks are overlain by the sandy claystones (lo- up to 560 m. cally with clayey sandstone interbeds and caliche Karlovo Basin. The lignite-bearing sediments profiles) of the Zainitsa Formation (35–60 m thick; are deposited within the NW-SE oriented Karlovo Pontian–Dacian). Late Pliocene to Pleictocene talus graben (Fig. 1b). The basin provenance is com- sediments of the Stranya Formation (50–80 m thick) prised predominantly of Archaean gneisses, schists, overlie the lignite-bearing succession towards the amphibolites, diabase, phyllites, late Palaeozoic northern margin of Staniantsi Basin. granite and granodiorite and Upper Cretaceous ar- Beli Breg Basin. The basin provenance is pre- gillaceous limestones and marlstones (Rousseva dominantly composed of Jurassic limestones and et al., 1994). The Miocene–Pliocene basin infill is Upper Cretaceous marlstones, limestones, andesit- subdivided into two formal lithostratigraphic units, ic and latitic volcanic and pyroclastic rocks, tuffs i.e. Iganovo Formation comprised of silty to sandy and tephra flysch (Zagorcev et al., 1995). Because clays with pebbly, sandy and clayey interbeds, cov- of the proximity to the large Sofia Basin (Fig. 1b), ered by the lignite-bearing Moskovetz Member Zagorcev et al. (1995) consider the sedimentary in- (diatomaceous clays with 3 lignite seams). These fill of Beli Breg Basin to represent a lateral exten- are overlain by the Romanian siliciclastic deposits sion of Lozenetz Formation from Sofia Basin. Sedi- (pebbles, sands and clays) of the Karavelovo For- mentation commenced with the deposition of up to mation (Rousseva et al., 1994). Alluvial and talus 60 m thick clayey sandstones, locally interbedded sediments cover most of the graben. by thin conglomerate beds at the bottom. These are overlain by up to 25 m thick lignite seam, followed by up to 40 m thick calcareous clays and marls. An- Material and methods other thin lignite seam is locally presented in the upper part of the succession. Fluvial channel and For the purpose of the present study 420 coal overbank siliciclastic sediments cover the Dacian samples and 75 carbonaceous shale samples from lignite-bearing succession. seven lignite basins from the Sofia coal province, Samokov Basin. The basin is formed within the were studied. The high temperature ash yield was E-W oriented Palakaria graben, south of the large determined following standard procedure (ISO Sofia Basin (Fig. 1b). The basement and provenance 17246:2010). The ash was further dried at 105 °С, are composed of Precambrian gneisses, schists and mixed with lithium tetraborate and melted in plati- amphibolites, Ordovician diabase and phyllites, Up- num crucible at 1600 °С. Subsequently, the alu- per Cretaceous siliciclastic and intrusive (diorite, mosilicate glass was dissolved in nitric acid (ISO quartz diorite, granodiorite) rocks (Zagorčev et al., 15587-2:2002), and the concentrations of K, Na, Ca 1994). The lignite-bearing succession is subdivided and Mg were determined according to ISO 17294- into one informal and two formal lithostratigraphic 1:2004 and ISO 17294-2:2016 standards using units, as part of the Palakaria Group (Antimova, ICP-VISTA-MPX SIMULTANEOUS CCD optical Kojumdgieva, 1991). The Neogene sedimentation emission spectrometer. commenced with the deposition of Pontian sands The results were statistically evaluated and the and conglomerates with sand interbeds (lower correlation coeficients, between the studied ma-

6 jor elements and ash yield, were determined using nous/groundwater supply from the catchment areas Excel™ Data Analysis toolpak. as equally important source of sodium in coal. For example, in a study of Canadian lignite Beaton et al. (1991) pointed out a continuous groundwater sup- Results and discussion ply from evaporitic and volcanic ash layers during peat formation, whereas in Australian low-rank coal The presence and distribution of the studied major deposits the major source of Na, Ca, Mg are consid- elements in the basins from Sofia coal province ered aquatic horizons with saline water underneath were mainly controlled by the following factors: the coal strata (Ward, 1992). Furthermore, deposits 1) the contents in the living plants; 2) the concen- of trona, glauberite and tenardite in the area of Bey- tration of the elements within the catchment areas; pazari (Turkey) are apparently responsible for the 3) the type (terrigenous/groundwater) and direction significant quantities of Na within the lignite seam of mineral influx; and 4) the settings of the peat- there (Querol et al., 1997b). For the Sofia Basin, the forming environments. Epigenetic mineralization is presence of sodium hydro-carbonate-sulphate wa- either insignificant or missing at all, thus suggest- ters, circulating in depth, is characteristic (Antonov, ing that neither the degree of endogenic/exogenic 1956). The latter might have influenced the Na con- cracking of the lignite beds, nor the composition of tents in Sofia lignites. the mineralized waters, played any significant con- Sodium contents in carbonaceous shale’s ash trol on the geochemical composition of the lignite. is above the Clarke values only in Sofia Basin and close to the Clarke in Beli Breg Basin (Table 2). The ashes of the rest of the carbonaceous shales show Contents and distribution of alkaline Na concentrations under the Clarke value, in Stani- and alkaline earth major elements antsi Basin being about 8 times lower (Table 2). The detected high Na contents in the coal and carbona- Sodium. The concentration of Na varies significant- ceous shale’s ashes from Sofia Basin might origi- ly between the studied lignite basins. The lowest nate from elevated sodium concentrations within values were detected in the lignite ashes from Stani- the catchment area, or within the groundwaters. antsi Basin (avg. 0.11 wt%), whereas the highest Similar conclusions could also be drawn for Beli values (avg. 2.4 wt%) are typical for the Sofia Basin Breg Basin. On the other hand, the low Na contents (Table 1). The rest of the basins are also charac- in Staniantsi Basin point towards sodium deficient terized by enhanced Na concentrations (0.75–1.19 provenance. wt%; Table 1). When recalculated to total organic For most of the studied lignite, the correlation matter, Na contents significantly exceed the World coefficients (ro) between Na and ash yields are neg- average values (Valković, 1983). Marine transgres- ative (Table 3). The highest values (ro > – 0.5) were sion is considered by Kessler et al. (1967) as the calculated for Chukurovo and Gabrovitsa deposits, main reason for increased Na contents in coal. In where Na most probably occurs in organically- addition, Slansky (1985) recognized the terrige- bound form. Predominant organic affinity can also

Table 1 Average concentration of alkaline and alkaline earth elements in lignite and lignite ash Таблица 1 Средно съдържание на алкални и алкалоземни елементи в лигнитите и тяхната пепел

Average concentration in lignite Average concentration in lignite ash Number of Basins (wt%) (wt%) samples Na Mg K Ca Na Mg K Ca Beli Breg 91 0.40 0.60 0.63 7.13 1.19 1.80 1.90 21.40 Staniantsi 38 0.04 0.71 0.12 7.84 0.11 2.00 0.35 22.40 Sofia 89 0.44 0.30 0.18 2.04 2.40 1.80 0.98 11.60 Samokov 31 0.28 0.32 0.68 2.14 0.82 1.50 1.98 6.20 Karlovo 23 0.19 0.36 0.45 1.14 0.97 1.80 2.23 5.70 Chukurovo 92 0.12 0.40 0.32 1.23 0.75 2.40 1.89 7.40 Gabrovitsa 46 0.23 0.74 0.58 2.27 0.82 2.60 2.07 7.70 World average* 0.02 0.02 0.01 1.01 * after Valković (1983)

7 Table 2 Average concentration of alkaline and alkaline earth elements in carbonaceous shale’s ash Таблица 2 Средно съдържание на алкални и алкалоземни елементи в пепелта на въглищните глини

Number Average concentration (wt%) Basins of samples Na Mg K Ca Beli Breg 8 1.00 1.50 2.80 9.30 Staniantsi 10 0.14 1.15 1.80 6.25 Sofia 19 1.54 1.10 1.40 6.40 Samokov 10 0.61 1.00 3.20 3.20 Karlovo 9 0.80 1.20 3.40 3.10 Chukurovo 11 0.31 2.10 3.90 5.80 Gabrovitsa 8 0.32 1.70 3.00 4.90 Clarke values for clays* 0.96 1.50 2.66 2.21 * after Turekian and Wederpohl (1961)

Table 3 Correlation coefficients between the element concentrations and ash yield Таблица 3 Корелационни коефициенти между съдържанията на елементите и пепелта

Pearson r Basins o Na/Ash Mg/Ash K/Ash Ca/Ash significance level Beli Breg ±0.20 –0.32 –0.31 0.33 –0.58 Staniantsi ±0.32 0.31 –0.42 0.78 –0.81 Sofia ±0.22 –0.41 –0.39 0.64 –0.43 Samokov ±0.35 –0.37 –0.42 0.53 –0.52 Karlovo ±0.41 –0.43 –0.49 0.52 –0.46 Chukurovo ±0.21 –0.59 –0.37 0.76 –0.44 Gabrovitsa ±0.25 –0.64 –0.46 0.49 –0.48

be suggested for Beli Breg, Sofia and Karlovo Ba- has also been reported by Beaton et al. (1993) for sins, although the lower ro values (< –0.5; Table 3) Canadian coals. indicate that minor amount of Na might also exists The organically bound sodium in coals can be in mineral form. Predominant organic affinity of Na derived from the living plants, in which its average in coal is reported by Miller and Given (1978), Bea- concentration is about 0.12% on dry base (Bowen, ton et al. (1991), and others. For example, Querol 1966). In addition, Voitkevich et al. (1983) point et al. (2001) establish that 40 to 98% of sodium in out that Na concentration in plant ash is typically different coals is bound to the organic matter. In ad- around 2%. However, according to Eskenazy and dition, Eskenazy and Ivchinova (1987) also estab- Ivchinova (1987), the main part of sodium occurs lished predominant organic affinity of Na in Bulgar- absorbed by the organic matter. Voitkevich et al. ian coals with ash yields in the range 10–20 wt%. (1983) consider the humic and fulvic acids as the Positive correlation between the ash yields and main Na absorbing organic compounds, whereas the Na contents was calculated only for Staniantsi Ward (1992) suggests that Na predominantly re- lignite. However, because of the poor correlation places the carboxylic functional groups in humic

(ro = 0.31; Table 3) it is highly probable that sig- acids. The mineral form of Na in coal is mostly re- nificant amount of Na exists in mineral form, and lated to the occurrence of clay minerals. According therefore mixed organic/inorganic afinity of Na can to Ward (1992) sodium associates mainly with kao- be suggested for this lignite. Similar observation linite, but other Na-containing minerals like albite,

8 analcime and clinoptilolite (Querol et al., 1997b) nificantly higher than the calculated World average can also be detected as certain coal. In addition, the concentrations (Valković, 1983). The carbonaceous lignite from Sofia Basin shows good positive cor- shales from Beli Breg, Samokov, Karlovo, Chuku- relation (ro > 0.5) between the sodium contents and rovo and Gabrovitsa deposits also contain increased the sulfide and carbonate minerals. potassium contents, compared to the Clarke values The main source of Na in the peat forming envi- for clays (Table 2). Based on the reported herein ronment is the basin’s catchment area, from which high potassium concentrations a provenance rich in the element is delivered through terrigenous sup- K-bearing minerals can be suggested. ply of siliciclastic grains or groundwater influx of Relatively significant positive correlation be- dissolved sodium ions. The environmental acidity, tween K contents and ash yields was calculated for however, has crucial control on sodium redistribu- most of the studied lignite (Table 3), arguing for tion and fixation. Because of the high mobility of predominant inorganic affinity of potassium. The sodium in exogenic environments, as well as its only exception is the lignite from Beli Breg Basin, high ion radius, Na incorporation into the clay min- for which the correlation is quite poor (Table 3) and erals structure is significantly limited (Perel’man, suggests mixed organic/inorganic mode of K occur- 1977). Furthermore, Querol et al. (1997b) pointed rence. The literature overview also provides data for out that alkaline peat forming environments inten- predominant inorganic incorporation of potassium sify Ca absorption instead of Na. The very low so- (Beaton et al., 1991, 1993; Crowley et al., 1997; dium contents in Staniantsi lignite, which is formed Warwick et al., 1997; Spears, Zheng, 1999; Liu et under neutral to weakly alkaline environment, are al., 2001). In addition, Querol et al. (1996, 1997a) therefore presumed to be a result of the limited el- and Alastuey et al. (2001) established positive cor- ement supply and limited chemical reactions. Fur- relation of K with the alumosilicate part of the min- thermore, the predominant inorganic affinity of the eral matter, whereas Querol et al. (2001) pointed out element there, might be related to the limited leach- that only minor amount of potassium (8–16 wt%) ing of sodium from the minerals under the alkaline can be organically bound. Furthermore, the study of conditions and the low concentrations of organic Еskenazy and Ivchinova (1987) on Bulgarian coals acids. On the other hand, in Chukurovo and Ga- also reveals high inorganic concentration of K. brovitsa lignite, which is formed under more acidic The main K-bearing mineral in coal is consid- environments, the main part of sodium is likely to ered to be illite. Spears and Martinez-Tarazona have organic affinity because of the relatively good (1993) use the increase of the ratio K2O/Al2O3 to in- negative correlations with the ash yields (ro = –0.59 fer the increasing amount of illite the coal relative to and ro = –0.64 respectively; Table 3). The organic that of kaolinite. The organically bound potassium, incorporation of Na in these lignite is most prob- as far as it is present in coal, can be either of bio- ably related to the more enhanced leaching of the genic origin, since the living plants contain about element from the sodium-bearing minerals. Howev- 1.4% K (Bowen, 1966), or exists absorbed with the er, the limited availability of Na within the basin’s humic and fulvic organic compounds (Voitkevich catchment areas is clearly marked by the relatively et al., 1983). low concentrations of Na in both Chukurovo and The main source of potassium for the peat-form- Gabrovitsa lignite (Table 1). The highest sodium ing environment are the rocks from the basin’s catch- concentrations were detected in Sofia lignite, which ment area, from where the element is supplied mostly formed under a wide range of pH settings. The poor through the terrigenous influx of clay-sized particles. negative correlation with the ash yields (Table 3) In addition, minor amounts of K may be supplied there indicates that Na occurs in both organic and through minerals like potassium feldspar and sericite. inorganic form, although slight predominance of the Furthermore, the weathering of the terrigenous min- latter can be suggested. From the above discussion erals might produce К-bearing sulfate minerals like it is apparent that the concentration and the form of jarosite and pollyhalite. The environmental setting in occurrence of sodium in peat-forming environments turn controls strongly the fixation of potassium with- are strongly dependent on element supply, pH set- in the coal. For example, the highest K concentra- tings and the chemical reactivity of the element. tions amongst the studied lignite seams were detected Potasium. The average element concentrations in in Karlovo Basin, where the acidic peat-forming en- the lignite ash from most of the studied basins vary vironment favored the neoformation of illite, which in the narrow range from 1.98 to 2.23 wt% (Table 1). concentrates the main part of the element contents. The lowest potassium contents were detected in On the other hand, the neutral to weakly alkaline en- Sofia and Staniantsi lignite (0.98 and 0.35 wt% re- vironment (Staniantsi and Beli Breg Basins) favors spectively; Table 1). Recalculated on organic matter the formation of montmorillonite. base, these values correspond to K concentrations in Magnesium. Element concentration in the lig- the range 0.12–0.68 wt% (Table 1), which are sig- nite ash from Staniantsi, Chukurovo and Gabrovitsa

9 deposits exceeds 2 wt% (Table 1). Recalculated on absorption reactions, and therefore are sedimented organic matter, Mg concentration significantly ex- in mineral form, or react with the organic substanc- ceeds the World average value (Valković, 1983), al- es within the peat-forming environment. though there might be some degree of inaccuracy in Calcium. It differs from the discussed above calculation of this value. The high Mg contents are major elements by its wide concentrations range considered to occur due to the presence of carbonate (Table 1). High calcium contents are typical for the rocks within the lignite-bearing sequence (Karayigit lignite from the whole Sofia province. The lignite et al., 2001). ash from Sofia, Staniantsi and Beli Breg deposits The carbonaceous shale’s ashes from Chuku- is characterized by Ca contents exceeding 10 wt%. rovo and Gabrovitsa deposits are characterized by The rest of the basins (i.e. Samokov, Karlovo, Mg contents above the Clarke value and close to Chukurovo and Gabrovitsa) are also characterized that value for Beli Breg Basin (Table 2). For the by abundant calcium concentrations (> 5 wt%; rest of the studied basins the element concentration Table 1). Similarly, when recalculated on organic is below the Clarke. It is therefore apparent that the matter, the Ca contents of Sofia province are greater variations in Mg contents are related to the element than the World average for coal (Valković, 1983). concentration in the catchment areas. The latter is especially valid for the Beli Breg and Based on the poor correlation with the ash yields Staniantsi Basins, for which more than seven times

(ro < –0.5; Table 3), predominant organic affinity higher concentrations were detected (Table 1). Car- of Mg in all studied lignite seams can be suggested, bonaceous shales from the studied deposits are also although the presence of Mg in mineral form cannot rich in calcium, with the concentrations being larger be excluded. Organic affinity of Mg is also previ- than the Clarke value for clays (Table 2). The re- ously reported by Gluskotter et al. (1977), Miller ported herein Ca abundance is presumed to result and Given (1987), Querol et al. (1996). from the carbonate-rich provenance. Organically bound Mg might originate from For all studied lignite, the correlation coefficient the plants themselves, as living plants are known of Ca and ash yield is negative (Table 3), thus sug- to contain about 0.32 wt% Mg (Bowen, 1966), gesting that the element occurs mostly in organic which accounts to about 7 wt% in plant ash (Voit- form. However, for most basins the correlations kevich et al., 1983). However, the element mostly are not very good (ro ~0.4–0.5; Table 3) and there- participates as metallic radical in humic and fulvic fore the existence of minerally-bound Ca can also substances (Voitkevich et al., 1983). According to be suggested. Predominant organic affinity of Ca Ward (1992), magnesium is mostly incorporated is previously reported by Crowley et al. (1997) and by replacement of carboxylic functional groups Liu et al. (2001), whereas Beaton et al. (1991, 1993) in organic acids. The mineral form of element oc- report mixed organic/inorganic element affinity. currence is predominantly carbonate. Many of the One of the possible sources of Ca in the peat- studied lignite seams contain minor amounts of forming environment are the plants themselves, Mg-bearing minerals like dolomite, kutnahorite and since the element concentration in plants is 1.8% magnesite. However, in most cases magnesium is (Bowen, 1966). However, the organically bound Ca delivered into the peat-forming environment with mostly exists in the form of humic and fulvic or- other carbonate minerals. For example, many lig- ganic compounds (Voitkevich et al., 1983) in which nite from the Sofia province are characterized by calcium typically replaces hydrogen in the function- the presence of high-Mg calcite (up to 5% Mg) or al groups of the organic acids (Beaton et al., 1991). siderite (up to 0.1% Mg; Kortenski, 1992). In ad- The mineral form of Ca occurrence is mostly related dition, magnesium often associates with allumo- to the carbonate (i.e. calcite, aragonite, dolomite, si- silicate minerals like illite and smectite, which are derite, ankerite, kutnahorite) and sulfate (gypsum, commonly found within the entire province. anhydrite) minerals (Kortenski, 1992). The main source of Mg in the peat-forming Apart from the plants, the main source of cal- environment are the rocks from the catchment ar- cium in the peat-forming environment are the rocks eas, from which the element is delivered either as from the catchment areas. Most of the Ca-bearing groundwater ionic solutions, or through the terri- silicates, but also part of the carbonate minerals, genous supply. Because of the relative stability of could be supplied through the terrigenous influx. the clay minerals in exogenic environments, it can However, the major part of the element is supplied be presumed that the main part of Mg introduced in the form of ionic solutions through surface or in mineral form most probably does not enter into ground waters. As a result, the lignite from Beli chemical reactions and is distributed within the ba- Breg, Staniantsi and partly Sofia Basins, for which sin together with the terrigenous particles. On the the catchment areas are dominated by carbonate other hand, the dissolved Mg ions that are delivered rocks, are characterized by significant amounts of through the groundwater influx, might participate in calcium.

10 Conclusions concentrations in its western part (Beli Breg, Stani- antsi Basins, and partly Sofia Basin) are 3–4 times The present study outlines the occurrence and dis- higher than in the rest of the basins. The latter is tribution of the alkaline and alkaline earth major presumed to be a result of the intensive Ca supply elements in the lignite seams from the Sofia coal from the carbonate-dominated catchment areas of Province. All studied elements are in concentra- these basins. For all lignite seams Ca, Na (except tions greater than the World average. Very low in Staniantsi Basin) and Mg are characterized with concentrations of both K and Na were detected in predominant organic affinity, whereas K shows ei- lignite and lignite ash from Staniantsi Basin. The ther mixed or predominant inorganic affinity. The ratio Na/K is less than 1 for all basins, except So- concentration of the studied major elements within fia Basin, in which the circulation of Na-bearing the catchment areas of the basins from the Sofia ground waters is presumed to result in the highest coal province and the environmental settings during sodium concentrations among the whole coal prov- peat formation, can be outlined as the main factors, ince. Magnesium and especially Ca concentrations controlling their occurrence and distribution within are high in all studied lignite seams, thus outlining the lignite seams. the presence of carbonate rocks in the catchment areas of all basins. However, distinctive distributional Acknowledgements: Financial support from the patterns were established. While Mg contents vary National Science-research Fund, project VU 05/06, only in narrow range throughout the province, Ca is greatly appreciated.

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Постъпила на 17.01.2019 г., приета за печат на 30.04.2019 г. Отговорен редактор Борис Вълчев