GeolCarp_Vol70_No5_433

GEOLOGICA CARPATHICA

, OCTOBER 2019, 70, 5, 433–445

doi: 10.2478/geoca-2019-0025

www.geologicacarpathica.com

Mineralogical and physico–chemical properties

of bentonites from the Jastrabá Formation

(Kremnické vrchy Mts., Western Carpathians)

MAREK OSACKÝ



, TOMÁŠ BINČÍK

, TOMÁŠ PAĽO

, PETER UHLÍK

JANA MADEJOVÁ

and ADRIANA CZÍMEROVÁ

Department of Economic Geology, Comenius University in Bratislava, Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, Slovakia;



mosacky@hotmail.com; t.bincik@gmail.com; peter.uhlik@uniba.sk

Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovakia; uachjmad@savba.sk,

adriana.czimerova@savba.sk

(Manuscript received February 6, 2019; accepted in revised form September 30, 2019)

Abstract: In the past years an increasing demand for bentonites resulted in the opening of new bentonite deposits in

the Jastrabá Formation. The shortage of information, in particular analytical data, on the bentonites from the newly

opened Jastrabá Fm. deposits was the motivation for the current study. Smectite is the predominant mineral in all bulk

bentonites from the new deposits. Its amount varied between 43 and 90 wt. %. The bulk bentonites also contain variable

amounts (10–57 wt. %) of mineral admixtures such as feldspars, mica, opal-CT, kaolinite, quartz and sometimes goethite.

The smectite mineral comprising the studied bentonites was montmorillonite. The octahedral Al in the structure of

montmorillonite was partially substituted by Mg, and to a lesser extent by Fe. The interlayer space of montmorillonite is

occupied predominantly by divalent exchangeable cations (Ca

and Mg

). The dehydroxylation temperature of smectites

(> 600 °C) determined on the DTG curves indicates the presence of the cis-vacant variety of montmorillonites. The mean

crystallite thicknesses of smectites (T

MEAN

) calculated by BWA analyses ranges from 7.2 to 11.5 nm. The shape of

the crystallite thickness distributions (CTDs) for smectites is lognormal in all cases. Cation exchange capacity (CEC)

and total specific surface area (TSSA) increases with increasing amount of smectite. The CEC of 101 meq/100g and

TSSA of 616 m

/g correspond to bulk bentonite from the Stará Kremnička III deposit containing 89 wt. % of smectite.

Keywords: Deposit, mineralogy, Kremnické vrchy Mts., Jastrabá Formation, bentonite, smectite, montmorillonite.

Introduction

Bentonite is a raw material composed predominantly of clay

minerals from the smectite group (e.g., montmorillonite,

beidellite, saponite, nontronite and hectorite) (e.g., Christidis

& Huff 2009). Smectites have characteristic 2:1 type of laye-

red structure consisting of one octahedral sheet interlayered

between two tetrahedral sheets. The octahedral sheet may be

either dioctahedral or, less commonly, trioctahedral. The non-

equivalent isomorphous substitution in the tetrahedral (Al

and/or Fe

for Si

) and octahedral (Mg

and/or Fe

for Al

)

sheets give rise to a charge imbalance which is compensated

by exchangeable cations (e.g., Na

, K

, Ca

, Mg

) located in

the interlayer space. The exchangeable cations are only loosely

held in the smectite interlayers and can be easily replaced by

other cations. As such, smectites have relatively high cation

exchange capacity (CEC = 80 –150 meq/100g; La Grega et al.

1994; Calarge et al. 2006). The swelling capacity of smectites

enables a reversible increase/decrease in the basal spacing

of the interlayer space. Smectites can increase their volume

12-times in contact with water (Galamboš et al. 2010).

The small size of smectite particles results in high total speci-

fic surface area (TSSA), which may reach 800 m

/g (Środoń &

McCarty 2008; Zhu et al. 2015).

Overall, the specific layered structure of smectites is respon-

sible for their unique properties, such as high CEC and TSSA,

swelling capacity, low hydraulic conductivity and high adsorp-

tion capacity. Due to these properties, smectites have been

studied for many potential environmental applications, for

example, as sealing material in landfill liners (Andrejkovičová

et al. 2008), backfill material for construction of high-level

nuclear waste (HLW) repositories (Pacovský et al. 2007;

Stríček et al. 2009; Osacký et al. 2013), wastewater treatment

(Viraraghavan & Kapoor 1994), adsorbent for heavy metals

(Sheta et al. 2003; Rao et al. 2006; Andrejkovičová et al. 2010;

Galindo et al. 2013) and organic compounds (Tiller et al.

1984). The bulk of the world’s bentonite production in 2018

(~28 million t) is used as pet waste absorbents, drilling mud,

foundry sand and ore pelletizing (U.S. Geological Survey

2019).

The formation of bentonite deposits is mainly associated

with alteration of volcanic glass-rich rocks (Kraus et al. 1994;

Moll 2001). The composition of the parent rocks (acidic rocks

— rhyolites, intermediate rocks — andesites and basic rocks

— basalts) may affect the crystal chemistry of smectites

(Christidis & Dunham 1997; Osacký et al. 2012). For instance,

the Fe

for Al

substitution controls the crystal chemistry of

smectites derived from intermediate rocks, whereas Mg

for

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, 2019, 70, 5, 433–445

substitution seems to be more important for smectites

derived from acidic precursors (Christidis & Dunham 1997).

Slovak bentonite deposits were formed mainly by an alteration

of rhyolitic (e.g., Stará Kremnička – Jelšový potok deposit) and

andesitic (e.g., Lieskovec deposit) volcanoclastics in a lacus-

trine environment (Kraus et al. 1994). The majority of Slovak

bentonite deposits are concentrated in the two areas in the

Western Carpathians: Central Slovakia Volcanic Field (e.g.,

Stará Kremnička – Jelšový potok, Kopernica and Lieskovec

deposits) and Eastern Slovakia Volcanic Field (e.g., Fintice,

Lastovce and Kuzmice deposits).

In the present study, we examine bentonites from different

deposits of the Jastrabá Formation (Fm.), located in the

south-western part of the Kremnické vrchy Mts. in the Central

Slovakia Volcanic Field. The Jastrabá Fm. is one of the most

promising areas for the occurrence of non-metallic raw mate-

rials in the whole Western Carpathians (Šamajová et al. 1992).

The economic accumulations of perlites, zeolites, bentonites

(clay rocks rich in smectite) and K-bentonites (clay rocks rich

in illite–smectite) on the south-western margin of the Krem-

nické vrchy Mts. were formed by an alteration of Miocene

acidic vitric volcanoclastics of the Jastrabá Fm. (Kraus et al.

1994) and margins of rhyolite domes (Demko et al. 2010).

With the annual bentonite production of about 200 kt and

total reserves more than 50,000 kt, Slovakia is one of the

world’s leading countries in bentonite exploitation (Baláž &

Kušík 2015). It should be noted, that 13 of 30 bentonite depo-

sits (43 % of all Slovak bentonite deposits) are located in the

Jastrabá Fm. (Record of Mining Areas 2016). The best grade

Slovak bentonites from the Stará Kremnička – Jelšový potok

and Kopernica deposits (both belong to the Jastrabá Fm.) con-

tain ~80 wt. % of smectite (Osacký et al. 2009).

Increasing demand for bentonites in the past years resulted

in the opening of several new bentonite deposits in the Jastrabá

Fm. (e.g., Stará Kremnička III, Lutila I, Lutila – Pod Klapou

and Bartošova Lehôtka), most of which have never been stu-

died in detail. As a consequence, the potential of recently

mined Slovak bentonites from new deposits is not fully

determined. We believe that bentonites from the newly opened

bentonite deposits of the Jastrabá Fm. are qualitatively equi-

valent to those from older bentonite deposits in this area (e.g.,

Stará Kremnička – Jelšový potok). However, this conjecture

needs to be supported by analytical data. The comprehensive

characterization of bentonites from new deposits is an essen-

tial primary step in assessing the qualitative and technological

parameters and the optimal application of these bentonites.

The main goal of the present study is a comprehensive cha-

racterization of bentonites from the new deposits and their

comparison with other Jastrabá Fm. deposits (e.g. Kopernica)

as well as world bentonite deposits (e.g., Gonzales County,

Texas; Crook County, Wyoming; Apache County, Arizona;

Otay San Diego County, California; Crook County, Wyoming;

all located in the United States). The obtained results provide

a set of original data on mineralogy, chemistry, thermal and

surface properties of bentonites from several new deposits

located in the Jastrabá Fm., which may help to find a proper

application for the bentonites, leading to a more rational and

efficient utilization of this kind of raw material.

Geological setting

All investigated bentonites come from deposits located in

the south-western part of the Kremnické vrchy Mts., in the

Western Carpathians and belong to the same geological for-

mation (Jastrabá Fm.) (Fig. 1). The Jastrabá Fm. consists of

rhyolitic volcanism products represented by volcanic extru-

sions, lava flows, tuffs and epiclastics, which form a conti-

nuous, 100–300 m thick complex (Lexa et al. 1998). Early

sporadic extrusive and explosive activity of rhyodacites is

followed by a widespread activity of plagioclase and pla-

gioclase–sanidine rhyolites; the late volcanic activity products

are represented by plagioclase–quartz–sanidine rhyolites

(Chernyshev et al. 2013). The biostratigraphic data suggest

the Late Sarmatian to Early Pannonian age of the Jastrabá Fm.

(Konečný et al. 1983). The results of K–Ar isotope dating of

Fig. 1. Simplified geological map of the south-western margin of the Kremnické vrchy Mts. (Jastrabá Fm.) (modified from Kraus et al. 1994;

Koděra et al. 2014). Bentonite deposits: Stará Kremnička – Jelšový potok (1); Stará Kremnička III (2); Lutila I (3); Lutila – Pod Klapou (4);

Bartošova Lehôtka (5); Kopernica (6) and Dolná Ves (7).

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the Jastrabá Fm. rhyolites yield ages ranging from 12.2 ± 0.3 Ma

to 11.4 ± 0.4 Ma (Chernyshev et al. 1995, 2013). Permeable

rhyolite volcanoclastic rocks of the Jastrabá Fm., rich in vol-

canic glass, are extensively altered into economic accumula-

tions of bentonites, K-bentonites and zeolites (Šamajová et al.

1992). The northern part of the Jastrabá Fm., in the vicinity of

the Kremnica ore vein system, is affected by regional illite–

smectite alteration forming K-bentonite accumulations (e.g.,

Dolná Ves deposit; Kraus et al. 1994; Šucha et al. 2001)

(Fig. 1). The alteration products in the southern part of the

Jastrabá Fm. are composed mainly of smectite (montmoril-

lonite), locally forming bentonite deposits (e.g., Stará Krem-

nička III, Stará Kremnička – Jelšový potok, Lutila I, Lutila – Pod

Klapou, Bartošova Lehôtka, Kopernica) (Fig. 1). Bentonites

consist predominantly of montmorillonite, accompanied by

variable amounts of opal-C/CT, quartz, feldspars, mica, zeo-

lites and kaolinite (Šamajová et al. 1992; Kraus et al. 1994;

Osacký et al. 2009; Uhlík et al. 2012; Górniak et al. 2016,

2017). The thickness of bentonite beds ranges from a few

metres up to 50 m (Šamajová et al. 1992). The bentonite beds

are often interbedded with limnic/lacustrine silicites which are

formed by SiO

discharge from the subsurface hydrothermal

fluids upon reaching the local limnic/lacustrine basins (Koděra

et al. 2014). The previous studies have reported that bentonite

deposits of the Jastrabá Fm. are primarily formed by alteration

of acidic vitric volcanoclastics during diagenesis of the volca-

noclastics in a freshwater environment in an open or semi-

closed hydrological system (Šamajová et al. 1992; Kraus et al.

1994). The results of recent studies have shown a strong effect

of subsurface hydrothermal fluids (deeply-circulating, mostly

meteoric waters, driven by heat from the contemporaneous

rhyolite magma chamber) on the bentonitization of rhyolitic

volcanoclastics from the Jastrabá Fm. (Demko et al. 2010;

Koděra et al. 2014).

The nature of the volcanic precursors of bentonites of the

Jastrabá Fm. has not been previously studied in detail. Kraus

et al. (1982, 1994) proposed that the precursors of the bento-

nites from the Jastrabá Fm. were mostly redeposited and partly

also autochthonous rhyolite tuffs. The results of recent studies

have demonstrated that the textural and compositional varia-

tion of bentonites from the Jastrabá Fm. (e.g., Kopernica

deposit) can be related to the formation of bentonites from

genetically diversified volcanic materials such as ignimbrites,

pyroclastic fall deposits and redeposited tuffs (e.g., Górniak et

al. 2016). Demko et al. (2010) reported that the best grade

bentonites (Stará Kremnička – Jelšový potok, Dolná Ves and

Kopernica deposits) were formed by the alteration of marginal

perlitic breccias of extrusive domes and cryptodomes.

Starting materials and methods

Starting materials

Eight bentonite samples were collected from the borehole

VSK-11 (

48°38’18.9” N, 18°53’9.8” E)

located in the Stará

Kremnička III deposit (1.5 m, 10.5 m, 16.5 m, 20.5 m, 30.5 m,

31.5 m, 40.5 m and 47 m). Point samples C87 and C88 also

come from the Stará Kremnička III deposit (

48°38’20.3” N,

18°53’7.1” E)

; point samples C74, C85 and C86 are from

the Lutila I deposit (

48°37’42.0” N, 18°52’19.3” E

); point

samples C75 and C90 are from the Lutila – Pod Klapou deposit;

point samples C76, C77 and C78 are from the Stará Krem-

nička – Jelšový potok deposit (

48°37’35.6” N, 18°53’12.0” E

);

and point sample C89 is from the Bartošova Lehôtka deposit

(

48°38’38.1” N, 18°53’35.5” E

The samples were dried for 3 days at 60 °C. Solids were

manually homogenized and separated into several size frac-

tions. Prior to the size separation, the bulk solids were soni-

cated in distilled water for 10 min and then stirred for

an additional 24 h at room temperature. The > 160 µm particle

size fraction was isolated from the bulk solids by wet sieving

using a Fritsch sieve shaker and distilled water. The sub-sieve

fraction (< 160 µm) was divided into two size fractions

(2–160 µm and < 2 µm) by settling in distilled water. All sepa-

rated size fractions were dried overnight at 60 °C, weighed

and analysed.

A portion of the < 2 µm fractions was treated three times

overnight with 1 M NaCl in order to prepare Na-saturated

smectites. Excess soluble salts were removed by centrifu-

gation, followed by dialysis. Na-saturated solids were dried

overnight at 60 °C, passed through a 250 µm sieve and ana-

lysed by BWA and thermal analyses.

Methods

The X-ray diffraction (XRD) patterns of oriented (air-dried

and ethylene glycol solvated) and randomly oriented prepara-

tions were recorded using a Phillips PW1710 diffractometer

with Cu Kα radiation and graphite monochromator at 20 mA

and 35 kV. The step size for all analyses was 0.02° 2θ.

The oriented preparations were made by dispersing 150 mg of

< 2 µm fraction in 2 ml of distilled water, pipetting the suspen-

sion onto a glass slide and drying at room temperature.

The XRD patterns of oriented preparations with an exposure

time of 0.80 s per step were utilized for clay mineral identifi-

cation (from the 00l series of reflections). The ethylene glycol

(EG) solvation of oriented preparations was carried out over-

night at 60 °C.

Quantitative X-ray diffraction (QXRD) was performed on

randomly oriented preparations and evaluated using the

RockJock software (Eberl 2003). RockJock determines the

quantitative content of minerals in powdered samples by com-

paring integrated reflection intensities of individual minerals

with the intensities for pure standard minerals and an internal

standard (corundum). Samples were prepared according to

the method modified by Omotoso & Eberl (2009) from that

reported by Środoń et al. (2001). The sample was passed

through a 250 μm sieve. Then 1 g of sample was mixed with

0.250 g of corundum and ground with 4 ml of denatured alco-

hol in a McCrone Micronizing Mill for 5 min using zirconia

grinding cylinders. The mixture was dried overnight at 60 °C.

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The sample/corundum mixture was then shaken for 10 min in

a plastic vial (25 ml) with 3 plastic balls (9 mm diameter)

using a Retsch MM 200 mill. Hexane was added to the mix-

ture in the ratio of 0.5 ml hexane to 1 g of pure clay and the

vial was shaken for an additional 10 min. The powder was

then side loaded into an XRD holder and X-rayed (from 4º to

65º 2θ, 2 s per step).

The chemical composition was determined at ACME

Analytical Laboratories Ltd. (Vancouver, British Columbia,

Canada) by inductively coupled plasma (ICP) analysis after

digestion of samples using lithium metaborate fusion, fol-

lowed by nitric acid leaching. The analytical procedure fol-

lowed the standard methods of the laboratories.

The structural formula of smectites was calculated using the

method outlined by Číčel & Komadel (1994). The input data

from the chemical analyses were corrected for the presence of

mineral admixtures determined by QXRD analyses (RockJock).

The mean crystallite thicknesses and crystallite thickness

distributions of smectite particles (Na-saturated < 2 µm frac-

tions) were calculated by the Bertaut-Warren-Averbach

(BWA) analysis (Drits et al. 1998) using the MudMaster pro-

gram (Eberl et al. 1996), from XRD patterns of oriented

EG preparations. This method is based on the observation that

XRD reflections are broadened regularly as a function of

decreasing particle size. The thickness of the coherently scat-

tering domains was derived from the 001 basal reflection of

smectites.

Cation exchange capacity (CEC) was determined by

the copper(II) triethylenetetramine [Cu(Trien)]

method.

The 0.01 M solution of [Cu(Trien)]

was prepared according

to Meier & Kahr (1999). An oven-dried sample

(105 °C/ overnight), approximately 120 mg in weight,

was added to 50 ml of distilled water and 10 ml of

the [Cu(Trien)]

. The suspension was dispersed by

an ultra sonic treatment for 5 min and shaken for

an additional 1 h. The suspension was then centrifuged

at 4500 rpm for 20 min and the concentration of

ions in the collected supernatant was determined

by UV-VIS spectrophotometry (Cary 100, Varian) at

578 nm. The CEC was calculated according to the

equation reported by Pentrák et al. (2012). The Pearson

correlation between CEC and smectite contents in

bentonites (QXRD analysis) was performed using

the Origin software.

Fourier transform infrared (FTIR) spectra in the mid-

dle infrared region (4000–400 cm

−1

) were obtained

using a Nicolet 6700 spectrometer. The KBr pressed-

disc technique was used for transmission measure-

ments. Samples of 1 mg were dispersed in 200 mg of

KBr to record optimal spectra. Discs were heated over-

night at 150 °C to minimize water absorption on KBr

pellets. For each sample 128 scans were recorded with

a resolution of 4 cm

−1

. Spectral manipulations were

performed using the OMNIC software package.

The assignment of the bands in FTIR spectra followed

Farmer (1974).

Total specific surface area (TSSA) of the samples was deter-

mined using the ethylene glycol monoethyl ether (EGME)

method. Samples, ~250 mg in weight, were placed into glass

weighing bottles and dried to a constant mass in a desiccator

over P

under evacuation. Then several drops of EGME

were added to the samples before storing them under vacuum

in a desiccator over ignited CaCl

. The samples were weighed

every 90 min until constant mass was achieved and the TSSA

was then calculated according to Novák & Číčel (1972).

Thermal analyses (TG, DTG and DTA) were performed

using a Netzsch STA 449 F3 Jupiter analyser. Prior to thermal

analyses, the samples (Na-saturated < 2 µm fractions) were

stored at ambient laboratory conditions (22 °C and ~ 30 % RH).

The moisture was removed by holding the sample in a Pt–Ir

crucible at 105 °C for 60 min. For all measurements, 40 mg

of samples was utilized and a heating rate of 10 K/min and

a N

flow rate 50 ml/min were maintained.

Results and discussion

Mineral composition

The XRD results revealed that the bulk bentonites from all

studied deposits consisted of similar mineral constituents, in

particular smectite, feldspars, mica, opal-CT, kaolinite, quartz,

sometimes goethite. The main differences were observed,

however, in the quantity of these mineral constituents for

the samples from different deposits and even for the samples

from a single deposit. Fig. 2 shows XRD patterns of randomly

Fig. 2. XRD patterns of randomly oriented preparations of bulk samples

VSK-11 10.5 m (Stará Kremnička III) (a), C86 (Lutila I) (b), VSK-11 30.5 m

(Stará Kremnička III) (c), C89 (Bartošova Lehôtka) (d), and C78 (Stará

Kremnička – Jelšový potok) (e). S — smectite, M — mica, K — kaolinite,

F — feldspars, Q — quartz, Op — opal-CT, G — goethite, * — corundum

(internal standard).

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oriented preparations of five representative samples to demon-

strate the main differences in mineralogy among the studied

bentonites. The results of quantitative mineralogy (QXRD

analysis) of all bentonites are reported in Table 1.

Smectite was the predominant mineral in all samples.

The majo rity of the studied bulk samples contained ≥ 80 wt. %

of smectite, as documented by bentonites from the Stará

Kremnička III (VSK-11 1.5 m, 10.5 m, 16.5 m; C87 and C88),

Stará Kremnička – Jelšový potok (C76 and C77), Lutila I (C85)

and Lutila – Pod Klapou (C75) deposits (Table 1). Bulk ben-

tonites, containing between 70 and 80 wt. % of smectite, were

found in the Stará Kremnička III (VSK-11 20.5 m, 31.5 m,

40.5 m and 47 m), Lutila I (C74) and Lutila – Pod Klapou

(C90) deposits (Table 1). Bulk bentonites with low amounts of

smectite (≤ 61 wt. %) were found in the Stará Kremnička III

(VSK-11 30.5 m), Stará Kremnička – Jelšový potok (C78),

Lutila I (C86) and Bartošova Lehôtka (C89) deposits (Table 1).

The bulk bentonites also contain admixture of feldspars

(5–15 wt. %), mica (0–5 wt. %), opal-CT (0–6 wt. %), quartz

(< 1–6 wt. %) and kaolinite (0–2 wt. %). A small amount

(4 wt. %) of goethite was detected only in VSK-11 30.5 m

(Table 1). In a few samples, elevated amounts of mineral

impurities were detected; 11–35 wt. % of opal-CT (C86, C78,

C89 and VSK11 30.5 m), 20–27 wt. % of feldspars (C89 and

C78), 9–13 wt. % of mica (C90 and 89), 8 wt. % of kaolinite

(C86) and 8 wt. % of quartz (C78). Overall, the QXRD results

revealed a heterogeneous nature of bulk bentonites from

the studied deposits.

Heterogeneous nature of bentonite beds, in terms of mineral

composition and thickness, was also reported for bentonites

STx-1 (Ca-montmorillonite, Gonzales County, Texas, USA)

and SWy-2 (Na-montmorillonite, Crook County, Wyoming,

USA) which are among the deposits producing the world’s

best-grade bentonites (Moll 2001). Elzea & Murray (1990)

reported that the characteristics of the Wyoming bentonite can

vary significantly across the deposit due to differences in the

volcanic ash composition, different depositional environment

and weathering conditions.

The bulk fraction of STx-1, SWy-2 and SAz-1 (Ca-

montmorillonite ‘Cheto’, Apache County, Arizona, USA) ben-

tonites contained 67, 75 and 98 wt. % of smectite, respectively

(Chipera & Bish 2001). The above smectite contents were

similar to those of Slovak bentonites examined in the present

study (Table 1) or reported previously (e.g., Górniak et al.

2016). The clay size fraction (< 2 μm) of SWy-2, SAz-1, SCa-3

(montmorillonite ‘Otay’, Otay San Diego County, California,

USA) and SWa-1 (ferruginous smectite, Grant County,

Washington, USA) bentonites usually contained ≥ 93 wt. %

of smectite (Osacký et al. 2013; Geramian et al. 2016).

The results obtained in the present study (Table 1) along with

published data (e.g., Osacký et al. 2013; Górniak et al. 2016;

Pentrák et al. 2018) show that the same size fraction (< 2 μm)

of the Slovak bentonites contained usually ≤ 93 wt. % of smec-

tite, due to higher amounts of mineral admixtures (e.g., feld-

spars, quartz, opal-CT, kaolinite and mica).

The particle size distribution results (Table 2) for bentonites

from the Stará Kremnička III deposit (VSK-11 samples) show

that the < 2 μm and 2–160 μm size fractions account for

15–34 wt. % and 42–69 wt. % of the total weight of the sam-

ples, respectively. Although the < 2 μm fractions of bentonites

had elevated smectite contents compared with the bulk sam-

ples (Table 1), the amount of this fraction was low. Substantially

higher amounts of the < 2 μm fractions were expected for

the studied bentonites. The low amounts of the < 2 μm

Sample

Quartz

Feldspars

Opal-CT

Kaolinite

Mica

Smectite

Goethite

Deposit

Bulk

VSK-11 1.5 m

−

Stará Kremnička III

VSK-11 10.5 m

−

Stará Kremnička III

VSK-11 16.5 m

−

Stará Kremnička III

VSK-11 20.5 m

−

Stará Kremnička III

VSK-11 30.5 m

Stará Kremnička III

VSK-11 31.5 m

−

Stará Kremnička III

VSK-11 40.5 m

−

Stará Kremnička III

VSK-11 47 m

−

Stará Kremnička III

C87

−

Stará Kremnička III

C88

−

Stará Kremnička III

C76

−

Stará Kremnička – Jelšový potok

C77

−

Stará Kremnička – Jelšový potok

C78

−

Stará Kremnička – Jelšový potok

C74

−

Lutila I

C85

−

Lutila I

C86

−

Lutila I

C75

−

Lutila – Pod Klapou

C90

−

Lutila – Pod Klapou

C89

−

Bartošova Lehôtka

< 2 μm

VSK-11 10.5 m

−

Stará Kremnička III

VSK-11 31.5 m

−

Stará Kremnička III

Table 1: Mineral composition (in wt. %) of the bulk and < 2 μm fraction of bentonites determined by RockJock.

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fractions indicate that some portion of smectite in bentonites

from the Stará Kremnička III deposit was present in the form

of aggregates (mainly between 2 and 160 μm in size) which

resisted the applied size separation process (drying, homoge-

nization, sonication and stirring). A similar behaviour was

observed for Slovak bentonites from the Hliník nad Hronom

(Uhlík et al. 2012; Górniak et al. 2017) and Kopernica depo-

sits (Górniak et al. 2016) and STx-1 bentonite (Chipera &

Bish 2001). The smectite aggregates in bentonites were often

coated and/or intergrown with silica minerals (e.g., opal-CT,

K-feldspars and zeolites) (Chipera & Bish 2001; Górniak et al.

2016, 2017). Furthermore, opal-CT and feldspars were com-

mon mineral admixtures identified in the studied bentonites

from different Slovak deposits (e.g. Stará Kremnička III,

Stará Kremnička – Jelšový potok, Lutila I, Bartošova Lehôtka),

sometimes present in significant amounts (e.g., 35 wt. % of

opal-CT for VSK-11 30.5 m, 27 wt. % of feldspars C78,

Table 1). Opal-CT, in amounts up to 45 wt. %, was reported in

bentonites from other Slovak deposits (Hliník nad Hronom

and Kopernica) (Uhlík et al. 2012; Górniak et al. 2016, 2017).

The presence of these minerals even in the < 2 μm fractions of

bentonites (up to 10 wt. %, Table 1) indicates their association

mainly with smectite.

The laboratory experiments simulating alteration of volca-

nic glass in open systems showed that the precipitation rate of

free SiO

decreased with the increasing rate of fluids flowing

through the volcanic glass (Daux et al. 1997). Analogically,

bentonites coexisting with SiO

polymorphs (e.g., opal-CT)

were preferentially formed from acidic pyroclastic rocks

deposited at low temperatures, which could not sustain high

rates of fluid flow migrating through pyroclastic rocks; as

a consequence, fluids have reached saturation by Si leached

from pyroclastic rocks and amorphous SiO

has precipitated

(Christidis & Huff 2009).

The formation of Slovak bentonite deposits of the Jastrabá

Fm. (Kremnické vrchy Mts.) is still not fully understood

mainly due to the complex geological–tectonic structure of

the studied area and several mutually overlapping alteration

processes. Kraus et al. (1994) proposed the model for the alte-

ration of the rocks from the south-western part of the Kremnické

vrchy Mts. (Jastrabá Fm.) based on the existence of three

mutually associated processes of different origin: the diage-

netic alteration of rhyolite tuffs, hydrothermal alteration of

rhyolites and hydrothermal alteration of andesites and rhyolite

tuffs in the vicinity of the Kremnica ore veins. According to

previous studies (Šamajová et al. 1992; Kraus et al. 1994)

the largest economic accumulations of bentonites exploited in

the Jastrabá Fm. (Stará Kremnička – Jelšový potok deposit)

were associated mainly with the diagenetic alteration of rhyo-

lite tuffs. The concept about diagenetic alteration of rhyolite

tuffs is based on the vertical mineral zonation (perlite, smec-

tite and zeolite zone) distinguished in the vicinity of the ben-

tonite deposit Jelšový potok (Šamajová et al. 1992, Kraus et

al. 1994). The same authors reported that the redeposited and

partially also autochthonous rhyolite tuffs were altered into

bentonites in a shallow water lacustrine environment (lakes

and swamps) in an open or semi-closed hydrological system.

Diagenetic mineral zones were formed as a result of gradual

changes in chemical composition and pH of pore solutions

percolating through vitric tuffs (Kraus et al. 1994). Due to the

gradual hydration and hydrolysis of the vitric component and

simultaneous formation of smectites in the upper zone, pore

solutions became enriched in alkalies and silicon eventually

attaining the mineralization and pH sufficient for clinoptilolite

crystallization in the lower zone (Šamajová et al. 1992).

New data on fluid properties of the Kremnica hydrothermal

system enabled reconstruction of the history of the spatial and

temporal fluid evolution of individual parts of the ore vein

system and shed new light on the formation of argillic altera-

tion products from the south-western part of the Kremnické

vrchy Mts. (Jastrabá Fm.) (Demko et al. 2010; Koděra et

al. 2014). Stable isotope data of clay minerals (illites and

illite–smectites) indicated isotopically homogenous sources of

fluids (deeply-circulating, mostly meteoric waters, driven by

heat from the contemporaneous rhyolite magma chamber)

which are associated with the formation of illites and illite–

smectites in the south-western part of the Kremnické vrchy

Mts. (Jastrabá Fm.) (Demko et al. 2010). The isotope geother-

mometry results indicated that the lateral mineral zonation in

the Jastrabá Fm., namely illite-smectite accumulations in

the northern part (e.g., Dolná Ves deposit) and smectite accu-

mulations in the southern part of the formation (e.g., Stará

Kremnička – Jelšový potok deposit), may be related to the

gradual decrease in temperature of the hydrothermal fluids

percolating through the rhyolite volcanoclastics from north to

south (Demko et al. 2010). This finding was in line with pre-

viously published data showing an increase in the expandabi-

lity of illite–smectites from north to south in the Dolná Ves

hydrothermal deposit (Šucha et al. 1992). Demko et al. (2010)

and Koděra et al. (2014) assume that the subsurface hydrother-

mal fluids migrating from north to south in permeable rhyolite

volcanoclastic rocks of the Jastrabá Fm. played a key role in

the alteration of the vitric component of rhyolite tuffs into

(smectites) bentonites.

Chemical composition of bentonites and smectite crystal-

chemistry

The position of the 060 reflection of smectite (d = 1.49 Å,

Fig. 2) indicates the presence of dioctahedral smectite in all

Sample

Size fraction (μm)

> 160

2–160

< 2

VSK-11 1.5 m

VSK-11 10.5 m

VSK-11 16.5 m

VSK-11 20.5 m

VSK-11 30.5 m

VSK-11 31.5 m

VSK-11 40.5 m

VSK-11 47 m

Table 2: Particle size distribution (in wt. %) of bentonites.

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studied samples. The 001 reflection of all studied smectites

observed near 15.3 Å (Fig. 3), indicates the predominance of

divalent exchangeable cations (most likely Ca

and Mg

) in

the interlayer space of smectites. The comparison of air-dried

and EG oriented XRD patterns showed that smectite was the

only swelling clay mineral identified in the studied samples

(Fig. 3). The observations above were in good agreement with

the structural formulas of smectites, calculated from the

< 2 μm fraction of VSK-11 samples from the Stará Kremnička

III deposit:
VSK-11 10.5 m

0.28

0.15

(Si

7.78

0.22

)(Al

3.14

0.22

0.64

(OH)

;

VSK-11 31.5 m

0.28

0.13

0.01

(Si

7.70

0.30

)(Al

3.24

0.23

0.53

(OH)

The formulas indicate that the layer charge of smectites

arises mainly from Mg for Al substitutions in the octahedral

sheet. Such dioctahedral Al-Mg rich smectites, with the layer

charge arising mainly from non-equivalent isomorphous sub-

stitution of cations in the octahedral sheet, can be classified as

montmorillonites (Brindley 1980). According to some authors,

the predominance of Ca

and Mg cations in the

interlayer space of montmorillonites may be

related to the shallow lacustrine environment in

which the vitric tuffs of the Jastrabá Fm. were

altered into bentonites (Kraus et al. 1982, 1994).

The comparison of the above smectite formu-

las with those reported for smectites from other

Jastrabá Fm. deposits, such as Stará Krem-

nička – Jelšový potok (Číčel et al. 1974, 1992;

Osacký et al. 2013) and Kopernica (Górniak

et al. 2016; Pentrák et al. 2018) shows no signi-

ficant differences. These findings indicate that:

(i) the crystal-chemistry of smectites from the

Jastrabá Fm. bentonite deposits is quite consis-

tent and did not change significantly over years

with bentonite exploitation, (ii) the similar crys-

tal-chemistry of smectites from the Stará Krem-

nička III, Stará Kremnička – Jelšový potok and

Kopernica deposits suggests a similar composi-

tion of the parent rocks (acidic rhyolitic tuffs)

and/or similar alteration conditions of bentonites

from these Jastrabá Fm. deposits.

Previous studies showed that the composition

of the parent rocks was an important parameter

affecting the composition of smectites (e.g.,

Christidis 1998, 2006). For instance, the crys-

tal-chemistry of smectites derived from acidic

precursors (e.g., rhyolite) was controlled mainly

by Mg for Al substitution, whereas Fe for Al

substitution seemed to be more important for

smectites derived from intermediate rocks (e.g.,

andesite) (Christidis & Dunham 1997). A typical

example of a Slovak bentonite deposit formed by

alteration of intermediate andesitic tuffs is the

Lieskovec deposit in the Abčiná Fm., located in

the south-eastern part of the Zvolen Basin (Šucha & Kraus

1999). Bulk bentonite from the Lieskovec deposit contained

from 51 to 65 wt. % of dioctahedral Al–Fe rich smectite

(montmorillonite) (Šucha & Kraus 1999; Andrejkovičová et

al. 2006; Osacký et al. 2012).

The chemical composition of the studied bentonites is

reported in Table 3. High SiO

and Al

contents in all sam-

ples correspond to high amounts of aluminosilicates namely

clay minerals (mainly smectite) and feldspars, and SiO

bearing mineral phases namely opal-CT and quartz. The ele-

vated SiO

content of VSK-11 30.5 m was due to an opal-CT

admixture. The increased Fe

contents may be related to the

presence of biotite which is a common admixture in bentonites

from the Jastrabá Fm. (Górniak et al. 2017) and/or montmoril-

lonite (octahedral Fe). However, the elevated Fe

content in

VSK-11 30.5 m results from the presence of goethite. Increased

O and K

O contents in VSK-11 31.5 m and 47 m corre-

spond to the higher amount of feldspars. Mica was another

possible source of K

O in the studied samples. The main

source of MgO and CaO was likely smectite. The presence of

Ti was likely due to the presence of accessory TiO

minerals

Fig. 3. XRD patterns of oriented air-dried (solid line) and ethylene glycolated

(dash line) preparations (Stará Kremnička III) of samples VSK-11 1.5 m (a),

10.5 m (b), 16.5 m (c), 20.5 m (d), 30.5 m (e), 31.5 m (f), 40.5 m (g), and 47 m (h).

S — smectite, K — kaolinite, Op — opal-CT.

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(e.g., rutile and anatase) coming from parent volcanic rocks.

The replacement of altered biotite crystals by Fe–Ti minerals

in bentonites from the Jastrabá Fm was previously reported by

Górniak et al. (2017). The LOI content is mainly related to

the dehydroxylation of clay minerals. Samples with higher

amounts of clay minerals (mainly smectite) show a higher

LOI than those with lower amount of clay minerals. Overall,

the results of chemical analyses are consistent with

the QXRD data.

Fourier transform infrared (FTIR) spectroscopy

FTIR spectra for all < 2 μm fractions of VSK-11 bentonite

samples (Fig. 4) display mainly bands related to the vibrations

of smectite. The absorption bands near 3628 and 3421 cm

–1

correspond to the OH stretching vibrations of the structural

OH groups and water molecules, respectively. The band at

3698 cm

–1

, observed in the spectra of the sample VSK-11

30.5 m, is attributed to the OH stretching vibrations of kaoli-

nite outer surface OH groups. The OH bending region of all

VSK-11 samples with the absorptions at 915 cm

–1

(AlAlOH),

845 cm

–1

(AlMgOH) and an inflexion near 882 cm

–1

(AlFeOH)

indicate that the octahedrally coordinated Al in the structure of

smectites was partially substituted by Mg, and to a lesser

extent by Fe. These findings are in good agreement with the

structural formulas of VSK-11 10.5 m and 31.5 m smectites

calculated from chemical analyses corrected for mineral

impurities. A strong complex band near 1042 cm

–1

was due to

Si-O stretching vibrations, whereas the bands at 524 and

468 cm

–1

are assigned to Al-O-Si and Si-O-Si bending vibra-

tions, respectively. The bands at 1107 and 794 cm

–1

in the

sample VSK-11 30.5 m indicate the presence of a silica

admixture. This finding is in good agreement with QXRD

analyses which confirmed 35 wt. % of opal-CT in this sample

(Table 1).

Cation exchange capacity (CEC)

CEC values determined by the [Cu(Trien)]

method for

oven-dried (105 °C/overnight) bulk bentonites and corre-

sponding < 2 μm fractions are shown in Table 4. CECs for

studied samples ranged from 57 to 106 meq/100g. A signifi-

cant positive correlation (p < 0.0001 and R

= 0.919, Fig. 5)

established between CEC values and smectite

contents indicates that the CECs increases with

increasing amount of smectite. The best-grade

bulk bentonites (VSK-11 10.5 m and C87) con-

tain 89 – 90 wt. % of Al-Mg montmorillonite and

have a CEC of 99 – 100 meq/100g (Tables 1

and 4). The best-grade < 2 μm fraction, obtained

from the bulk sample VSK-11 10.5 m by conven-

tional gravitational settling in water, contains

slightly higher smectite content and CEC values

(93 wt. % of Al–Mg montmorillonite and

CEC = 105 meq/100g, Tables 1 and 4). However,

it should be noted that, in general, the amounts of

< 2 μm fractions of bentonites from the Jastrabá Fm. are low

(Table 2) due to the presence of smectite aggregates coated/

intergrown with silica minerals (e.g., feldspars and opal-CT)

(e.g., Górniak et al. 2016, 2017).

For comparison, the < 2 μm fraction of the SWy-2 bento

nite with a similar smectite content (94 wt. % of smectite,

Geramian et al. 2016) had lower CEC (93 meq/100 g, Pentrák

et al. 2012) than the < 2 μm fraction of VSK-11 10.5 m. This is

due to the lower total layer charge of the SWy-2 montmoril-

lonite, i.e. −0.70 for SWy-2 (Geramian et al. 2016) vs. −0.86

for VSK-11 10.5 m per O

(OH)

. On the other hand, the

< 2 μm fraction the of SAz-1 bentonite, containing 97 wt. % of

smectite (Osacký et al. 2013) had higher CEC (121 meq/100 g,

Pentrák et al. 2012) than the < 2 μm fraction of VSK-11

10.5 m, due to slightly higher smectite content, and more

importantly, substantially higher total layer charge of SAz-1

montmorillonite, namely –1.13 for SAz-1 (Osacký et al. 2013)

vs. −0.86 for VSK-11 10.5 m per O

(OH)

In addition, we have calculated theoretical CECs from

structural formulas (CEC

) of smectites VSK-11 10.5 m

and 31.5 m (based on the exchangeable cations content).

The CEC

of pure, dehydrated smectites VSK-11 10.5 m and

31.5 m were 124 and 121 meq/100 g, respectively. The CEC

values were then compared with the CECs determined by

the [Cu(Trien)]

method for the < 2 μm fractions of VSK-11

Fig. 4. FTIR spectra of < 2 μm fractions (Stará Kremnička III) of sam-

ples VSK-11 1.5 m (a), 10.5 m (b), 16.5 m (c), 20.5 m (d), 30.5 m (e),

31.5 m (f), 40.5 m (g), and 47 m (h).

Sample

SiO

MgO

CaO

TiO

LOI

Bulk

VSK-11 10.5 m

53.88

18.20

2.11

0.12

0.89

3.24

1.72

0.15

19.4

VSK-11 30.5 m

61.86

14.51

5.37

0.20

0.77

1.79

1.12

0.09

14.1

VSK-11 31.5 m

56.48

18.23

2.10

0.31

1.42

2.49

1.56

0.15

17.1

VSK-11 47 m

55.59

18.80

1.99

0.32

1.47

2.68

1.57

0.13

17.9

< 2 μm

VSK-11 10.5 m

52.41

18.88

1.86

0.03

0.29

3.42

1.78

0.11

21.0

VSK-11 31.5 m

52.41

20.80

1.95

0.07

0.38

2.86

1.71

0.12

20.2

LOI — 1000 °C

Table 3: Chemical composition (in wt. %) of the bulk and < 2 μm fraction of bento-

nites determined by ICP.

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10.5 m and 31.5 m, recalculated (based on the QXRD data,

Table 1) to CEC of pure smectites (113 meq/100 g for pure

VSK-11 10.5 m and 109 meq/100 g for pure VSK-11 31.5 m).

The comparison shows that the CEC

values are slightly

higher than the CECs measured by [Cu(Trien)]

(the diffe-

rence being < 10 %). This small discrepancy is likely mainly

because CEC

was calculated from an absolutely dry (ignited

at 1000 °C) basis, while CEC determined by [Cu(Trien)]

were measured after the sample was dried at 105 °C/overnight,

cooled in a desiccator, and then weighed. As a consequence,

the H

O left in the sample (mainly in smectite) at 105 °C

reduced the CEC values compared to CECs calculated from

the H

O-free basis (Środoń & McCarty 2008).

Total specific surface area (TSSA)

TSSA values for selected bulk bentonites and corresponding

< 2 μm fractions are reported in Table 4. TSSA increases with

increasing amount of smectite in the studied bentonites. TSSA

values gradually increase from 462 to 565 and 616 m

/g for

bulk bentonites with the smectite content of 52 wt. % (VSK-11

30.5 m), 78 wt. % (VSK-11 31.5 m) and 89 wt. % (VSK-11

10.5 m), respectively. Similar TSSA values (624–632 m

/g)

were reported for bulk Wyoming bentonites containing

~ 89–91 wt. % of smectite (Kiviranta & Kumpulainen 2011).

Thermal analysis

The results of thermal analyses (TG, DTG and DTA) for the

selected bentonites (Na-saturated < 2 μm fractions) are shown

in Figure 6. For all samples, the mass loss from room tempera-

ture to 1000 °C can be divided into three steps in the < 300 °C,

300–550 °C and 550–700 °C regions. At temperatures < 300 °C,

the mass loss accounted for 8.9–11.8 % on the TG curves

of the studied samples (Fig. 6) and it is mostly related to

the desorption of surface H

O (e.g., H

O on exterior surfaces)

and dehydration (e.g., interlayer H

O) of clay minerals

(mainly smec tite). At elevated temperatures, 300–550 °C and

550–700 °C, the mass loss was mainly due to the dehydroxy-

lation (release of structural OH groups) of clay minerals, in

particular smectite. The dehydroxylation of smectite was indi-

cated by one broad peak with maximum at ~ 640–660 °C on

DTG curves (Fig. 6) suggesting the presence of cis-vacant

dioctahedral smectites (Drits et al. 1995; Wolters & Emmerich

2007) in the studied bentonites. Generally, the cis-vacant

smectites are primary products of weathered or hydrother-

mally altered volcanoclastic rocks of rhyolitic composition.

Fig. 5. Cation exchange capacity (CEC) plotted versus smectite

content of the studied bentonites.

Sample

CEC (meq/100g)

TSSA (m

/g)

MEAN

(nm)

Deposit

Bulk

< 2 μm

Bulk

< 2 μm

VSK-11 1.5 m

96 ± 5

102 ± 2

−

9.8

Stará Kremnička III

VSK-11 10.5 m

101 ± 1

105 ± 1

616

684

11.1

Stará Kremnička III

VSK-11 16.5 m

91 ± 1

99 ± 2

−

10.7

Stará Kremnička III

VSK-11 20.5 m

89 ± 4

102 ± 3

−

11.5

Stará Kremnička III

VSK-11 30.5 m

61 ± 1

79 ± 3

462

574

9.3

Stará Kremnička III

VSK-11 31.5 m

84 ± 2

96 ± 2

565

659

11.2

Stará Kremnička III

VSK-11 40.5 m

90 ± 2

99 ± 1

−

9.5

Stará Kremnička III

VSK-11 47 m

85 ± 1

102 ± 1

−

11.2

Stará Kremnička III

C87

99 ± 3

100 ± 3

−

9.1

Stará Kremnička III

C88

93 ± 2

98 ± 5

−

8.6

Stará Kremnička III

C76

94 ± 1

105 ± 2

−

8.7

Stará Kremnička – Jelšový potok

C77

87 ± 3

97 ± 3

−

7.4

Stará Kremnička – Jelšový potok

C78

57 ± 2

96 ± 8

−

7.7

Stará Kremnička – Jelšový potok

C74

85 ± 2

105 ± 4

−

9.9

Lutila I

C85

95 ± 2

106 ± 3

−

8.4

Lutila I

C86

72 ± 3

84 ± 2

−

7.2

Lutila I

C75

94 ± 2

101 ± 2

−

8.7

Lutila – Pod Klapou

C90

92 ± 2

102 ± 4

−

9.2

Lutila – Pod Klapou

C89

64 ± 2

85 ± 1

−

9.0

Bartošova Lehôtka

Table 4: Cation exchange capacity (CEC), total specific surface area (TSSA) and mean crystallite thickness (T

MEAN

) of the bulk and < 2 μm

fraction of bentonites.

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This conjecture is in line with the geological origin of the stu-

died bentonites from the Jastrabá Fm. (e.g., Kraus et al. 1982,

1994; Koděra et al. 2014). The DTA curves of studied samples

showed two endothermic events at 465–473 °C (mass loss =

1.2–1.9 %) and 630–655 °C (mass loss = 2.6–3.1 %) associated

with the dehydroxylation of smectites (Fig. 6). Some contribu-

tion from kaolinite and mica was also possible because these

are often present in the < 2 µm fractions of studied bentonites

in low amounts (~ 1 wt. %, Table 1).

The 739–762 °C endothermic events in the DTA of all sam-

ples (Fig. 6) were probably artefacts, related to the pretreat-

ment of the samples (Meyer 1972; Wolters & Emmerich 2007)

and/or could represent carbonates (dolomite and/or calcite)

decarbonation (Rowland 1955; Guggenheim & Van Groos

2001). The presence of carbonates was not confirmed by XRD

and FTIR in the studied samples. In addition, no carbonates

were previously reported for bentonites from the Jastrabá Fm.

deposits. Thus, the endothermic events at 739–762 °C are

most likely artefacts due to the pretreatment of the samples

(size fractions separation, Na-saturation). Wolters & Emmerich

(2007) reported that sodium might act as a flux causing partial

sintering and retardation of released OH groups.

A weak endothermic event at 840–870 °C in the DTA curves

for VSK-11 10.5 m and C85 (Fig. 6) is associated with the

breakdown of anhydrous smectite (montmorillonite) structure

to an amorphous material (Bradley & Grim 1951).

Bertaut–Warren–Averbach (BWA) analysis

The mean crystallite thicknesses (T

MEAN

) and the crystallite

thickness distributions (CTDs) for smectites (Na-saturated

< 2 μm fractions) are shown in Table 4 and Figure 7. The T

MEAN

values calculated for smectites from studied bentonites range

from 7.2 to 11.5 nm. The highest T

MEAN

values (11.1–11.5 nm)

were calculated for smectites from the Stará Kremnička III

deposit. Besides that, no apparent relationships were found

between the T

MEAN

of smectites and the studied bentonite

deposits. Figure 7 shows CTDs for seven representative sam-

ples with distinct T

MEAN

values to demonstrate changes in the

shape of CTDs due to different T

MEAN

. The shape of CTDs for

smectites is lognormal for all studied samples. The maxima of

the CTDs gradually decreases in frequency, broadens and

shifts to higher thicknesses with the gradual increase of T

MEAN

values of smectites (Fig. 7).

The crystallite thickness of smectites depends on several

factors. The structure of smectites is labile along the c direc-

tion due to the ability of smectites to accommodate water

Fig. 6. TG, DTG and DTA curves of Na-saturated < 2 μm fractions of

samples C85 (Lutila I), C77 (Stará Kremnička – Jelšový potok) and

VSK-11 10.5 m (Stará Kremnička III).

Fig. 7. Crystallite thickness distributions (CTDs) of smectites

(Na-saturated < 2 μm fractions) from the Lutila I (C86, C85), Stará

Kremnička – Jelšový potok (C77, C78) and Stará Kremnička III

deposits (VSK-11 10.5 m, 20.5 m and 40.5 m) and corresponding

mean crystallite thicknesses (T

MEAN

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molecules in the interlayer space. The subsequent change in

the basal spacing due to the swelling produces changes in the

crystallite thickness of smectites (e.g., Moore & Hower 1986;

Mystkowski et al. 2000). At high humidity, smectite adsorbs

water which splits some layers or sets of layers and this in

turn, lowers the mean thickness which is a statistical descrip-

tion of the associations of layers that exist in a sample

(Mystkowski et al. 2000). The effect of different pretreatment

of smectitic samples on the mean crystallite thickness was pre-

viously studied by Mystkowski et al. (2000). The authors

showed that different size fractions (< 2 μm vs. < 0.2 μm) and

exchangeable cations (Na- vs. Ca-saturated forms) may also

produce changes in the mean crystallite thickness of smectites

calculated by BWA analyses.

The smectite crystallite thickness does not reflect a growth

mechanism of smectite crystals (e.g., Mystkowski et al. 2000;

Christidis 2001) as is the case of crystals of other clay mine-

rals (e.g., illites, kaolinites), according to Eberl et al. (1998).

The use of lateral a–b dimensions (length and width) instead

of c dimension (thickness) of smectite particles was proposed

by Christidis (2001) to study the growth mechanisms of smec-

tites in bentonites. Mystkowski et al. (2000) showed a rela-

tionship between the a–b and c dimensions of the smectite

crystallites; bigger crystallites (i.e. larger a–b dimensions)

were statistically more likely to associate into thicker sets (i.e.

larger c dimensions). In addition, the same authors believe that

high thicknesses and large lateral dimensions of beidellite

crystallites are related to the hydrothermal conditions in which

the samples have crystallized. Šucha et al. (1996) observed

that the redeposition of bentonites at a short distance reduced

the crystallite thickness of smectites in comparison with the

original bentonites formed by in situ alteration of andesitic

volcanoclastics from the Zvolen Basin (Western Carpathians).

Simić and Uhlík (2006) reported that the distinct smectite

crystallite thicknesses determined for Serbian bentonites can

be related to different geological environments in which the

bentonites originated.

It would be interesting to discover the reason for the diver-

sity of smectite crystallite thickness for the bentonites exa-

mined in the present study. All samples were pretreated in

the same way (< 2 μm fractions, Na-saturation) thus there must

be some other factor than the samples pretreatment, which

affects the smectite crystallite thickness of the studied

samples.

Conclusions

The bulk bentonites from the Jastrabá Fm. consist of widely

variable amounts of smectite, feldspars, mica, opal-CT,

kaolinite, quartz, and sometimes goethite. Smectite was the

predominant mineral in all samples and its amount varied

between 43 and 90 wt. %. The clay size fractions (< 2 μm)

isolated from the bulk bentonites contain up to 93 wt. % of

smectite. However, the amounts of the < 2 μm fractions in the

studied bentonites are rather low, compared to other studied

fractions, due to the presence of smectite aggregates asso-

ciated with silica minerals (mainly feldspars and opal-CT).

The calculated structural formulas revealed that smectites

comprising the studied bentonites can be classified as mont-

morillonites. The layer charge of montmorillonites arises

mainly from the Mg for Al substitutions in the octahedral

sheet. The interlayer space of montmorillonites is occupied

predominantly by divalent exchangeable cations (Ca

and

). The results of thermal analyses indicate the presence

of cis-vacant variety of montmorillonites in the studied ben-

tonites. The mean crystallite thickness (T

MEAN

) of smectites

calculated by BWA analyses ranges from 7.2 to 11.5 nm.

The shape of crystallite thickness distributions (CTDs) for

smectites is lognormal in all samples. Both cation exchange

capacity (CEC) and total specific surface area (TSSA) increase

with the increasing amount of smectite in the studied bento-

nites. The CEC of 101 meq/100g and TSSA of 616 m

/g corre-

spond to bulk bentonite from the Stará Kremnička III deposit

containing 89 wt. % of smectite.

The general consensus is that bentonites from the Jastrabá

Fm. resulted from alteration of volcanoclastics of rhyolitic

composition. The transformation mechanism, however, is con-

troversial. The largest economic accumulation of bentonites in

the Jastrabá Fm. (Stará Kremnička – Jelšový potok deposit)

may be associated with the diagenetic and/or hydrothermal

alteration of rhyolite tuffs whereas smaller occurrences of

bentonites may be related to hydrothermal rather than diage-

netic alteration. Further studies on the genesis of bentonite

deposits are required. More comprehensive knowledge on

the formation of bentonite deposits may contribute to

the discovery of other prospective bentonite deposits on

the south-western margin of the Kremnické vrchy Mts.

Acknowledgements: This study was supported by the Slovak

Grant Agency VEGA (1/0196/19 and 2/0156/17) and Slovak

Research and Development Agency (APVV-0339-12 and

APVV-17-0317). The authors are grateful to Zora Lukáčová

for her help with TSSA measurements. Our special thanks go

to K. Górniak and G.E. Christidis for their constructive com-

ments which have improved the quality of the manuscript.

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