Characterization of two forms of sepiolite and related Mg-rich ...

Characterization of two forms of sepioliteand related Mg-rich clay minerals from

Yenidogan (Sivrihisar, Turkey)

M. YENIYOL*

University of Istanbul, Department of Geology, 34850 Avcilar, Istanbul, Turkey

(Received 12 March 2013; revised 13 February 2014; Editor: George Christidis)

ABSTRACT: An Early Pliocene sedimentary succession in the Yenidogan area, Sivrihisar, Turkey,

consists of sepiolite, stevensite, kerolite, dolomite and magnesite. The geology, mineralogy and

geochemistry of the succession was examined by extensive field work along several trenches and a

representative measured section, followed by X-ray diffraction (XRD), Fourier transform infrared

spectroscopy (FTIR), scanning electron microscopy (SEM), thermal, and chemical analyses.

Structurally, two distinct forms of sepiolites were distinguished by XRD: (a) well crystallized

sepiolite with a 110 reflection at 12.07�12.3 A, and (b) poorly crystallized sepiolite in which the 110

reflection occurs at 12.7�13.0 A (denoted as sepiolite-13A). Differences in crystal chemistry,

thermal and morphological properties of these forms, the vibrational spectra and XRD

characterization of the related phyllosilicates were also documented.

Stevensite, kerolite and sepiolite were formed by direct precipitation from alkaline lake water rich

in Mg and Si. Sepiolite-13A was probably formed by transformation from precursor smectite via

dissolution-precipitation, more likely during early diagenesis. Environmental conditions such as ion

concentration, salinity and variations in pH may have controlled the formation of the phyllosilicates.

KEYWORDS: sepiolite, stevensite, kerolite, magnesite, dolomite, Yenidogan, Turkey.

Nodular sepiolite (Meerschaum) is a well known

clay that has been exploited in Eskis� ehir province

(Turkey) for centuries. Since 1967, several layered

sepiolite occurrences have been discovered and

documented in Neogene lacustrine sediments near

Eskis� ehir (Akıncı, 1967; Bilgin, 1972; Oncel &

Denizci, 1982; Yeniyol, 1992, 1993b, 2012;

Gencoglu et al., 1993; Ece & Coban, 1994),

Konya (Karakaya et al., 2004), Denizli (Akbulut

& Kadir, 2003) and Ankara (Karakaya et al., 2011).

Of these, four layered sepiolite occurrences are

currently mined around Eskis� ehir, and low grade

sepiolite clays are exploited for the production of

cat litter.

The sepiolite occurrence found SW of Eskis� ehir(Fig. 1) is the largest deposit of commercial value

discovered in Turkey. So far the geology, miner-

alogy and genesis of this deposit have been

described in a general sense (Yeniyol, 1992).

Recently, detailed studies which were made by

the present author led to the distinction of a poorly

crystallized sepiolite for the first time, besides the

widely known sepiolite. In addition to these two

forms of sepiolite, stevensite and kerolite are Mg-

rich phyllosilicate minerals found in association in a

sequence with dolomite. The objective of this study

is to describe the geology, mineralogy, geochem-

istry of the distinct sepiolite forms and related clay

minerals present in this deposit.* E-mail: [email protected]: 10.1180/claymin.2014.049.1.08

ClayMinerals, (2014) 49, 91–108

# 2014 The Mineralogical Society

GEOLOGICAL SETT ING

The basement lithology of the area is represented

by rock units that range from Palaeozoic to

Cretaceous and Upper Cretaceous. Palaeozoic to

Cretaceous marbles, recrystallized limestones and

schists crop out to the north and notheast of the

area together with Upper Cretaceous granitoids

(MTA, 2002). In the southwest of the study area,

recrystallized limestones (occasionally dolomitic)

are present (Yeniyol, 1982). They contain schist

layers of Cretaceous age, and ophiolitic rocks

including serpentinites and gabbroic blocks that

were emplaced as large overthrust slabs over the

older lithology during the Upper Cretaceous.

The region has been affected by extensional

tectonic movements between the Lutetian and

Neogene (Erentoz, 1975), producing northwest-

southeast trending regional normal faults (Yeniyol,

1982) and depressions that were bordered by these

tectonic lines. Miocene-Pliocene lacustrine sedi-

ments, extending over a large area at the vicinity of

Eskis� ehir, have been deposited in these depressions

and overlie older rocks unconformably. These

sediments are represented mainly by conglomerates

at the base and margins of the basin. The upper part

of the Miocene consists of claystone, marl,

dolomite, tuff and gypsum beds (Yeniyol, 1992).

During the Late Miocene�Early Pliocene,

volcanism provided materials of variable composi-

tion including dacites, trachytes, trachyandesites,

rhyodacites and tuffs (Kulaksız, 1981).

The upper part of Miocene-Pliocene sequence is

represented by Early Pliocene lacustrine sediments

where chemical precipitation has predominated

(Yeniyol, 1992). Due to the presence of two distinct

sepiolite forms and related Mg-rich minerals, it was

considered important to study these sediments in

depth.

FIG. 1. Location and geological map of the study area (adapted from 1/500,000 scale geological map of Turkey;

MTA, 2002).

92 M. Yeniyol

In addition to the field observations and studies,

160 samples were collected from the open sepiolite

quarry faces, from trenches and from a representa-

tive measured stratigraphic section for analytical

purposes.

The analyses were performed on raw and on

purified samples. Purification involved removal of

carbonates (fine-grained dolomite) using sodium

acetate-acetic acid solution buffered at pH 5

(Jackson, 1969). After washing several times, the

samples were dispersed by mechanical stirring and

ultrasonic treatment alternately, after which <2 mm

fractions were separated by centrifugation. Oriented

samples were prepared by smearing the clay residue

on the glass slides. The remaining material was

dried at room temperature for X-ray diffraction

(XRD), Fourier transform infrared (FTIR) spectro-

scopy, thermal (DTA-TGA) analysis, and chemical

analyses.

The XRD analyses were carried out on 80

samples (14 of dolomite, 40 of sepiolite and 26 of

smectite), using random powders of both raw and

purified samples, and on oriented samples: air-

dried, ethylene glycol-solvated and heated at 550ºC.

A Philips PW 3020 diffractometer with Cu-Karadiation and a graphite monochromator, operating

at 40 kV and 20 mA, in the range 3�65º2y with a

step size of 0.04º2y min�1.

Thermal analyses were performed on two pure

sepiolite samples by TA SDT Q600 thermal

analyser. Differential thermal (DTA) and thermo-

gravimetric (TG) analyses were simultaneously

carried out over the 25�1000ºC range using

20 mg of sample under 100 mL/min air flow and

10ºC/min heating rate, utilizing Al2O3 as a

reference material.

Six naturally pure (confirmed by XRD) samples

of sepiolite ignited at 1000ºC were analysed by a

combination of atomic absorption spectrometry

(AAS) and wet chemistry (volumetric and gravi-

metric) techniques. Forty five samples were

analysed in order to calculate the dolomite content

of the carbonate-bearing layers. Dolomite was

extracted from representative samples using 1 M

hot (80ºC) HCl. Then, solutions were analysed

using a Rank Hilger 1550 atomic absorption

spectrometer. The samples analysed do not

contain Ca-bearing minerals such as calcite or

gypsum (confirmed by XRD). Therefore dolomite

percentages were calculated theoretically from the

chemical analyses by assigning all CaO to dolomite.

Since the mineralogical composition may vary

within the same layer, semi-quantitative estimations

of the non-carbonate minerals, by comparison of the

integrated peak areas of their strongest peaks (110

for sepiolite and 001 for stevensite and kerolite)

(Brindley, 1980) from the randomly oriented XRD

patterns. The calculated percentages were rounded

off to the nearest 5 wt.% and the approximate

mineral compositions are shown in Fig. 2 in

relation to the corresponding layers of the

stratigraphic section.

The structural formulae of the sepiolites were

determined from chemical analyses of the <2 mmfractions of 2 representative pure samples

(confirmed by XRD) by inductively coupled

plasma emission-spectroscopy (ICP-AES) at

ACME Analytical Laboratories Ltd. Vancouver,

Canada.

Fourier transform infrared (FTIR) spectroscopic

study was carried out on two sepiolite samples

using a Perkin Elmer Precisely Spectrum One FTIR

spectrometer. The samples were first diluted with

KBr at 1:200 ratio and subsequently compressed in

the form of discs. Samples were scanned in the

frequency range 4000 to 450 cm�1.

Scanning electron microscope (SEM) analyses

were performed on gold-coated chips of 12 samples

with a Jeol JSM-6335F field emission SEM

equipped with an Oxford Inca energy dispersive

spectrometer (EDS).

RESULTS

Description of lithological units

Four lithological facies have been distinguished

in the Early Pliocene sediments; carbonate facies,

sandstone facies, sepiolitic facies and smectitic

facies. Carbonate facies consist of dolomite,

clayey dolomite and dolomitic marl occupying the

major part of the succession. It starts with a light

green-white coloured, loose and >2 m thick

magnesite-dolomite mudstone (Fig. 2a), followed

by a white-grey, relatively hard micritic dolomite,

soft chalky dolomite and dolomitic marl layers in

alternation. The thicknesses of carbonate layers

range from 0.5 to 1.5 m.

Sepiolite facies occur in two separate levels

consisting of massive sepiolite, layered sepiolite

and dolomitic sepiolite (Fig. 2a). The lower sepiolite

level with a total thickness of 3 m consists of three

sepiolite layers. A laminated and light beige

coloured sepiolite (Fig. 2a; YD8) occurs in a

Sepiolite from Yenidogan, Turkey 93

50�60 cm thick layer, between two sandstone beds,

which are composed of rounded and sandy-sized

dolomite intraclasts embedded in the sepiolite matrix

or vice versa. The thickness of this level decreases

laterally, and its composition grades to dolomitic

sepiolite. The upper sepiolite level occurs after

massive clayey dolomite layers with 7 m total

thickness. This level comprises almost pure sepiolite

lenses, either massive or layered, alternating

dolomitic sepiolite and dolomite layers. Nearly

pure sepiolite occurs in lenses as thin as 0.5 m.

The positions of the lenses change both vertically

and horizontally. When they are superimposed the

total thickness of sepiolite reaches up to 3.5 m.

Both massive and layered sepiolites are wax-like

when wet. When dry, sepiolite is light-weight with a

light beige colour, turning to dark brown or even

black when organic material is present. In places,

sepiolite is laminated or thin layered. Rounded

intraclasts of sepiolite up to 2 cm across, representing

the reworked material may be occasionally found

embedded in sepiolite clay matrix. In some places,

sepiolite shows a massive structure where well

developed layering is lacking. Also, thin tuff

(0.5�10 cm), opal bands and nodules, and diatomac-

eous earth accompany the sepiolite in places.

Dolomitic sepiolite and sepiolitic marl layers

0.4�2.6 m thick alternate with sepiolite (Fig. 2a).

FIG. 2. (a) Measured stratigraphic section and (b) compositional diagram of corresponding layers. Sample

locations are denoted by YD. Key to the symbols. T = talc; Ke = kerolite; S = sericite; Q = quartz; F = feldspar;

Ce = celestine (adapted from Yeniyol, 1992).

94 M. Yeniyol

They are massive in appearance and vary in colour

from white to light beige according to the sepiolite

content. These layers often show a detrital texture

and contain dolomite and sepiolite intraclasts up to

1 cm in size. In addition, casts of roots, burrowed

organisms, fenestrae structures and fissures which

are filled with carbonate are often recognized in

sepiolite-bearing materials. The upper sepiolite

level extends throughout the study area, comprising

a total thickness of ~10 m.

After the upper sepiolite level, smectitic facies

occurs in dolomitic clay layers, in which smectite is

the predominant clay mineral. These layers alternate

clayey dolomite layers constituting the middle part

of the sequence (Fig. 2a; from YD20 to YD28). The

dolomitic clay layers have colours varying from

white to light green and even light beige. They are

massive showing scarce fine rounded or angular

intraclasts, rare fenestrae structures, desiccation

fissures and traces of bioturbation and are

0.8�1 m thick.

Clay mineralogy

Structurally, two distinct sepiolite forms were

identified by XRD from purified <2 mm fractions of

the samples. The first one is typical sepiolite,

characterized by a sharp and intense (110) peak at

12.3 A with a 1º2y full width at half maximum

(FWHM) and well resolved remaining reflections,

indicating the presence of well ordered sepiolite

(Fig. 3, YD50). The second sepiolite type displays

an XRD pattern fairly similar to the well crystal-

lized sepiolite. However, the 110 reflection is

relatively weak and broad with 1.5º2y FWHM and

is shifted to 12.7 A; reflections which typically

occur between 12�22º 2y are absent (Fig. 3; YD8).

Also, reflections between 26�30º2y are not well

resolved, appearing as a convoluted peak with two

maxima at 3.3 and ~3.2 A. In addition, some of the

remaining reflections are different, both in terms of

their shapes and d values. This form of sepiolite is

denoted herein as sepiolite-13A.

The oriented and air-dried samples of well

crystallized sepiolite are dominated by a 110

reflection at 12.3 A, which slightly shifts to

12.52 A upon glycolation and contracts to 11.44 A

after heating at 550ºC (Fig. 4a). In addition, a new

reflection appears at 8.14 A which may be attributed

to the formation of sepiolite anhydride (Nagata et al.,

1974). The oriented and air-dried sample of sepiolite-

13A is dominated by a broad 110 reflection of low

intensity at 12.8 A, which shifts slightly to ~12.94 A

after ethylene glycol solvation and contracts to

11.47 A after heating at 550ºC (Fig. 4b).

Smectite is always found with sepiolite-13A in

mechanical mixture that makes it impossible to

obtain a pure monomineralic sample. In the XRD

pattern of randomly oriented samples in which

smectite predominates, the (001) basal reflection is

weak and asymmetric and is centred at 12.5 A

(Fig. 3; YD28). The reflections at higher º2y occur

as two-dimensional hk bands due to the turbostratic

structure of this mineral. The 061 band centred at

1.52 A is symmetrical and sharp, indicating a

trioctahedral structure with b0 of 9.12 A.

FIG. 3. XRD patterns of randomly oriented samples of well crystallized sepiolite (YD50), sepiolite-13A (YD8)

and stevensite (YD28).


In oriented and air-dried samples a broad

reflection occurs at 13.2 A (Fig. 5; YD28), which

upon ethylene glycol solvation shifts to 17.4 A. A

second reflection appears at 12.9 A, indicating the

presence of sepiolite-13A. The higher-order reflec-

tions of smectite appear at 8.8, 5.7, 4.4 and 3.4 A.

The slight deviation from a rational series, high d

value and low peak/saddle ratio of 001 reflection,

and 1.6º2y FWHM indicate a small number of

layers per coherent scattering domains (CSD)

(Tettenhorst & Moore, 1978; Yeniyol, 2007). Due

to the relatively low d001 values ranging from 12.5

A to 13 A and the trioctahedral character this

smectite is assigned as stevensite. Ethylene glycol

solvation also produces a weak and broad peak

visible at 10.0 A which may correspond to kerolite.

Upon heating at 550ºC, these peaks shift to 10.0 A,

indicating a talc-like phase formed after collapse of

the stevensite structure. This thermal behaviour is

typical of stevensite (Christidis & Koutsopoulou,

2013).

Kerolite was occasionally detected in very small

amounts locally associated with talc in some layers

of the sequence. The relatively sharp reflection at

FIG. 4. XRD patterns of oriented samples of (a) well crystallized sepiolite and (b) sepiolite-13A. (AD) air-dried,

(EG) solvated with ethylene glycol and (550) heated at 550ºC.

96 M. Yeniyol

9.40 A was assigned to talc, whereas kerolite was

identified by the broad reflection in the

9.53�9.82 A range, particularly from the XRD

patterns of randomly oriented samples. In addition,

except for some celestine which was found in a

layer at the bottom of the sequence, other minerals

were not detected by XRD.

Bulk mineralogical composition

Dolomite is the main phase in the sedimentary

succession (Fig. 2b). The dolomite-bearing layers

range from pure dolomite to clayey dolomite and

even to dolomitic marl. Dolomitic marls contain

dolomite and clay minerals in approximately equal

proportions, whereas the clay mineral content of

clayey dolomite is less than 50%. In two distinct

layers, magnesite occurs in association with

dolomite. The lower magnesite-bearing layer

(Fig. 2b; YD1) with a 30/60 dolomite/magnesite

ratio also contains small amounts of sepiolite and

trace celestine, and the upper layer (Fig. 2b; YD21)

has a 35/25 dolomite/magnesite ratio and also

contains stevensite and sepiolite.

Sepiolite is the principal clay mineral in

association with dolomite in the lower half of the

succession, occurring at two separate levels. In the

lower level, nearly pure sepiolite-13A is present

between two dolomitic sepiolite layers (Fig. 2b;

YD7 and YD9) that contain over 70% sepiolite. The

upper sepiolite level contains well crystallized

sepiolite (Fig. 2b; YD11 to YD19). Sepiolite

layers vary in composition from almost pure

sepiolite to that containing less than 50% dolomite.

In some places, pure sepiolite also contains up to

10% organic matter. Besides, trace amounts of

feldspar, quartz grains, biotite, sericite and thin

opal-CT bands or nodules are associated with

sepiolite in places.

In the middle part, only sepiolite-13A is present.

It is always associated with stevensite with the

exception of one layer consisting of ~20%

FIG. 5. XRD patterns of oriented samples of stevensite (YD-28). (AD) air-dried, (EG) solvated with ethylene

glycol and (550) heated at 550ºC. St = stevensite; Se = sepiolite; Ke = kerolite.


sepiolite-13A and 80% dolomite (Fig. 2b; YD22).

In the remaining layers the smectite content varies

from 15% to 70% (Fig. 2b; YD24 to YD28), whilst

the sepiolite-13A content ranges from 15% to 30%.

Kerolite and talc are also present in traces (less than

10%) in the middle part and in some layers of the

lower half of the succession.

FTIR spectra

FTIR spectra are typical of sepiolite, and two

distinct sepiolite samples examined are similar in

general (Fig. 6). Over the region 3700�3000 cm�1,

the spectra of the well crystallized sepiolite (Fig. 6;

YD50) show a weak band at 3688 cm�1

corresponding to stretching (nOH) vibrations of

OH groups (Mg3OH) (Nagata et al., 1974). The

band at 3622 cm�1 assigned to H�O�H stretching

vibrations of water molecules are weakly hydrogen

bonded to the Si�O surface (Ozcan et al., 2006).

The bands at 3558 cm�1 and 3430 cm�1 correspond

to OH stretching of bound water and zeolitic water,

respectively. Absorption corresponding to the

bending vibrations of the bound water molecules

occurs at 1660 cm�1 and 1700 cm�1 (Frost et al.,

1998, 2001). Two weak bands at 1424 cm�1 and

1372 cm�1 are assigned to the bound water

coordinated to Mg in the octahedral sheets (Sabah

& Celik, 2002; Wouters et al., 2010). The weak

band at 1208 cm�1 is attributed to Si�O�Sistretching between alternate ribbons (Suarez &

Garcia-Romero, 2006) and the intense band at

1020 cm�1 with a shoulder at 982 cm�1 is assigned

to the Si�O stretching. The bands at 783 cm�1,

692 cm�1 and 649 cm�1 correspond to Mg3OH

bending vibrations. In addition, a bending band

occurs at 477 cm�1.

The infrared spectrum of sepiolite-13A (Fig. 6;

YD8) differs from the well crystallized sepiolite by

the absence of 3688 cm�1, 1700 cm�1, 1424 cm�1

FIG. 6. FTIR spectra of well crystallized sepiolite (YD50), sepiolite-13A (YD8) and stevensite (YD28).

98 M. Yeniyol

FIG. 7. DTA and TG analyses of (a) well crystallized sepiolite (YD50), (b) sepiolite-13A (YD8) and (c) stevensite

with sepiolite-13A (YD28).


and 1372 cm�1 bands and the weak shoulder at

982 cm�1. Moreover, the 3442 cm�1 band is more

intense than 3566 cm�1 when compared with the

corresponding bands of the well crystallized

sepiolite. The positions of the Mg�OH bending

vibrations differ slightly and appear at 788 cm�1,

658 cm�1 and as a shoulder at 524 cm�1.

Differences in vibrations above ~700 cm�1 (up to

1500 cm�1) are dominated by atomic displacements

within the silicate sheets, and below these values by

motions among the octahedral and silicate sheet

(McKeown et al., 2002).

The FTIR spectrum shown in Fig. 6 (YD28) is

characteristic for stevensite. An asymmetrical and

strong band occurs at 3443 cm�1 which may be

attributed to the OH stretching vibration of

adsorbed water (Imai et al., 1970; Elton et al.,

1997; Buey et al., 2000; Benhammou et al., 2009)

and masks the other vibrations that appear in this

region. Bending vibrations of water molecules

appear at 1635 cm�1, whereas Si�O stretching

absorption occurs as a strong and sharp band at

1015 cm�1. In addition, there are bands at

658 cm�1 and 470 cm�1 which are attributed to

Si�O�Mg vibration (Christidis & Mitsis, 2006).

The weak bands at 1224 cm�1 and 788 cm�1

correspond to traces of sepiolite-13A present in the

sample.

Thermal analyses

The DTA curve of the well crystallized sepiolite

shows the first endothermic peak at 92ºC due to

loss of zeolitic water (Fig. 7a). Two broad and

weak endothermic peaks occurring at 281ºC and

466ºC are attributed to the loss of the first and

second halves of coordinated water, respectively.

Removal of coordinated water in these two steps

results in folding of the sepiolite structure and

formation of sepiolite anhydride (Nagata et al.,

1974; Serna et al., 1975; Rautureau & Mifsud,

1977).

A fourth endothermic peak appears at 765ºC

which is related to the loss of the hydroxyls

(structural water). Finally, the sharp exothermic

peak at 836ºC indicates the collapse of the sepiolite

structure and formation of enstatite. Similar thermal

reactions occur at sepiolite-13A (Fig. 7b). The

second endothermic peak occurs at 293ºC i.e. at

slightly higher temperature than the well crystal-

lized sepiolite, and the third and fourth endothermic

reactions occur at 445ºC and 730ºC respectively.

The temperatures of these reactions and the

exothermic reaction that appears at 826ºC are

slightly lower than the corresponding reactions of

well crystallized sepiolite.

The water loss is continuous for both forms of

sepiolite. The first water loss (25�164ºC) belongs

to both hygroscopic and zeolitic water (10.8 wt.%,

Fig.7a). The first half of the coordinated water that

is rather weakly bounded at the structural channels

is released from 164ºC to 334ºC (3.04 wt.%),

whereas the loss of the second half occurs at

334�548ºC (3.24 wt.%). The remaining water,

which corresponds to the loss of hydroxyls, is

released at 548�836ºC (2.11 wt.%). The total

amount of weight loss is 19.19 wt.%. Water loss

for sepiolite-13A (Fig. 7b) takes place in four

subsequent steps (Nagata et al., 1974; Serna et

al., 1975; Frost & Ding, 2003): 25�174ºC,174�348ºC, 348�512ºC and 512�826ºC, corre-

sponding to 11.33 wt.%, 2.74 wt.%, 2.30 wt.% and

2.97 wt.% water loss, respectively. The total

amount of water loss for the sample calculated

from the TG curve is 19.07 wt.%. The differences

in thermal reactions and water losses described

above are attributed to the different crystallinity of

the two sepiolite types.

The DTA and TG curves of sample YD28

containing stevensite and minor sepiolite-13A are

FIG. 8 (facing page). SEM images of sepiolite and related Mg-rich phyllosilicates: (a) Randomly oriented flakes

and laths formed by parallel fibres of well crystallized sepiolite (YD17). Dolomite appears as scarce and fine

rhombs 0.5 mm in size, formed together with sepiolite. (b) Wrinkled planar structures resembling smectite (YD8).

(c) High magnification image of (b) showing that entire photo is occupied by subparallel or randomly oriented

and tightly interwoven delicate fibres of sepiolite-13 A. (d, e) Sepiolite-13A (YD22) occupying the entire photo

but displaying honeycomb texture that was inherited from the precursor smectite. (f) Stevensite with honeycomb

texture (YD28). (g) High magnification image of stevensite showing honeycomb texture similar to (e) with scarce

sepiolite-13A fibres resting on stevensite substrate (arrow) (YD21). The honeycomb texture is similar to (e)

which consists only of sepiolite-13A. (f) Thick kerolite flakes with slightly undulated edges (YD24). Rectangles

in (f) and (h) indicate the areas where EDS analyses were made. Do = dolomite.

100 M. Yeniyol

shown in Fig. 7c. These curves are rather similar to

the well crystallized sepiolite and sepiolite-13A.

The first endothermic peak at 88ºC corresponds to

loss of the majority of interlayer water of stevensite

and zeolitic water of sepiolite-13A. The main

difference is the presence of an endothermic peak

at 825ºC which is attributed to the loss of the

remaining hydroxyl water of stevensite (Imai et al.,

1970), whereas the exothermic peak at 808ºC

indicates the collapse of the sepiolite structure and

formation of enstatite. The water loss for this

sample corresponds to different types of water in

the sepiolite and the zeolitic, interlayer waters and

hydroxyls of the stevensite. The total amount of

water loss is 19 wt.%.

SEM analyses

In the SEM the well crystallized sepiolite appears

in bundles of parallel and straight fibres in the form

of planar structures and laths up to 5 mm long,

occasionally associated with fine dolomite rhombs

(Fig. 8a; YD17), suggesting that it was formed by

precipitation from the solution with the dolomite.

Sepiolite-13A appears in the form of wrinkled

flakes resembling smectite (Fig. 8b; YD8). Under

higher magnification, it is clear that the flakes

consist of sub-parallel or randomly oriented

interwoven delicate sepiolite fibres ~1 mm long

(Fig . 8c) . In another layer where only

sepiolite-13 A coexists with dolomite (Figs. 8d

and e; YD22), sepiolite displays a honeycomb

texture resembling smectite. However, the entire

material consists of randomly oriented short

sepiolite fibres. In both samples, fibres of a late

generation up to 3 mm long also occur in the

interstitial spaces projecting from the tips of the

flakes. They are curled, and in some cases are

interlocked with each other, displaying a lacework

texture (Fig. 8d and e).

Stevensite occurs in the form of large or fine

crinkled planar structures or uneven flakes, forming

a typical honeycomb texture (Fig. 8f and g).

Although stevensite is the dominant constituent, it

is always associates with sepiolite-13A. As a rule,

sepiolite fibres of varying amounts rest on the

stevensite substrate (Fig. 8g; YD21).

Under the SEM, accessory kerolite is always seen

to be present in association with stevensite.

Morphologically it is rather different from steven-

site by appearing in the form of small, even thick

flakes with slightly undulating edges (Fig. 8h;

YD24) whereas stevensite appears in the form of

thin and larger undulated flakes (Fig. 8f; YD28).

Qualitative EDS analyses provided MgO and SiO2

values approximating to the composition of talc and

thus suggesting the presence of kerolite.

Chemical analyses

The major element oxide analyses of the samples

are given in Table 1. Analyses using wet chemical

methods were performed on some well crystallized

sepiolite samples that were pure in the raw state

(SS8 and SY16/8). In these samples, SiO2 varied

from 53.30 wt.% to 54.04%. Al2O3 is present in

small amounts being less than 2.85%. Also, all

samples showed low Fe2O3 and TiO2 contents.

To compare the crystal chemistry of well

crystallized sepiolite and sepiolite-13A, ICP

analyses were performed on essentially monomi-

neralic samples (YD8 and YD50) (Table 1).

Sepiolite-13A (YD8) has Al2O3, Fe2O3, and TiO2

contents rather lower and MgO content higher than

the well crystallized sepiolite (YD50). The struc-

tural formulae calculated per half unit cell (32

oxygen a toms) a re : (Ca0 . 0 5Na0 . 0 8K0 . 0 7 )

(Mg7.17Al0.32Fe0.11Ti0.02&0.38)(Si11.96Al0.04)

O30(OH)4 for sepiolite-13A and (Ca0.05Na0.09K0.10)

(Mg5.90Al0.96Fe0.36Ti0.04&0.74)(Si11.81Al0.19)

O30(OH)4 for well crystallized sepiolite. The

principal cation in the octahedral sheet is Mg.

Small amounts of Al, Fe and Ti substitute for Mg in

the octahedral sheet of sepiolite-13A. The numbers

of structural vacancies (shown by squares), which

are common in sepiolite structures, are lower in

sepiolite-13A than in well crystallized sepiolite. The

latter has a lower Mg content than the ideal

composition, which is due to higher substitution

by Al, Fe and also Ti; this increases the number of

octahedral vacancies (Garcia-Romero & Suarez,

2010). Depending on its high Al content, it can

be termed an Al-rich sepiolite.

Stevensite is always found in association with

sepiolite-13A and minor kerolite. Therefore, it was

not possible to obtain pure phases for chemical

analyses. However, it was considered convenient to

evaluate the chemical analyses obtained by EDS on

isolated particles as an approximation to the crystal

chemistry of these minerals (Fig. 9f and h). Kerolite

(Table 1; YD21) has a lower SiO2 and higher MgO

content than stevensite (Table 1; YD28). The CaO

content originates from the presence of dolomite

that was probably masked by kerolite particles.

102 M. Yeniyol

After assigning the the CaO content to dolomite,

the theoretical formula of Mg3Si4O10(OH)2 can be

calculated for kerolite. The structural formula is

similar to talc without vacancies.

Besides MgO and SiO2 stevensite also contains

Al2O3 and Fe2O3 in very small amounts. The

structural formula of stevensite calculated on the

basis of O10(OH)2 is: (Ca0.56)(Mg2.47Al0.07Fe0.03&0.43)Si4.16O10(OH)2. After making correc-

tions for the excess SiO2 (0.16 atoms), which were

assigned to amorphous silica, the following

s t r u c t u r a l f o r m u l a w a s o b t a i n e d :

Ca0.06(Mg2.82Al0.08Fe0.04&0.06)Si4O10(OH)2. In

both formulae, Mg is the principal octahedral

cation, with small amounts of Al and Fe

substituting for Mg in the octahedral sheet.

Although this structural formula is indicative, with

high Mg and low Al and Fe contents, the absence

of tetrahedral substitution and the octahedral

vacancies suggest the presence of stevensite, in

accordance with the XRD and FTIR data.

D I SCUSS ION

Two distinct forms of sepiolite were distinguished

in this study: a well crystallized and a poorly

crystallized sepiolite, the latter being referred to as

sepiolite-13A. The well crystallized sepiolite shows

a sharp and strongest peak at ~12.3 A (110), and

well resolved remaining reflections similar to

references in the literature with respect to their

shapes, d values and intensities. The overall XRD

pattern of sepiolite-13A, although rather similar to

the widely-known sepiolite, has some characteristics

typical of poor crystal order.

The first description of poorly-ordered sepiolite

was made by Brindley (1959) for a sample which

was obtained from Eskis� ehir (most probably

sepiolite of the meerschaum type). It was a very

poorly crystallized sepiolite with fairly broad and

poorly resolved reflections and with the 110

reflection appearing at 12.3 A. Sepiolites with

fairly similar XRD patterns with higher d values of

the 110 reflection were reported in later studies

(Yeniyol & Oztunalı, 1985; Yeniyol, 1986, 1993a;

Ece & Coban, 1994; Yalcın & Bozkaya, 1995;

Chahi et al., 1997; Ece, 1998). These works

describe poorly-ordered sepiolite having broad 110

reflections with high d values, showing a convo-

luted single reflection in the 26�30º2y region and

remaining reflections which are not well resolved,

rather similar to the work of Brindley (1959).

Sepiolite-13A also shows some characteristics

similar to these poorly-ordered sepiolites. The 110

reflection is wide (1.5º2y), appearing at ~13 A with

an intensity that is considerably weaker than the

corresponding reflection of the well crystallized

sepiolite. The high d value of this reflection

resembles loughlinite (Na-rich variety of sepiolite),

although the present sepiolite is Na-deficient and

has a different XRD pattern from loughlinite

(Yeniyol, 1997; Kadir et al., 2002). Three

reflections which typically occur in the 12�22º2yregion were not observed. Moreover, two relatively

strong reflections occurring at 4.52 and 4.31 A

either show nearly equal intensity or the second one

TABLE 1. Chemical analyses of sepiolite, kerolite and stevensite.

Wet chemical analyses (wt.%) – ICP – – EDS –SS8 SY16/8 YD8 YD50 YD21 YD28

SiO2 54.04 53.30 52.39 53.18 58.23 66.52Al2O3 2.85 1.14 1.46 4.39 � 0.99SFe2O3 1.08 0.90 0.64 2.15 � 0.68TiO2 n.d.* n.d. 0.08 0.24 � �MgO 20.70 22.29 21.08 17.83 38.73 26.48CaO 0.19 0.72 0.20 0.19 3.04 0.83Na2O 0.10 0.26 0.19 0.21 � �K2O 0.68 0.38 0.25 0.36 � �LOI 20.10 21.43 23.20 21.10 � �Total 99.7 100.42 99.49 99.65 100 100

* n.d., not determined; SFe2O3, total iron; LOI, loss on ignition; SS8 and SY16/8, raw sepiolites; YD8,sepiolite-13A; YD50, well crystallized sepiolite; YD21, kerolite; YD28, stevensite.


is less intense than the first one, in contrast to the

well crystallized sepiolite where the second

reflection is more intense than the first.

Furthermore, the convoluted reflection at

26�30º2y and the remaining reflections are not

well resolved. These data demonstrate the poorly-

crystallized nature of sepiolite-13A, with a struc-

tural organization poorer than the typical sepiolite.

Similar XRD data relevant to sepiolite-13A were

obtained from the analyses performed on numerous

monomineralic samples that were collected from

different layers containing sepiolite of this form.

Mineral impurities other than sepiolite (e.g.

stevensite and/or kerolite, which frequently coexist

with sepiolite) were not detected by XRD.

FTIR spectra of both types of sepiolite are

identical in general and fairly similar to those

reported elsewhere (Sabah & Celik, 2002; Wouters

et al., 2010; and Tunc et al., 2012). However, some

weak bands are absent, and the positions and

intensities of some bands are slightly different in

sepiolite-13A, indicating its poor crystallinity.

Neither the overall spectrum nor the position or

shape of the individual bands are identical to the

FTIR spectra, indicating the presence of stevensite

and kerolite (e.g. Imai et al., 1970; Davidson, 1989;

Elton et al., 1997; Christidis & Mitsis, 2006; Dekov

et al., 2008; Benhammou et al., 2009).

The thermal curves of well crystallized sepiolite

are rather similar to those shown in previous work

on thermal reactions and water loss steps (Nagata et

al., 1974; Serna et al., 1975; Perez-Rodriguez &

Galan, 1994; Singer et al. 1998, among others). The

DTA and TG curves of sepiolite-13A are almost

identical to well crystallized sepiolite. However

some differences in thermal properties arise from its

poor crystallinity and thereby the presence of some

weaker structural bonds. The second half of

coordinated water and structural hydroxyls are

rather weakly bonded to the structure. Therefore,

the last two thermal reactions involving the losses

of these types of water appear fairly continuous and

at lower temperatures. Also, the exothermic

reaction at 826ºC, corresponding to collapse of the

sepiolite structure and subsequent formation of

enstatite, occurs at slightly lower temperature than

the well crystallized sepiolite, due to the poor

crystallinity.

Well crystallized sepiolite contains variable

proportions of Al2O3. However, some samples are

richer in Al2O3 and poorer in MgO than the ideal

sepiolite. A high Al content is probably associated

with devitrification of the volcanic glass which was

then transferred from solution to the sepiolite

structure. Sepiolite-13A (sample YD8) has low

Al2O3, Fe2O3, TiO2 and higher MgO contents than

the Mg-rich typical sepiolite.

Stevensi te almost a lways occurs with

sepiolite-13A and kerolite. SEM observations and

crystal chemical data indicate a high Mg content,

insignificant Al and Fe contents, absence of

tetrahedral substitution and the presence of

structural vacancies in the octahedral sheets.

Kerolite was distinguished by the fairly broad

001 reflection appearing at 9.53�9.82 A in the

XRD patterns. Thick and rather flat flakes with

morphology distinct from that of stevensite under

the SEM were assigned to kerolite. In addition, the

theoretical formula which was calculated from EDS

analysis from the selected area under the SEM is in

accordance with kerolite.

The Miocene-Pliocene sediments of the study

area were deposited in an alkaline lake environ-

ment. Stratigraphic and lithological features indicate

that semi-arid or arid climatic conditions with short-

lasting wet seasons had prevailed (Yeniyol, 1992).

During the Early Pliocene, an alkaline shallow lake

environment rich in Mg, Ca and Si was developed

in which chemical precipitation dominated. All

mineral phases present are authigenic except for

traces of feldspar, quartz, biotite and sericite that

are present in some layers.

The sepiolitic and smectitic facies of the Early

Pliocene are interpreted as palustrine to mudflat

deposits formed in the alkaline lake margin where

high Mg and Si were available by feeding of the

surface waters (Pozo & Casas, 1999; Galan & Pozo,

2011). Massive and layered sepiolites precipitated

at short-lived ponds at the edges of main water

bodies and the marshlands (palustrine), at which

short-lived plants had grown. Sepiolite was

deposited in dry periods following the formation

of dolomite (Galan & Ferrero, 1982). Well crystal-

lized sepiolite was formed by direct precipitation

from lake water or by crystallization from pore

solutions during diagenesis under low Al, high Si

and Mg activities, and a pH ranging between 8.0

and 9.5 (Wollast et al., 1968; Khoury et al., 1982;

Galan & Castillo, 1984; Abtahi, 1985). Opal-CT

bands and nodules found in association with

sepiolite indicate a high activity of SiO2 that

favours the formation of sepiolite (Birsoy, 2002).

In those layers consisting only of sepiolite-13A

(samples YD8 and YD22), randomly oriented or

104 M. Yeniyol

parallel delicate fibres occupy the entire mass.

However, the wrinkled planar or honeycomb texture

which is characteristic for smectite was inherited

and was remarkably preserved. This relationship

may suggest an epitaxial growth on a smectite

substrate, but SEM and XRD data reveal that the

material consists entirely of sepiolite without a

planar structure which might indicate the presence

of stevensite or kerolite precursor minerals. Based

on these relationships, it is suggested that this form

of sepiolite was developed by in situ transformation

via a dissolution-precipitation mechanism from the

precursor stevensite during diagenesis, as suggested

previously (Eberl et al., 1982; Khoury et al., 1992;

Chahi et al., 1997 and Pozo & Casas, 1999). The

relative SiO2 enrichment during the dry periods

resulted in low pH and low salinity that should

decrease the stability of stevensite and probably

favoured the formation of sepiolite-13A (Khoury et

al., 1982; Galan & Pozo, 2011). This type of

sepiolite is present associated with stevensite,

indicating that the dry episodes did not last

enough to complete this transformation (Chahi et

al., 1997).

It is noteworthy that all poorly ordered sepiolites

mentioned in the literature were formed by

transformation of pre-existing minerals such as

magnesite (Yeniyol & Oztunalı, 1985; Yeniyol,

1986, 1993a,b; Ece, 1998) or stevensite (Chahi et

al., 1997). It is thus suggested that the poorly

ordered sepiolite-13A may be due to the transfor-

mation from stevensite, probably involving struc-

tural reorganization, in contrast to the mechanism

described by Guven & Carney (1979) for the

sepiolite to stevensite transformation.

Stevensite-bearing layers were probably depos-

ited in a mudflat environment under relatively wet

climatic conditions by direct chemical precipitation

from alkaline lake water due to the high pH and

salinity, and depending on high Mg and Si activities

(Siffert, 1962; Birsoy, 2002). Kerolite was also

formed in mudflat and palustrine environments as

was suggested by Pozo & Casas (1999) by direct

precipitation from the solution at pH<8 (Stoessell &

Hay, 1978) under conditions of lower salinity and

higher Mg activity than that required for stevensite

(Jones, 1986).

CONCLUS IONS

A poorly crystallized sepiolite form was discovered

in a sepiolite deposit which also contains the widely

known well crystallized sepiolite. This form was

denoted herein as sepiolite-13A, having structural,

crystallochemical, thermal and textural properties

similar to typical sepiolite, although differences in

these properties are in accordance with the poorly

crystallized nature of this type of sepiolite.

The two distinct sepiolite forms occur in separate

levels as natural almost pure materials or associated

with stevensite, kerolite and dolomite. Sepiolites

and related Mg-rich phyllosilicates were formed in

mudflat and palustrine environments of a shallow

alkaline lake margin depending on the changes in

salinity and pH, and on the activities of Al, Si and

Mg. The well crystallized sepiolite was formed by

direct precipitation from the lake water rich in Si

and Mg ions during dry periods and from pore

waters during diagenesis. Sepiolite-13A was prob-

ably formed at the expense of precursor stevensite

via a dissolution-precipitation mechanism during

diagenesis.

Stevensite is always associated with sepiolite-13A

and traces of kerolite and was formed by direct

precipitation during relatively wet periods. The

formation of kerolite was favoured by rather

similar conditions and by a decrease in pH.

ACKNOWLEDGMENTS

The present work was supported by the Research Fund

of Istanbul University Project No. UDP-6650/

26032010. I would like to thank A. Kas� goz for FTIR

analysis (Istanbul University, Laboratories of

Advanced Analyses) and I. Yusufoglu (Istanbul

University, Department of Metallurgical and

Materials Engineering) for providing the thermal

analysis. I am also grateful to the editor for his

constructive support and for making corrections to the

manuscript, to E. Garcıa-Romero and to two anon-

ymous referees for valuable comments they made when

reviewing this manuscript.

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Characterization of two forms of sepiolite and related Mg-rich ...

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