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Ž .Applied Clay Science 14 1999 121–140

An X-ray, EPMA, and oxygen isotope study ofvermiculitized micas in the ultramafic rocks at

Askos, Macedonia, Greece

Ananias Tsirambides ), Kleopas MichailidisDepartment of Geology, Aristotle UniÕersity of Thessaloniki, Thessaloniki 540 06, Greece

Received 4 March 1998; revised 17 September 1998; accepted 6 November 1998

Abstract

Extensive metasomatic zones of vermiculite-, tremolite-, chlorite-, and talc-rich rocks havebeen developed at the contacts of serpentinized ultramafic bodies and surrounding two-micagneisses in the Askos area, Macedonia, Greece. These zones are probably related to the intrusionof acid magmatic bodies in the area. X-ray and EPMA studies confirmed the formation ofvermiculite through a layer-by-layer transformation of original micas. In decreasing abundance,the following mixed-layer and discrete phases were identified: biotitertrioctahedral vermiculiteŽ . Ž .hydrobiotite , biotitersmectite, trioctahedral chloritertrioctahedral vermiculite corrensite , ver-miculitersmectite and discrete biotite, vermiculite, chlorite and talc. The 2–20 mm fraction of the

Ž .vermiculitic samples consists mostly of biotitervermiculite )40% with the biotite percentageŽ .dominating in the mixed phase. Lower abundances 20–40% of biotitersmectite and

Ž .chloritervermiculite occur in both finer fractions 2–20 and -2 mm of all vermiculitic samples.Vermiculitersmectite is very abundant in the -2 mm fractions of most vermiculitic samples. TheDTA curves of the samples analyzed are characteristic of Mg-vermiculites. The electron micro-probe analyses show a gradual K leaching from precursor mica with increasing degree ofweathering. Oxygen isotope results confirm this assumption. Initially, hydrothermal fluids derivedfrom the neighboring granitic intrusions, were responsible for the micatization of the primaryminerals of the ultramafic bodies. Hydrothermal activity was also responsible for the partialformation of corrensite. Consequently, water moving downwards was very important for theformation of the vermiculite and other clay mineral mixed-layer phases through the alteration ofmicas and chlorite. The low relief and the long-lasting tectonic stability of the area were essential

) Corresponding author. Fax: q30-31-998-568; E-mail: [email protected]

0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-1317 98 00054-4

( )A. Tsirambides, K. MichailidisrApplied Clay Science 14 1999 121–140122

for the development of a significant thickness of the vermiculite zones. These vermiculites haveproperties desirable in certain building, agriculture, and horticulture products. q 1999 ElsevierScience B.V. All rights reserved.

Keywords: vermiculitized micas; ultramafic rocks; Askos; Macedonia; Greece

1. Introduction

Vermiculite is usually considered to be the product of a mica alteration, oftenwith interstratified micarvermiculite or chloritervermiculite as transitional

Ž .phases Ross and Kodama, 1974; Ross et al., 1982 . The precursor of vermi-culite formed from mica can be identified from the inherited parent mineralpolytypic structure. Moreover, such vermiculites on saturation with Kq ions,

Ž .readily revert to the polytype of the parent mica de la Calle et al., 1975 .The problem of the origin of vermiculites is still a subject to discussion. The

major commercial deposits of vermiculite are in ultrabasic and basic rockswhere it is found in large crystalline plates. These macroscopic vermiculites are

Ž .of secondary origin and result from the alteration of micas biotite or phlogopite ,chlorites, pyroxenes, amphiboles, etc., by weathering, hydrothermal action,

Ž .percolating ground water or a combination of these three effects Basset, 1963 .Supergene vermiculitization of trioctahedral micas through intermediate stagesof interstratified micarvermiculite has been identified as one of the main

Žmineral weathering sequences reported in the literature April et al., 1986;.Ildefonse et al., 1986; Buurman et al., 1988; Fordham, 1990; Moon et al., 1994 .

The combination of diffractometry and spectroscopy is suggested by de la CalleŽ .and Suquet 1988 to be the most efficient and accurate methods to study

vermiculitic mineral phases.A number of vermiculite occurrences, some of which are considered work-

able deposits in Greece, are found usually associated with ultramafic rocks.Ž .Skarpelis and Dabitzias 1987 described the occurrence of vermiculite at the

Zidani Kozani asbestos deposits, where gneisses host the serpentinite body andŽ .both rocks are intensively sheared. Dabitzias and Perdikatsis 1991 and Dab-

Ž .itzias and Kougoulis 1994 referred to the presence of vermiculite ore in theAskos area and gave preliminary data concerning its mineralogy and formation.

Ž .Zhelyaskova-Panayotova et al. 1992 found that vermiculites in the Vavdos andGerakini areas of Chalkidiki occur in mafic and ultramafic rocks along theircontacts with intruding pegmatite dykes.

This study considers the alteration zones along the contacts between gneissesand ultramafic rocks in the Askos area, Macedonia, Greece. It focuses mainly onthe mineral constituents of the vermiculite zones. Using a variety of petro-graphic, mineralogical, DTA, EPMA, and oxygen isotope techniques, we haveattempted to elucidate the processes through which these vermiculitic phases

( )A. Tsirambides, K. MichailidisrApplied Clay Science 14 1999 121–140 123

were formed. The possible industrial application of the Askos vermiculites isalso considered.

2. Geological setting

The studied area belongs geotectonically to the Serbo–Macedonian massifŽ .which consists mainly of metamorphic rocks Fig. 1 . It is divided into two

units: the eastern, older unit of Kerdilia and the western, younger unit ofŽ .Vertiskos Kockel et al., 1977 . Gneisses, schist–gneisses, amphibolites, meta-

ophiolitic bodies, as well as small marble occurrences constitute the Vertiskosunit, part of which is the Askos area. Extensive investigations, including isotopicstudies, indicate that the rocks of the Serbo–Macedonian massif have undergoneseveral metamorphic and deformational events that occurred from pre-Carbonif-

Žerous to post-Jurassic times Kockel et al., 1977; Chatzidimitriadis et al., 1985;.Papadopoulos and Kilias, 1985; De Wet et al., 1989; Kourou, 1991 . According

Ž .to Kourou 1991 , three tectono-metamorphic events affected the mineralogyand structural features of the region: a Permo-Triassic medium- to high-pressure

Ž .amphibolite–facies metamorphism Ts500–6408C, Ps5–8 kbar , a lateŽ .Jurassic upper greenschist–facies Ts440–5208C, Ps5–8 kbar , and a post-

Ž .Jurassic to Tertiary series of retrogressive stages Ts300–5508C, P-5 kbar .During Late Jurassic, an S-type, syn-collision granite 155 my old, intruded the

Ž .metamorphic rocks of the Vertiskos unit De Wet et al., 1989 .The serpentinized ultramafic rocks of the Askos area are part of a discontinu-

ous mafic–ultramafic belt with a SE to NW orientation and form concordantlens-shaped or elongated bodies within two-mica gneisses. More than 70ultramafic bodies of relatively small dimensions exist in the adjacent areaŽ .Dabitzias and Perdikatsis, 1991 . Their sizes range from 100 to more than 1000m in length and from 20 to 200 m in thickness. The contacts between theultramafic bodies and the gneisses are always sharp and sheared. Thermalaureoles are absent. The ultramafic rocks are antigoritic serpentinites and displaymetasomatic zones in their contacts with the adjacent metamorphic rocksŽ .Michailidis, 1991 . Three different metasomatic zones are found: towards thehost metamorphic rock, a vermiculite zone developed. A zone of tremolite andchlorite follows, and in the contact with the antigoritic serpentinite, the talc zoneoccurs. These zones were formed before the latest thrusting event as is shown bythe intensively bent megacrysts of tremolite and flakes of vermiculite. All

Ž .textural types of vermiculite ores massive, stockwork, veins, irregular massesare well-developed at the upper parts of the profile.

Ž .According to Michailidis 1991 , the silicate mineral assemblage in the Askosmeta-ultramafics denotes metamorphic conditions of Ps4 kbar and Ts450–5508C, which correspond to the transition from the upper greenschist to loweramphibolite facies.

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Ž .Fig. 1. Petrographic sketch map of the Askos area after Michailidis, 1991 .


3. Materials and methods

Ž .Vermiculite ore samples were collected from four different sites V –V of1 4Ž .the Askos area Fig. 1 . Each sample was crushed in a mortar and the chunks

Ž .up to 1 cm in size were disaggregated and size-separated prior to X-rayŽ .diffraction XRD analysis. Disaggregation was done gently in order to retain, to

the extent possible, the intrinsic grain sizes of the particles. Each sample wasplaced in H O in a plastic bottle for two months, during which it was shaken2twice a day. Samples were dried overnight in an oven at about 658C. A 10-gsplit of the -1 mm fraction of each sample was subjected to the following

Ž .chemical treatments Jackson, 1974 to remove the nonsilicate phases: 1 NŽ .sodium acetate–acetic acid buffer solution pHs5.0 for carbonate removal;

30% H O for organic matter and Mn-oxides removal; and 1 M NaHCO –0.32 2 3Ž . Ž .M sodium citrate buffer solution pHs7.3 , to which 1 g increments up to 3 g

of Na S O were periodically added during digestion in a water bath at2 2 475–808C, to remove free Fe-oxides and interlayer Fe- and Al-hydroxides.

ŽThe cleaned residues were separated into four size fractions )63, 20–63,.2–20 and -2 mm by wet sieving, gravity settling, and centrifugation and were

dried overnight in an oven at about 658C. X-ray diffraction was done using aPhilips diffractometer with Ni-filtered CuK radiation. Both randomly orientedasamples and samples with preferred orientation were scanned over the interval2–408 2u at a scanning speed of 18rmin. Some samples were re-analyzed afterglycolation. The recognition of interstratified phases was based on the XRD

Ž .patterns of Moore and Reynolds 1997 . Semi-quantitative estimates of theabundances of the minerals were made from the XRD data using the methods of

Ž . Ž . Ž .Schultz 1964 , Perry and Hower 1970 , and Moore and Reynolds 1997 .Ž .Dehydration and Differential Thermal Analysis DTA were carried out in a

Rigaku PTC-10 A thermoanalyser, using calcined Al O as reference, static air2 3atmosphere, and a heating rate of 108Crmin.

Thin sections were prepared from ultramafic rocks and metasomatic zones foroptical examination. Polished thin sections of vermiculites were made forpetrographic and subsequent electron microprobe examination. Mineral analyseswere obtained by a JSM-840 electron microprobe, equipped with a LINK ANsystem energy dispersion analyzer. Operating conditions were: accelerating

Žvoltage at 15 kV, beam current -3 nA low enough to permit a reasonable.analysis without damaging these minerals , surface electron beam of 1 mm in

Ž .diameter, and counting time of 100 s. Scanning electron microscope SEMphotomicrographs were taken using the same apparatus. Corrections were madeusing the ZAF-4rFLS software provided by LINK.

Oxygen isotope analyses were made on 10–15 mg splits of some fractions.The structural oxygen was liberated by reaction with BrF at about 5758C and5

Ž .converted to CO for mass spectrometric analysis Clayton and Mayeda, 1963 .2Results are reported in d-notation as per mil deviations from the SMOW


standard. The measured d-values were related to SMOW through repeatedŽ .analyses of the National Bureau of Standards NBS isotopic reference material

No. 28 for which we adopted a value of q9.51‰. The oxygen isotopecomposition of the samples was measured in a double collecting mass spectrom-eter apparatus.

4. Results

4.1. Petrography

Vermiculitic samples are semi-friable, with an obvious schistosity and consistŽ .of large crystalline flakes sometimes up to 3 cm in diameter . Under the

polarized light microscope, they present bronze–brown color.Particle size distribution of the samples disaggregated by soaking in H O and2

after chemical treatments is given in Table 1. In consideration of these data, afurther insight comes into the nature and abundance of weathering products.

Ž .From these data, we conclude that: 1 the average amount of the sum ofŽ .carbonatesqorganic matterq iron oxides and hydroxides is high 6–15% ,

Ž .indicating an environment of low oxidation potential Eh during the weatheringŽ . Ž . Ž .processes Degens, 1967 ; and 2 the sand size fraction )63 mm of all

samples mostly predominates, signifying a mild intensity of weathering.

4.2. X-ray mineralogy

The results of XRD analyses of some size fractions are given in Table 2 andrepresentative XRD patterns are shown in Fig. 2. The dominant minerals presentin the vermiculitic samples are mixed-layer phases of phyllosilicates. The 2–20

Ž .mm fraction consists mostly of biotitervermiculite )40% with the biotiteŽ .percentage dominating in the mixed phase. Lower abundances 20–40% of

Žbiotitersmectite and chloritervermiculite occur in both finer fractions 2–20.and -2 mm of all vermiculitic samples. Vermiculitersmectite is very abun-

dant in the -2 mm fractions of most vermiculitic samples. As the grain size

Table 1Ž . Ž .Grain size mm distribution wt.% of the samples analyzed

a b cSample COI )63 20–63 2–20 -2

V 6 88 4 2 01V 14 79 6 1 02V 15 77 7 1 03V 6 70 12 10 24

aLetter V denotes mineral vermiculite. Indexes denote different collecting sites.bTotal percentage of carbonatesqorganicsqiron oxides and hydroxides.c Ž .Wt.% less than 0.4 except V .4


Table 2Ž . Ž . Ž .Mineralogical wt.% and oxygen isotope ‰ composition of separated size fractions in mm of

the samples analyzed18Sample Size BrV BrS ChrV VrS B V Ch Tc d O

V )63 4.98120–63 5.22

Ž .2–20 A 80 I L T I T T T 5.59Ž .-2 A 10 L L A T 13.68

V )63 6.00220–63 5.92

Ž .2–20 A 50 L L L 6.78Ž .-2 A 15 L L A 17.91Ž .V 2–20 A 50 L L L3Ž .-2 A 10 L L A

V 20–63 10.464Ž .2–20 A 85 T I T T 11.12Ž .-2 A 70 T A T 17.58

BrV s biotitervermiculite; BrS s biotitersmectite; ChrV s chloritervermiculite; VrS svermiculiter smectite; Bsbiotite; Vs vermiculite; ChsMg-rich chlorite; Tcs talc. Mineralo-

Ž .gical composition wt.% : A )40%; Is20–40%; L s5–20%; T-5%. Numbers in parenthesesdenote percentage biotite in mixed-layer biotitervermiculite.

decreases, the amount of vermiculite percentage in the mixed-layerbiotitervermiculite phase increases. The same trend is noticed for smectite inthe mixed-layer phases of smectitervermiculite and biotitersmectite. In de-creasing abundance, the following mixed-layer and discrete phases were identi-fied.

Ž .- Mixed-layer biotitertrioctahedral vermiculite known as hydrobiotite- Mixed-layer biotitersmectite

Ž- Mixed-layer trioctahedral chloritertrioctahedral vermiculite known as cor-.rensite

- Mixed-layer vermiculitersmectite- Traces of discrete biotite, vermiculite, chlorite, and talc in some fractions

Ž .According to calculated patterns of Reynolds 1980 , all the above mixed-layerphases are regularly interstratified. Well-ordered hydrobiotites consist of aregular alternation of biotite and vermiculite layers. Hydrobiotites are consideredhigh-temperature alteration products of biotite andror phlogopite in which the1:1 stacking sequence represents the attainment of homogenous equilibrium at

Ž .the time of alteration Brindley et al., 1983 .

( )4.3. Differential Thermal Analysis DTA

Dehydration curves of discrete vermiculites show a large water loss belowabout 1008C and a gradual loss from this temperature to about 8508C, where


Ž .Fig. 2. Representative XRD patterns of two fractions sample V of the vermiculites analyzed.1Ž . Ž .a,c Air-dried; b,d ethylene-glycolated. Symbols as in Table 2.

dehydration is substantially complete. Natural discrete vermiculites show twolarge endothermic reactions with peaks at about 150–2008C and 250–2758C,

Ž .respectively Grim, 1968 . These reactions vary depending on the relativeŽhumidity and on the degree of interstratification with other phyllosilicates i.e.,

.biotite .


Dehydration and Differential Thermal Analysis was performed on four vermi-culitic mixed-layer samples and their curves are shown in Figs. 3 and 4. Themixed-layer biotitervermiculite of our study shows the large water loss at about1208C. Additionally, the curves of samples V , V , and V show another flexure,1 2 3indicating more rapid dehydration in the vicinity of 2208C. The curve of sampleV presents this second rapid water loss at about 5508C. The DTA curves of the4samples V , V , and V show large endothermic peaks at about 125–1358C1 2 3together with smaller ones at 210–2308C. The DTA curve of sample V shows4the first two endothermic peaks at 100 and 1808C, respectively. A third very

Žbroad one is shown at about 6108C. These values are below those 150–2008C.and 250–2758C, respectively reported for pure magnesium-saturated vermi-

culites. This effect is probably due to the high micarvermiculite interstratifica-Ž .tion Justo et al., 1989 . The destruction of the silicate structure of the

Ž .mixed-layer phase studied takes place at about 8408C third endothermic peak .The two endothermic effects at low temperature and the high temperature effects

Fig. 3. Dehydration curves of the vermiculitic mixed-layer phases.


Ž .Fig. 4. Differential thermal analysis DTA curves of the vermiculitic mixed-layer phases.

Žin the region of 800–9008C are characteristic of Mg-vermiculites Mackenzie,.1970 .

( )4.4. Electron Probe Micro Analysis EPMA and Scanning Electron Microscope( )SEM study

The combined EPMA and SEM investigation of vermiculite samples revealeda complex patchy appearance illustrated in Fig. 5a–c, where positions ofselected analysis points are also indicated. Numbers correspond with analyses inTable 3 and refer to the sample V . Additionally, representative analyses of1vermiculites of the other three samples are listed in Table 4.

The electron microprobe analyses confirmed the presence of the mixed-layerphase of biotitervermiculite. An SEM study in the backscattered electron image

Ž . Ž .mode BEI Fig. 5 confirmed the assumption that vermiculitization can beinterpreted in terms of a layer-by-layer transformation from the original biotite

Ž .to vermiculite see also Ruiz-Amil et al., 1993 .The structural formulae were calculated on the basis of 22 oxygens on the

assumption that the octahedral sites were completely occupied. If the sum of


Ž . Ž .Fig. 5. a–c Backscattered electron images BEI of mixed-layer biotitervermiculite crystals fromŽ .site V . The lighter areas represent concentrations of K partly altered mica . Numbers correspond1

with analyses in Table 3.


Table 3Representative electron microprobe analyses of biotitervermiculite mixed-layera from sample V1

1 2 3 4 5 6 7 8

SiO 39.82 37.79 39.97 40.26 39.11 37.81 35.87 36.902TiO 0.68 0.56 0.70 0.65 0.49 0.47 0.59 0.512Al O 14.59 14.02 14.99 14.79 14.36 13.90 13.45 13.492 3Cr O 0.29 0.00 0.12 0.25 0.14 0.22 0.17 0.152 3

bFe O 9.22 8.50 8.50 8.16 8.34 8.50 7.76 8.542 3MnO 0.08 0.00 0.27 0.14 0.00 0.20 0.13 0.00MgO 18.85 19.09 20.98 21.19 21.53 21.90 21.72 21.61NiO 0.22 0.04 0.00 0.18 0.30 0.00 0.34 0.05CaO 0.18 0.11 0.13 0.17 0.08 0.25 0.20 0.15Na O 0.00 0.00 0.00 0.00 0.00 0.00 0.12 0.222K O 9.72 7.12 5.88 5.30 3.51 1.80 0.69 0.292Total 93.35 87.23 91.54 91.09 87.86 85.05 81.04 81.91

[ ]Cations on the basis of 22 O atomsSi 5.766 5.775 5.762 5.805 5.790 5.737 5.681 5.755

Ž .Al IV 2.234 2.225 2.238 2.195 2.210 2.263 2.319 2.245Z 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Ž .Al VI 0.257 0.301 0.309 0.320 0.297 0.224 0.192 0.236Ti 0.074 0.064 0.076 0.071 0.055 0.054 0.070 0.060Cr 0.033 – 0.014 0.028 0.017 0.026 0.022 0.019

3qFe 1.005 0.978 0.922 0.886 0.929 0.971 0.925 1.003Mn 0.010 – 0.033 0.017 – 0.026 0.017 0.000Ni 0.025 0.005 – 0.021 0.036 0.041 0.044 0.007Mg 4.068 4.348 4.507 4.553 4.666 4.658 4.730 4.675Ž .S VI 5.472 5.696 5.861 5.896 6.000 6.000 6.000 6.000

Mg – – – – 0.085 0.295 0.397 0.348Ca 0.028 0.018 0.020 0.026 0.012 0.041 0.034 0.024Na – – – – – – 0.036 0.067K 1.796 1.388 1.081 0.976 0.664 0.348 0.139 0.058Ž .S Int. 1.824 1.406 1.101 1.002 0.761 0.684 0.606 0.497

aSee Fig. 5 for positions of analyses.bIron of the probe analysis is assumed to be in the Fe3q state.

octahedral cations exceeded 6, the excess Mg contents were treated as interlayercations. Vermiculites, like trioctahedral smectites, consist of talc-like layers inwhich the deficiency of the positive charge is compensated by the presence ofsome interlayer cations. The most common cations in smectite are Na and Caand sometimes Mg. In vermiculites, a larger charge deficiency is causedprincipally by tetrahedral substitution of Al or Fe3q for Si and is generallycompensated by about 0.7 divalent cations, or their equivalent, between thelayers. In natural samples, these are most commonly Mg ions, though Ca andvery rarely Na also occur. The structural formula of a standard vermiculite may

Ž . Ž . Ž . Ž .be written approximately as follows: Si,Al Mg,Al,Fe O OH Mg H O ,4 3 10 2 x 2 n


Table 4Representative electron microprobe analyses of biotitervermiculite mixed-layer from samples V2Ž . Ž . Ž .1–3 , V 4,5 , V 6–93 4

1 2 3 4 5 6 7 8 9

SiO 33.90 35.86 35.50 37.87 36.72 39.57 38.53 38.52 36.112TiO 0.92 1.10 1.06 1.10 0.91 1.76 1.72 1.54 1.292Al O 11.25 11.70 12.06 12.54 12.39 16.99 16.84 17.17 17.142 3Cr O 0.12 0.09 0.04 0.16 0.09 0.33 0.44 0.33 0.402 3

aFe O 9.90 10.51 9.83 10.08 10.50 14.11 14.15 13.91 13.222 3MnO 0.09 0.19 0.20 0.04 0.02 0.11 0 0.02 0.16MgO 17.35 21.41 21.88 19.79 22.27 13.88 13.51 12.92 14.16NiO 0.31 0 0.36 0 0.12 0.13 0.06 0.04 0CaO 0.34 0.78 0.27 0.16 0.36 0.59 0.70 0.93 1.04Na O 1.54 0.26 0.26 0 0 0 0 0 02K O 0.54 0.32 0.20 0.10 0.19 5.87 4.73 4.25 3.532Total 76.26 82.22 81.66 81.84 83.57 93.34 90.68 89.63 87.05

[ ]Cations on the basis of 22 O atomsSi 5.964 5.825 5.780 6.077 5.831 5.876 5.864 5.902 5.695

Ž .Al IV 2.036 2.175 2.220 1.923 2.169 2.124 2.136 2.098 2.305Z 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Ž .Al VI 0.296 0.065 0.095 1.448 0.151 0.851 0.886 1.003 0.881Ti 0.122 0.135 0.130 0.133 0.109 0.196 0.197 0.177 0.153Cr 0.016 0.011 0.005 0.021 0.011 0.039 0.053 0.040 0.050

3qFe 1.310 1.285 1.205 1.217 1.255 1.577 1.621 1.604 1.570Mn 0.013 0.026 0.028 0.005 0.002 0.013 0 0.002 0.022Ni 0.044 0 0.047 0 0.015 0.015 0.008 0.004 0Mg 4.299 4.478 4.490 4.176 4.457 3.074 3.064 2.950 3.324Ž .S VI 6.000 6.000 6.000 6.000 6.000 5.765 5.829 5.780 6.000

Mg 0.252 0.705 0.822 0.557 0.815 – – – 0.004Ca 0.064 0.136 0.047 0.027 0.061 0.095 0.114 0.153 0.176Na 0.526 0.081 0.081 0 0 0 0 0 0K 0.121 0.066 0.042 0.021 0.039 1.112 0.919 0.831 0.710Ž .S Int. 0.963 0.988 0.992 0.605 0.915 1.207 1.033 0.984 0.890

aIron of the probe analysis is assumed to be in the Fe3q state.

with 0.9)x)0.6, where xs layer charge per formula unit. The ranges ofsubstitution which occur in most natural vermiculites are indicated by the

Ž .following formula Deer et al., 1996 .

The studied vermiculites present AlIV ) two atoms. The total charge ofŽ .interlayer cations including the excess of Mg ranges from 0.43 to 0.93 and is

almost equal to the total charge of octahedral and tetrahedral layers.


From the data of Table 3, it is clear that the transformation of biotite tovermiculite proceeds slowly and results from the gradual leaching of K from theinitial biotite. This is compensated by the increase of Mg in octahedral sites and

Ž .of H O in interlayer ones low total in the analysis . Chemically, the studied2mixed-layer vermiculites may be characterized as Mg-vermiculites with verylow Ca and Na content. In the coordinates Al–Fe–Mg shown in Fig. 6, the

Ž .studied vermiculites from the three sites V , V , and V plot in the field of1 2 3Ž .trioctahedral vermiculites Foster, 1963 , while those from site V are outside4

from this field due to their higher Fe and Al contents.

4.5. Isotopic analysis

Isotopic compositions of detrital minerals can be used as indicators of theirprovenance. Unaltered gabbros and basalts as well as ultramafics have verysimilar d18O values ranging from q5.0 to 8.0‰, while massive ophiolite

Fig. 6. Differences in chemical composition of the studied vermiculites in the system Al–Fe3q –MgŽ 3q . Ž .total iron expressed in Fe . The field of trioctahedral vermiculites Foster, 1963 is also givenfor comparison.


18 Žcomplexes and altered seafloor basalts present a tendency to O enrichment up. Ž .to about q15‰ Cox et al., 1979 . The oxygen in the clay minerals is richer in

18O than those of nearly all the igneous and metamorphic rocks. Hassanipak andŽ . 18Eslinger 1985 found that the d O values of the residual Georgia kaolins

Ž .fraction -2 mm range from q18.5 to 23.1‰. Tsirambides and MichailidisŽ .1990 , studying the residual kaolins of Leukogia, Macedonia, Greece, foundthat their d18O values range from q14.5 to 18.7‰. The above residual kaolinswere collected from outcrops in regions of temperate climate and were probablyformed in freshwater environments.

Unweathered minerals in soil profiles developed on igneous rocks undergo noappreciable oxygen isotope exchange with meteoric water in the weathering

Ž . 18 16environment Lawrence and Taylor, 1972 . Thus, Or O ratios may be used todistinguish authigenic from detrital mineral phases because the second originat-ing from igneous and metamorphic rocks present lower such ratios than most

Ž .minerals formed at low temperatures Hoefs, 1980 .The isotopic composition of separates of some samples examined is shown in

Table 2. The most significant trend is the increase in d18O values as the grainsize decreases. The three coarser fractions of V and V , enriched in mixed-layer1 2biotitervermiculite, have values of d18O less than q6.8‰. The almost doublevalues of d18O of sample V are due to the different compositions of its4fractions. The low d18O value of the coarsest vermiculitic fractions, which arericher in biotite, are indicative of an igneous intrusion influence in the adjacent

Ž . Ž .area i.e., Arnea granite apophyses . However, the clay fraction -2 mm of allthe vermiculite-rich samples presents d18O values between q13.68 andq17.91‰ which are within the expected range of supergene vermiculites. Ouroxygen isotope data agree almost completely with those mentioned by TaylorŽ . Ž .1968 . Savin and Epstein 1970 suggest that the isotopic composition ofambient water and not of parent rock, is the primary factor in determining theisotopic composition of the weathering product. Thus, during the formation ofthe vermiculite, isotopic equilibration with the ambient water was approached.

5. Discussion

Extensive metasomatic zones of vermiculite, tremoliteqchlorite, and talchave been developed at the contacts of the serpentinized ultramafic bodies andthe hosted gneisses. These zones are the result of a hydrothermal metasomaticreplacement of serpentine by magmatic or metamorphic fluids infiltrated along

Ž .the contacts of ultramafic and gneissic rocks. Dabitzias and Kougoulis 1994attributed the micatization phenomena and the development of the metasomaticzones to metamorphic fluids related to the late Jurassic–Cretaceous metamor-phic episode, which affected the Vertiskos unit.


wThe same zoning pattern, however, ultramafic–talc–actinolite–chlorite–bio-Ž .xtite or vermiculiteqchlorite is usually found at the contact of an ultramafic

Ž .rock with an acid magmatic intrusion Kuzvart, 1984 . Thus, the intrusion of theLate Jurassic granite bodies in the area might be responsible for the development

Ž .of these zones. Zhelyaskova-Panayotova et al. 1993 , using the K–Ar method,found that the micatization in the Askos district took place 104–105 my agoŽ .during the Cretaceous period .

The presence of biotite andror phlogopite in close intergrowth with vermi-culite suggests that vermiculite was formed by hydrothermal reaction or super-gene alteration from precursor micas. In the case of hydrothermal formation, thetemperature of the phlogopite to vermiculite transformation should not haveexceeded 200–3008C, the upper stability limit of vermiculite under hydrother-

Ž .mal conditions Komarneni and Roy, 1981; Justo et al., 1987 . However, in ourcase, the material is mostly a mixed-layer biotitervermiculite. Vermiculitizationof micas had been the result of later weathering. Water moving downwards wasvery important for this process. Trioctahedral micas in basic igneous rocks arenot stable under earth-surface conditions. They weather rapidly, mainly byprogressive hydration of interlayer cations.

Weathering profiles, in which vermiculitic material is dominant, develop onbiotite-rich metamorphic rocks, where in their )75 mm fraction, the following

Ž .sequence was observed Wilson, 1970 .

biotite™biotitervermiculite™chloritervermiculite™vermiculite

Whereas mixed-layer BrV and vermiculite are present in sand and silt sizefractions, VrS and smectite may be the dominant phases in the clay fractions.VrS is very abundant at the -2 mm fractions of most samples.

Ž .As stated also by Weaver 1989 , the alteration of basic igneous rocks mayprovide single mixed-layer phases of vermiculite with micas andror chloritewhen the chemical weathering is reasonably mild or slow. The following is themineral transformation that is commonly observed.

chlorite™chloritervermiculite™vermiculite™smectite

The weathering sequence in both cases moves from left to right withincreasing intensity of weathering, increase in age, and from bottom to the topof a soil profile.

Ž .Dubinska and Wiewiora 1988 concluded that phyllosilicates may be alteredthrough the opening of the mica structure to permit formation of vermiculite andminor amounts of interstratified vermiculiterchlorite.

Corrensites and chloriterexpandable minerals may be formed by the oxida-tive weathering of chlorite in low pH soil environments. The result is theremoval of alternate brucite-like interlayers from the chlorite structure. Theoxidation of Fe2q lowers the silicate layer charge to that of vermiculite. If mostof the Fe is contained in the silicate layer, chlorite is transformed into corrensite.


Alternatively, an Fe-rich hydroxide layer is quickly oxidized and removed byŽ .the acidic solution to produce vermiculite or smectite Reynolds, 1988 . Mixed-

Ž .layer chloritervermiculite corrensite , although formed commonly by weather-ing of basic igneous rocks, may be formed additionally by hydrothermal activityon basaltic or ophiolitic rocks with a temperature range of 100 to 3008CŽ .Kristmannsdottir, 1978; Evarts and Schiffman, 1983 .

Mica and chlorite usually develop separately in environments with differingconcentrations of Kq and Mg2q ions, respectively. Rocks rich in micasŽ . Žultramafics in our case were probably formed close to a source of K Arnea

.granite, apophyses of which extend till Askos , whereas chlorite was theessential mineral in rocks close to serpentinite.

Thus, the formation of vermiculite from the alteration of the meta-ultramaficrocks at Askos, by both hydrothermal and weathering processes, is evident. Itcan be assumed that the altering fluids passed through internal crystal fracturesand cracks of primary minerals of the ultramafic bodies. Hydrothermal activityis also responsible for the partial formation of corrensite. Consequently, watermoving downwards was very important for the formation of the vermiculite andother clay mineral mixed-layer phases through the alteration of micas andchlorite.

The low relief and the long-lasting tectonic stability in the Askos district wereessential for the significant thickness of the vermiculitic zones. Additionally, theextended presence of mixed-layer phases, as well as the predominance of

Ž .sand-sized grains flakes in the vermiculites studied, confirms low intensityleaching. The weathering processes, which prevail under cool temperate climaticconditions, basically are responsible for the formation of the altered 2:1 clay

Ž .minerals, dominated by vermiculite mixed-layers Chamley, 1989 .

5.1. Quality eÕaluation

The applications of vermiculites in various fields of industry are closelyrelated to their structure, composition, and physico-chemical properties. Onrapid heating to about 8708C, the contained water evaporates and exfoliates thevermiculite resulting in an 8- to 12-fold expansion and a decrease in density

3 Ž .from 640–960 to 56–192 kgrm Harben, 1992 . Consisting of 90% trappedair, expanded vermiculite is lightweight, presents good thermal and acousticinsulation, high refractoriness, and is chemically inert. Depending on the finaluse, its quality demands include particle size, exfoliation efficiency, and densityafter exfoliation.

ŽThe degree of expansibility of the studied mixed-layer vermiculites is 7 at. Ž .8708C according to Dabitzias and Perdikatsis 1991 . Their expansibility is

Ž .probably higher at even higher temperatures. Justo et al. 1989 found that purevermiculites present lower expansibilities relative to interstratified ones withmicas. The thermal expansion of vermiculites although mainly related to loss of


water, is dependent on chemical composition, loss of OH groups, and mineralog-ical composition of the impure phases.

The mineralogical, DTA, and expansibility characteristics of the Askosvermiculites indicate that they have properties desirable in certain building,agriculture, and horticulture products.

Acknowledgements

A. Tsirambides is indebted to S. Savin for providing hospitality during hissabbatical at the Department of Geological Sciences, Case Western ReserveUniversity, Cleveland, Ohio, where most of the analytical work was carried out.We express our gratitude to L. Lumbert, assistant to the above Dept., whothoroughly checked and improved the English of the manuscript. The manuscriptbenefited from reviews by A. Justo, D.D. Eberl, and C.C. Harvey to whom weare very grateful.

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