997

TheCanatlian Mineralo gist Vo].37, pp. 997-1006 (1999)

THESTRUCTURAL FORMULA OF TALCFROM THE TRIMOUNS DEPOSIT, PYRENEES.FRANCE

FRANQOIS MARTIN$ ANDPIERRE MICOUD

Inboratoire de Mindralogie,Universitd Paul Sabatier-Toulouse III, UMR 5563du CNRS,39, Alldes Jules Guesde, F-31000 Toulouse. France

LUC DELMOTTE, CLAIRE MARICHAL ANDRONAN LE DRED LaboratoireMatdrieux Mindraux, URA 0428, CNRS, Ecole Nationale Supdrieure de Chimiede Mulhouse, j, rueAlfred Werner,F-68093 Mulhouse Ceder, France

PHILIPPE DE PARSEVAL Laboratoirede Mindralogie,Universitd Paul Sabatier-Toulouse III, UMR 5563du CNRS,39, All4es Jules Guesde, F-31000 Toulouse. France

ALAIN MARI ktboratoire de Chimiede Coordination,CNRS,205 Route de Narbonne,F-31077 Toulouse Cedex, France

JEAN-POL FORTUNE, STEFANO SALVI ANDDIDIER B6ZIAT Laboratoirede Mindralogie,Universitd Paul Sabatier ToulouseIII, UMR 5563du CNRS,39, All4es Jules Guesde, F-31000 Toulouse, France

OLIVIER GRAUBY Centrede Recherche de la Moddlisationet de la CroissanceCristalline, CNRS, Campus de Luminy,Case 913, F-13288Marseille Cedex 9. France

JOCELYNEFERRET Talc de Luzenac 5.A., BP 1162, F-31036 Toulouse Ceder, France

Assrnacr

A sample of talc (Tl 11) from the Trimouns deposit, in the Pyr6n6es,France, was studied by various spectroscopicmethods (FTIR, NMR, EPR and M

Keywords: talc, tetrahedral site, octahedral site, substitutions, spectroscopy,charge deficiency, polymer, Trimouns, Pyr6n6es

SovuaInB

Un 6chantillon de talc (T111) provenant du gisement de Trimouns (Pyr6n6es,France) a 6t6 6tudi6 en d6tail d l'aide des spectroscopiesFTIR, NMR, EPR et Mijssbauer, du microscope 6lectronique d transmissionet de la diffraction des rayons X afin de quantifier les faibles taux de substitution dans les sites tdtraddriqueset octa6ddques.Les r6sultats monffent que le feuillet du talc Tl 1I est 6lectriquement neutre, avec un d6ficit de charge t6traddrique compens6par un excds de charge octa6drique, ceci 6tant d0 aux nombreusessubstitutions. Le d6ficit local de surface de la couche t6tra6driquepeut expliquer les interactions actives entre le talc, utilis6 comme charge min6ra1e,et les polymdres.

Mots-clis: talc, site tefa6drique, site octa6drique,substitutions, spectroscopies, d6ficit de charge,polymdres, Trimouns, Pyr6n6es.

s E-mail address: [email protected] 998 THE CANADIAN MINERALOGIST

INrnooucrroN due to inclusions of graphite. The total iron content of the talc ore is about 0.6 ! O.2VoFe2O3, but no separate Talc ore is used in paper coating, and its properties iron-bearing phase has been detected. make it useful in the paint, ceramics, and polymer industries. In the automotive industry, talc is added to ANarvrrcer MBrnoos polymers to stabilize and harden parts of automobiles like fenders, dashboard,and steering wheel. However, Chemical analyses various problems appear with these materials, notably an undesirable coloring in ceramics and polymers Whole-rock analysis were performed by Inductively (blush), and variable behavior in polymer applications. Coupled Plasma (ICP) at the CRPG in Nancy. The To improve the use of various types of talc in specific chemical composition of the talc was determined using industrial applications, it is necessary to take into a CamecaSX-50 electron microprobe at the Universitd account the physical and chemical propertiesofthe talc. Paul Sabatier in Toulouse. Operating conditions were In this paper, we characterizethe chemical composition 15 kV and 10 nA, and natural and synthetic standards of a sample of pure talc (T111) from the Trimouns were used. deposit,the largestofthe world. The resultsofthis study X-ray dffiaction (XRD) improve our knowledge of the extent of substitutionsin talc from this deposit by detailed characterization of The talc from the Trimouns deposit was analyzedby element distributions (Fe2+,Fe3+, Al, Mn2+, F), using X-ray diffraction (XRD). The spectrawere collected in variousspectroscopic techniques. reflection mode using a Philips PWl130 diffractometer with Ni-filtered CuKct radiation (30 kV, 30 mA). The BecrcnouNo lNpouaarron spectrawere recorded for 3.5 s per step of 0.0005" from 2' to 80' (e). With this approach,the limit of detection Most samples of talc (2:7 layer silicate) typically of a phasewas establishedat0.5Vo. have a near-end-member composition [Mg3Si4O1e (OH)zl, and in many casesno interlayer exists because Fourier transform infrared spectroscopy( FTIR) the 2:7 layer is electrostatically neutral. However, ma- j or amounts of Fe, minor amounts of Al and F, and faces Fourier transform infrared (FTIR) spectra were of Mn, Ti, Cr, Ni, Na and K have been reported in talc recorded in transmission mode in the 4000-400 cm 1 (Heller-Kallai & Rozenson 1981, Noack et al. 1986, range on a Nicolet 510 FTIR spectrometer.The pellets Abercrombie et al. 198'7, Aramt et al. 1989, Coey et al. were prepared by mixing 4 mg sample with 300 mg L997, de Parsevalet al. 1991,1993). These authors KBr. documented the various potential substitutions in the tetrahedral(Si replacedby Fe3+,Al) and octahedralsites s7Fe Mdssbauer spectroscopy (Mg replaced by Fe2*, Fe3*, A1), but did not establish charge balance of these natural samplesof talc. A 57FeMossbauer absorption spectnrm was recorded over the range!4 mm/s with 512 channelsin the Labo- GBorocrcer- Coxrsxr ratoire de Chimie de Coordination in Toulouse. The Mrissbauer spectrometeris composed of a compact de- The Trimouns talc and chlorite deposit is located rn tector "y-systemfor high count-ratesand a conventional the French Pyr6n6es, 100 km south of Toulouse, at an constant-accelerationMtissbauer device (WISSEL). A 57Co altitude of 1,700 m. It occurs along the eastem side of 1in Rh) source with nominal activity of 50 mCi the Hercynian Saint Barth6l6my Massif of gneissic was used. Powders were finely ground under acetone rocks. The mode of formation of the talc and chlorite rn (to minimize possible oxidation of Fe) and placed in a the deposit is now well established(Fortun6 et a|.7980, plexiglass sample holder. The spectrawere obtained at Moine et al. 1982, 1989, de Parsevalet al. 1993). These room temperature, 80 K and 4 K, and, were recorded minerals originated by hydrothermal alteration of with a Canberra multichannel analyzer, coupled to a wallrock in a zone of intenseshearing between the Saint computer. The isomer shift was recorded with respect Barth6l6my dome and the low-grade Paleozoic meta- to cr-Fe metal. As advocatedby Rancourt et al. (1993), morphic cover. Dolostones of the Paleozoic cover were the absorber thickness ofthe talc sample was approxi- transformed to talc, whereas siliceous and aluminous mated to that of micas in the phlogopite-annite series; rocks (mica-bearing schists and granitic pegmatites) in particular, phlogopite has a Fe content similar to that were completely transformed to chlorite-dominant ore of talc T1 I l. This was done to minimize the width of characterizedby well-defined metasomatic zones. The the absorptionlines in the^spectra.The values are around transformation of dolostone to talc involved addition of 200 mg of mineral per cm'. Lorentzian line-shapeswere Si, leaching of Ca, and loss of CO2during the alteration. assumedfor deconvolutions.based on least-squares The talc ore exhibits facies that vary in color from pure fitting procedures.The r!2 and misfrt values were used white to grey and to black, the latter being principally to measurethe goodnessof the computer fit. THE STRUCTURAL FORMULA OF TALC FROM TRIMOUNS, FRANCE 999

Electron paramagnetic resonance( EPR) spectroscopy TABLE 2 CHEMICAT COMPOSITIONO}- TALC TI I ]

All EPR spectrawere recorded at room temperature with a Bruker ESP-300e spectrometeroperating at AlrO: 009 019 X-band frequency (9 GHz). A mercury lamp was used Fe2OrG) 059 068 as UV radiation source (200 W), iradiating the ESR MnO o02 fr 023 Mco 3t 1l 3152 TI cavity through the medium of a quaftz optical fiber. This CaO 002 002 4 60 4.84 procedureallowed us to follow the evolution ofthe ESR Na2O 001 Tr 9961 99 03 on l0 points spectrum of the sample during the UV irradiation. ICP : lnductively Coupled Plasma

Nuclear magnetic resonance( NM R) spectroscopy Rssur-rs The solid-state nuclear magnetic resonanceexperr- ments were performed as follows: 29SiNMR measure- Chemicnlcomposition ments were carried out on a Bruker MSL-30 '7.1 spectrometeroperated atBo - T (u6= 59.63 MHz) The talc sample was selected from a vein of pure with B0 the magnetic field. The NMR spectra of 27Al talc; it is representativeof ore in the Trimouns deposit, and leF were recorded on a Bruker DSX-400 spectrom- and was carefully hand-picked. Its chemical composr- eter operating at B0 = 9.4 T (vo= 104.263 and 376.49 tion (Table 2) indicates a virtually monominerallc MHz, respectively).Single-pulse magic-angle spinning sample, with no accessoryminerals and no Fe-bearins (MAS) spectra were obtained using a Bruker MAS minerals, such as sulfides. probe with a 7 mm Zro2rotor for the 2eSiexperiment and a 4 mm rotor for the 27Al and l9F experiments. XRD Acquisitions parameters are summarized in Table 1. Chemical shifts of ze5i, zr41 and 1eFwere externally The powder-diffraction pattern is characteristic of compared to TMS (tetramethylsilane), CFCI3 (trichlo- pure talc; no other phases (> 0.5Va) were detected rofluoromethane), and aqueous Al(H:O)o3+. respec- (Fig. 1). The powder pattem exhibits sharp hkl rcflec- tively. The deconvolutions were carried out with tions. indicative of a well-ordered three-dimensional WINFIT (Bruker program) by using the Gaussian structure, with coherency of domains on the scale of model. 1100A along the c* axis (Fig. l).

FTIR TABLE I NMR ACQL,TSITION PARAMETERS FOR TALC T111 FTIR spectroscopyis a very sensitive probe of the --Sl t'Al possible talc-chlorite association,especially in the OH-

Spiming speed (kHz) 41210 stretchingregion; three vibration bands are expectedfor chlorite, and only one for talc. According to Farmer Pulse length (flip mgle) 3 25 (n/4) 3 6 (n/2) 1 ps (n/12) tLs t\s (1958), Vedder (1964), Farmer & Russell (1967), Recycle delay zffi lOs ls Farmer et al. (1968), Ishii er al. (1967) and Grauby et al. (1991), the vibrations ofpure talc are as follows: 1) 11andv2for Si-O stretching vibrations at 1040 and 687 cm-r, respectively,2) 4 for Si-O-Si stretching vibra- H i gh- r eso lution transmiss ion ele c t ron mic ro sc opy tions at 1014 cm-r, 3) v4 for Si-O-Si bending vibra- (HRTEM) tions around 460 cm r, 4; rr5for M-OH L (octahedral

For observationsusing high-resolution transmission electron microscopy, slices of the specimen containing optically selectedareas were removed from thin sections and ion-thinned with an argon beam following the pro- cedure described by Amouric & Parron (1985). Elec- tron microscopy was performed at Centre de Recherche de la Mod6lisation et de Ia Croissance Cristalline (CRMC2, Marseille, France) with a JEOL-2000 FX instrument, equipped with an energy-dispersionX-ray spectrometerfor chemical micro-analysis. The micro- scope was operated at 200 kV for HRTEM imaging, 15 14 13 12 11 l0 9^ 8 7 6 (A) electron diffraction and micro-analysis (Baronnet e/ d al. 1993). Frc. 1. X-ray-diffraction pattern of talc Tl I 1 1000

3660 cm-t. It is invariably observed in pure talc, with an intensity depending on the extent of substitutions in the octahedral sites (F. Martin and S. Petit, in prep.). This band is not observed in chlorite-group minerals.

The 1200-300 cm-t region

In this complex region, the spectrum shows well- defined absorption bands characteristic of pure talc (Fig. 3), with u2 at 668 cm I and 6OH at 685 cm r. No vibration bands of chlorite are detected.

3700 3690 3680 36'70 3660 3650 3640 HRTEM Warenumbers(cm-r) Figures 4a and 4b show the electron-diffraction pat- Frc 2 Infraredspecrra oftalc T1 I 1 in the stretchingregion. tem and the lattice image of massivetalc Tl11 from the Trimouns deposit. The diffraction pattem corresponds to the one-layer triclinic structure, with the c* axis per- oxygen atoms and hydroxyl groups, vibrations of octa- pendicular to the electron-diffraction image. The lattice hedra moving in opposition to octahedrally coordinated image consists of a single set of lattice fringes of pure cations) at 535 cm 1, 5) v6 for M-OH I (octahedrally talc that is well correlated with the diffraction pattern. coordinatedcations moving in phasewith their adjacent The stacking faults observed were induced durins sheetsof oxygen atoms, relative to sheetsof silicon and sample preparation. surfaceoxygen atoms) at 259 cm 1,6) u7 for trans (OH) // (translatoryvibration ofhydroxyl groups,out ofphase Mtissbauer spectra with the other oxygen atoms in this sheet) at 465 cm r, and 7) bOH for OH librations at 669 cm 1. Table 3 lists the possible site-occupanciesin talc and a listing of the D and A values determined on various The OH-stretching region (3800-2500 cm-t) minerals by Rancourt et al. (1992, 1994b) and Rancourt (1994b), and taking into account the quadrupole split- The vibrational spectrum(Fig. 2) conespondsto that ting distributions (Rancourt 1994a). of pure talc and is very simple in this wavenumber Ideal talc is conventionally representedby the for- range. It displays only one sharp band located at mula Mg3SiaOro(OH)2,with Fe replacing Mg in the 36'76cm-r, indicating an extremely regular environment octahedral sites M

talc Tl l I

v1 Arv+

Ms-(OH),,'u

6OH V5 Mg-(oH)

1200 1100 1000 900 800 100 600 Wavenumbers(cm-r)

Frc. 3. Infrared spectra oftalc Tl 11 in the 1200-400 cm I region. THE STRUCTURAL FORMULA OF TALC FROM TRIMOUNS. FRANCE 1001

Ftc.4. a. Electron-diffractionpattern of massivetalc T11l b Lattice image of massive talc Tl I1 Hieh-resoiutlon transmission electron microscopv.

Aramu e/ al. 1989, Coey et al. 1991). These authors The deconvolution in Lorentzian doublets (Fig. 5b) is have proposed a single doublet for octahedrally coordi- made with two components: one according to Fe" nated Fez+.Recent studies (Rancourt et al. 1992,7993, in octahedral sites, with a large quadrupole splitting 1994b, Rancourt (1994a, b) have demonstratedthat (Table 4), and one inside the first absorption band, with paramagneticstate Mtjssbauer spectroscopy of 2: 1 layer a small quadrupole splitling and a low isomer shift silicates cannot resolve the octahedrally coordinated (Table 4), attributed to Fer+in the tetrahedralsite (Dyar Fe2* in cis (M2) and trans (Ml) sites. Mdssbauer stud- & Burns 1986,Rancourt et a|.1992,1993). No orienta- ies ofthese materialscan neverthelessindicate the over- tion effects are Dresent in soectra (the electric field all proportion and the coordination of Fe2* and Fe3* gradients61lolp.'i+. l+1p.3+ unj lolFe2*have different ori- (Table 3). entatlons) The spectrum of talc Tlll at room temperature The same sample was recorded at 80 K (Fig. 6a) to (Fig. 5a) is similar to spectra obtained for annite by benefit from the second-order Doppler effect. The Rancolrrtet al. (1994).lt shows a difference in intensity deconvolution (Fig. 6b) is made with various contri- between the two absorption bands, and an important butions corresponding to those obtained at room tem- shoulder on the side of the peak located at -0.5 mm/s. perature, and with two doublets whose Mossbauer parameters correspond to Fe3* in octahedral sites (Blaauw et al. 1980, Dyar & Bums 1986, Rancourt e/ TABLE 3 DIFFERENT STTE-OCCI,?ANCIES OF IRON IN TALC T11I al. 1992,Dyar 1993,Rancourt 1993).The iron is dis- AND THE PHLOGOPITE-ANNITE SERIES AND MOSSBALIER PARAMETERS USING THE OSD MODEL tributed (except for Fe2+in the tetrahedral site) in both sites, Fer+ being the predominant form (687o), indica- Sitc occupmcies Phlogopite* tive of the redox conditions during the formation of talc. 6A In Table 4, we report M

1.001

1.000 - F a a 0.999 a a -t lt a -a 0.998 t 3+ t< lr F [4]Fe F 0.997 0.996 b 0 995- 0 995f -4 -3 -2 -1 0 1 2 -4 -3 -2-101234 Velocity (mm/s) Velocity (mm/s)

Ftc. 5. a. Room-temperature experimental Mtissbauer spectrum of talc T111. b. Calculated spectrum of talc T111 at room temperature.

Solid-state NMR cates, characteristichigh-field shifts are observed with a decreasing number of next-near-neighbor Al atoms zeSi The leF MAS-NMR spectrum of talc Tl11 exhibits (Kinsey et al. 1985). The MAS-NMR spectrumof one resonance at -176.2 ppm (Fig. 7), indicating that talc T111 shows an asymmetricalpeak at -96.8 ppm, only one site is present in the structure (Huve et al. attributed to Q3 species,characteristic of clay minerals. 1992). This signal has already been observed for The shoulder observedat low field can be explained by trioctahedral 2 :l layer silicate and is attributed to increasing incorporation of Al atoms. 27Al F atoms bonded to three atoms of Mg (Mg-Mg-Mg It is well known that MAS-NMR spectroscopy configuration). is a powerful tool with which to distinguish readily and The 2esi isotropic shift is strongly influenced by the unambiguously between four- and six-fold-coordinated chemical environment of the silicon nucleus and is aluminum atoms because of well-separated shifts in highly corelated with the degree of polymerization of range(Barron et al.1985, Herrero et al. l985a,b).For 21 the SiO4tetrahedra (Lippmaa et al. 1980, l98l,Magi et the quantitative aspect of Al NMR, selective condi- al. 1984, Kirkpatrick 1988). Furthermore, in layer sili- tions of excitation are induced by taking very short du-

[6]Fe2* 1.002

a 1.000 a - O 0 998

k F o.996

0 994

0.992

0.990 -4-3-2-1012 -4 -3 -2 -1 0 1 2 Velocity (mm/s) Yelocity (mm/s)

FIc. 6 a- Experimental M

,,Si tnF

176.16

too o -80 _120 -r20 pp- ppm ppm

Frc. 7. Si, F, andAl MAS NMR spectraof talcT1 l l rations of impulse (0.7 ps) with very-weak-field radio- located at g = ), which is assignedto Mn:* in the octa- frequencies(flip angle = n/12). Under theseconditions, hedral site. A possible assignmentof Fer* in the tetra- all the types of aluminum in the sample are excited in hedral or octahedral site is indicated by modeling the an equivalent way. The 27Al MAS-NMR spectrum of EPR powder spectrausing numerical diagonalization of talc Tlll showstwo resonancesat 8.9 and 64.1 ppm, the spin hamiltonian (Morin & Bonnin 1999).Under UV correspondingto Al in octahedral and tetrahedral sites, radiation and aggressivetreatments with water, no evo- respectively.The integratedarea under eachNMR peak lution in ESR soectra is evident. This observation (Fig. 7) show that both t4lAl and t6lAl are present in indicates that the Fe3+content reveals the redox condi- roughly equivalent amounts. tions during the formation of the talc.

Electron paramagnetic resonancespectroscopy Suvlaanv op Specrnoscoplc INvesncatIoNs aNo ELscrnoN-Mrcnopnoee Dera Electron paramagneticresonance spectroscopy is used to detect unpaired electrons, such as in Fel* and XRD, HRTEM and FTIR investigations show that Mn'*. and to characterizetheir chemical environments. talc sample T1 I I is a pure phase.Small amounts of Fe, The EPR spectrum of talc T111 at room temperature Al, Mn, and F are incorporated into the structure. Ac- (Fig. 8) shows a sextet at g = 2.0 characteristicof Mn2+ cording to Mcissbauer,NMR and EPR spectrocopies,it in the octahedral site and a small signal around g = 4 is possible to determine site assignmentsfor these ele- (Table (Goodman & Hall 1994). But at a low temperature ments 5). (4 K), a well-resolvedspectrum is obtained(Fig. 8), with The structural formula obtained by these various two distinct signals,one locatedat g= 4.3, characteris- techniquesand basedon results ofanalyses oftalc T1 I I (taking tic of Fert in the tetrahedral or octahedral site. and one with an electron microprobe into account the standard deviation of each element), according the electroneutrality of the structure and on the basis of (O TABLE 4 MOSSBAIER PARAMETERSOF TALC T1I1 AT ROOM TEMPEMTre AM4K +OH+F)=12,is:

[Mg2 eTsF-e2+s619Mn2*s s01Fe3*o oosAlo oor]:: ISi3e6aFer*0 egaAlo ooz]>+Oro(OH)r sszFo oae

008 040 r47 14) In the tetrahedral sheet, a charge deficit appears: 0.011.In the octahedralsheet, there is a chargeexcess of 0.012. However, it seemsclear that a deficiency in tetrahedralcharges due to the replacementof Si by Fer* and Al is compensatedby an excess in octahedral 1004

' i 1$ '/'t'r-"iii'ilti.'ir

rj- '{!'irY s- 4.3 Room temperature

0 1000 2000 3000 4000 5000 6000 7000

Frc. 8. ESR spectra of talc T1 1I at room temperature and 4 K.

charges generatedby the replacement of Mg by triva- monomineralic and does not contain any Fe-bearing lent cations Fe and A1. phasessuch as sulfides.Using appropriatespectroscoprc techniques,the locations of minor elements can be DrscussroNaNn CoNcr-usroNs determined. The resulting spectral signaturesare characteristic of those obtained in similar clay miner- The XRD, FTIR and HRTEM data obtained on a als. On this basis,we considerall the Al, Fe, Mn, and F carefully hand-picked sample of talc indicate that it is to belong to the structure of the talc. Mossbauer spectroscopyis very useful for a charac- terization of the distribution and valence of Fe in min- TABLE 5 CONTENT AND LOCATION OF VARIOUS ELEMENTS IN in This showsthat Fel+ occurs TETRAIIEDRAL A\D OCTAHEDRAL SITES OI TALC T1 11 erals,specially talc. study Occupmcy of sites* : Content Nomalized contml in the tetrahedral position in talc. Moreover, we report the systematic presence of t4lFe3+(14.7Ed, t6lpe3* Fe2* Fcr' Fer Fer* (17.87a), f6lFe2+ 4t14 6;o nlo and 67.5Vo),with valuesthat are simi- "[67 5] 81 ors; oosl oo+1 117 1o lar to those encounteredin Mcissbauerspectra of annite (Rancourl et al. 1994a). Another major conclusion rs the high [Fe2*/(Fe2*+ Fe3*)] value. approxim ately687a, '[50] oto "[50] 1ooorl oorl indicative of the redox conditions during the formation of talc.

Mn2* The NMR resultsfor Si, Al, and F indicate the nature

0 001 "f0 00ll of substitutions in the structure. For Si. the sDectrum clearly demonstratesa Q3 environment, charaiteristrc of phyllosilicates. The shoulder on the peak indicates F minor replacementof Si by Al or Fe (or both). For the F [o 04E] spectroscoplc anion, the single sharppeak shows the existenceof only Cation contents in atoms per one OH site being substituted,located in the large hex- THE STRUCTURAL FORMULA OF TALC FROM TRIMOUNS, FRANCE 1005 agonal cavity in rings oftetrahedra. But the major result, AMouRrc M. & PennoN, C. (1985): Structure and growth establishedhere for the first time. is the location of Al mechanism of glauconite as seenby high resolution trans- in both octahedral and tetrahedral sites. The Al content mission electron microscopy. Clays Clay Minerals 33, of both sitesis equivalent. 4'73-482. The EPR spectra confirm the presenceof Fe'* and Aneuu, F, Mextrv, G. & DELUNAS,A. (1989): Mijssbauer Mn'* in talc T111. The structural formula is written on spectroscopy of talc minerals Nuovo Cimento llD, the basis of spectroscopicand chemical results. The 891-896. actualchemical composition of sampleT11l indicates S. & KANG,Z.C. (1993):Layer stack- a deficiency in tetrahedral charges and an excess in BARoNNET,A., Ntrscrs, ing microstructures in a biotite single crystal. A combined octahedral charges, due to the various substitutions. HRTEM-AEM sltrdy. Phase Trans. 43, 10'7-128. The deficiency in tetrahedralcharges is compensatedby the excesscharge on the sheet of octahedra.The major BennoN, PF., Sranr, P. & FRosr, R.L. (1985): Ordering of remark concernsthe local deficiency on the sheetof tet- aluminum in tetrahedral sites in mixed-layer 2:1 phyllo- rahedra. This local deficiency createsan active site on silicates by solid-state high-resolution NMR. J. Pftys. the surfaceofthe talc. The local deficiency and the rela- Chem.89.3880-3885. tively high content of Fe2*give a great oxidation poten- Bt-e,cuw, C., SrnorNr, G. & LEIPER,W. (1980): Mossbauer tial to the talc. analysis of talc and chlorite. J. de Phys. l, 411-412. In various industrial applications (e.g, paper manu- facturing, paints, ceramics, polymers), talc is used as a CoEv, J.M.D. (1980): Clay minerals and their transformations filler to reinforce the polymer structure.However, prob- studied with nuclear techniques: the contribution of Atom. Energy Rev. 18,'73-124 lems with aging and photo-oxidative degradation of Miissbauer spectroscopy polymers have been encountered. A change in color Blras. T & Guccr,NnrIu, S. (1991): Mdssbauer appearson the polymer surface upon exposureto solar spectra of minnesotaite and ferrous talc. Am. Mineral.76, radiation for long periods. Previous studieshave shown I 905-1 909. that the presence of fillers may enhance the rate of P., FouRNEs,L., FonruNE, J.-P., MorNs, B. & degradation (Rabello & White 1996). To reduce these ln PnnsrveL, FsnREr, J. (1991): Distribution du fer dans les chlorites par effects, TiO2, carbon black, and HALS (Hindered spectrom6trieM6ssbauer (57Fe):Fe3* dans les chlorites du Amino Light Stabilizer), for example, are added to the gisement de talc-chlorite de Trimouns (Pyr6n6es,France). polymers. These additives exert a real influence, nota- C. R Acad. Sci Paris 312, Sdr. II, 1321-1326. bly on the depth of the degraded layer (Rysavy & Tkadleckova 1992). There is, however, little informa- Mowe, B., FonruN6,J.P. & FBnnBr,J (1993) tion on the interactions among all the components Fluid-mineral interactionsat the origin ofthe Trimouns talc (Pyr6n6es, France). 1n Current present in the polymers. The degradation effects may and chlorite deposit Research in Geology Applied to Ore Deposits (P. Fenoll result from charge deficiencies on surfaceof the fillers, Hach-Ali, J. Torres-Ruiz & F. Gervilla, eds.). Univ. of and especially on the surfaceoftalc, which may contain Granada, Granada, Spain (205-209). active sites. The ultimate goal of this project is to understandthe Dvan. M.D. (1987): A review of Mdssbauerdata on causeof the degradationof talc-bearing polymers, par- trioctahedral micas: evidence for tetrahedral Fe3* and ticularly with the help of spectroscopic evidence. For cation ordering Am. Mineral.72, 102-112. example, in the automotive industry, talc is added to (1993): Mtjssbauer spectroscopy of tetrahedral polymers to stabilize and harden various parts of auto- Fe3+in trioctahedral micas. Discussion. Am Mineral.78, mobile bodies. We hope to understand the role of talc 665-668. during degradationof polymers by natural UV irradia- tion. ESR and NMR spectroscopiesare planned to study & BuRNS,R G. (1986): Mtissbauer spectral study of trioctahedral micas.Am. M ineral. 7 l, free radicals produced during UV iradiation. Thesefree femrginous one-layer 955-965 radicals could be the vector of various types of damage causedin filled polymer structures. FARMER,V C. (1958): The infrared spectraof talc, saponiteand hectorite Mineral. Mag 31,829- 845. AcrnowreocBupNrs & Russnr-r-,J.D. (1967): Infrared absorption 15, Supportfor this researchwas provided by the Soci6td spectrometry in clay studies Clays CIay Minerals Anonyme Talc de Luzenac. We thank the anonymous 12t-t42. reviewers and R.F. Martin for their conshuctive comments. & Anrnrcns.J L. (1968):Characteiza- tion of clay minerals by infrared specEoscopy.Trans. 9th RBpeneNces Int Congr. Soil. Sci.3, 101-110.

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