Canadian Mineralogist Yol. 27, pp. 225-235(1989'1
CHARACTERIZATIONOF LEPIDOLITESBY RAMAN AND INFRAREDSPECTROMETRIES. I. RELATIONSHIPSBETWEEN OH.STRETCHING WAVENUMBERS AND COMPOSITION
JEAN-LOUIS ROBERT, JEAN.MICHEL BENY, CLAIRE BENYANI MARCEL VOLFINGER GroupementScientilique, Centre National de la RechercheScientiJique - Bureau de Recherches Gdologiques et Minidres,1A, ruede la Fdrollerie,45071 Orldans Cedex 2, France
ABSTRACT ephesite).The highestknown v - OH value (3755cm-') correspondsto hydroxyl gxoupsfree of any OH...O inter- action. Four partial solid-solutionsthat affect lepidolite com- positions have been investigated by Raman scattering and infrared absorption spectrometries, in the wavenumber Keywords: Raman scattering, infrared absorption, hy- range of the OH-stretching vibration (y - OH); in every droxyl stretching,lepidolite, syntheticmicas. case, both methods gave the same values, within ex- perimentalerrors. The compositionssynthesized belong to Souuarnn the solid solutions betweenphlogopite and eachof the end members: taeniolite, trilithionite and polylithionite (all purely trioctahedral) and to the j oin tetrasilicic magnesium Quatre solutions solides parlieues de lepidolites hydro- mica - taeniolite, which exhibits a partial dioctahedral xyl€esont dtd 6tudi6espar diffusion Raman et absorption character. On the first three joins, changesin the evolution infrarouge, dans le domaine des vibrations d'dlongation ofspectrareflect the compositionalevolution and especial- desgroupements hydroxyles (z - OH); danstous lescas, les ly the increase in lithium contenU all the bands observed valeursde nombre d'ondes obtenues par lesdeux m6thodes belong to the trioctahedral type. Three subtypesare char- sont les m€mes, aux incertitudes experimentalesprbs. Ces acterized by the bulk number of charges of the three ad- l€pidolites appartiennent aux solutions solides entre la jacent cations: "5-chargebands", dueto OH groupsbond- phlogopite et chacun des p6les l6pidolitiques taeniolite, ed to Mg2Li or Li2Al (range3755-3740 cm-r); "6-charge trilithionite et po$thionite (cesmicas sont tous pruement bands', due to OH groups bonded to Mg3 or AlMgLi trioctaddriques),arnsi qu'au joint mica t6trasiliciquema- (range3740-3715 cm-1) and "7-chargebands", dueto OH gndsien- taeniolite, qui poss€deun caracterepartiellement groupsbonded to Mg2Al or Al2Li (z < 3700cm-l). On the dioctaddrique.Sur lestrois premiersjoints, 1'6volutiondes basis of these observations, a new nomenclature is pro- spectresreflCte l'dvolution des compositions et en par- posed, relevant to any mica. On the join tetrasilicic mag- ticulier I'augmentation de la teneur en lithium; toutes les nesium mica - taeniolite, an additional dioctahedral-type bandesobserv6es sont de type trioctaddrique.On montre band observedat 3595cm-l is dueto OH groupsbonded to que ces bandes appartiennent d trois sous-types, 2Mg and adjacent to an octahedral vacancy. This wave- caractaris€spar le nombre de chargesportees par les trois numberis constantalong the join, indicatingthe exclusion cationsli6s i OH: bandesde type "5 charges",dues i des of Li+ from the proximity ofthese hydroxyl groups. For OH li6s i Mg2Li ou Li2Al (domaine 3755-3740cm-'); all the triocrahedral-type bands, a systematic shift is ob- bandesde type "6 charges",dues d desoH li6s d Mg3 ou servedtoward low wavenumbers,as the bulk aluminum AlMgLi (domaine3740-3715 cm-'), et bandesde type "7 content (Alro) increasesin the mica. This phenomenon, charges"pour lesOH li6sd MgzAl ou Al2Li (<37@ cm-'). already known in phlogopite solid-solutions, is related to Cetteobservation seft de base aune nouvelle nomenclature, changein mica composition and is explained by changesin valable pour toul mica. Sur le joint mica t€trasilicique OH...O interactionsby weak hydrogenbonding between magndsien- taeniolite, on observeen plus, une bande the hydroxyl proton and the apical oxygen atoms of sur- caraderistique d'un environnementdiocta6drique (OH 1i6s rounding tetrahedra. Thesevariations can be expressedby d 2Mg et adjacents d une lacune octa6drique), ir tris bas tlree equations,valid for light elements:(l) z = -9.33Alot nombre d'ondes: 3595cm-'. La constancede ce nombre + 3754 (S-chargebands); (2) / : -l6.95Altot + 3741 d'ondesrlelong dujoint, indiqueque le cationLi + estexclu (6charge bands), and (3) v = -29.66.A{.t + 3719(7
(A1,o)augmente dans le mica. Ceph6nomdne ddji connu hydroxyl-bearing lepidolites were investigated by dansles solutions solides de la phlogopiteest expliqu6 par vibrational spectrometries,Raman scattering and la modificationdes interactions OH...O par liaison infrared absorption. Three of the four lepidolitic hydrogbnefaible, entrele proton et lesatomes d'orygene series investigated here are purely trioctahedral: apicauxdes tdtrabdres, lorsque les compositions 6voluent. phlogopite-polylithionite, phlogopite-trilithionite Trois €quationsrelient les nombres d'ondes aux composi- (Robert Volfinger 1979), phlogopite-taenio- tions par (l) : -9.33A1ro, & and desmicas, exprim6es Alro, z + partial 3754(bandes 5 = -l6.95Atot lite (Robert 1981);the fourth serieshas a i charges);Q) v + 3741 join (bandesd 6 charges); v : -29.66A161+ 3719(bandes i dioctahedral character and belongsto the tetra- Q) - 7 charges).Cette dernidre €quation inclut I'ensembledes silicic magnesianmica taeniolite (Robert 1981).A phlogo- donn6esdisponibles, relatives aux micas potassiques Q€pi- comparison with data available on synthetic doliteset solutionssolides de la phlogopite),ainsi qu'aux pite solid-solutions (Li-free trioctahedral micas) micassodiques (preiswerkite et ephesite).La plus haute @obert & Kodama 1988)also is presented. valeurconnue de z - OH (3755cm-l; a 6te observ6e au cours de cettedernibre €tude; elle correspondd desgroupc SeMpr-ssSruorso hydroxyleslibres de toute interaction OH.,.O. The samplesstudied were prepared by hydrother- Mots-cl€s:diffusion Raman,absorption infrarouge, hy- mal synthesis.The starting products used were gels droxyle,l€pidolite, micas de synthbse. of appropriate compositions. Experimental details are given in Robert & Volfinger (1979). The maxi- INTRoDUCTIoN mum extent of solid solution occurs at low tempera- ture. For this reason, sampleswere obtained at Naturally occurring lepidolites generally have a 500oCand below, at2kbar P(HzO);X-ray-diffrac- high fluorine content; therefore, little is known tion resultsindicate excellentcrystallinity despitethe about the spectroscopicproperties of hydroxyl relativelylow temperatureof synthesis. groupsin this group of micas. The limited infrared On the join phlogopite (Phl) KMg3(Si3AI) absorption data available on the environmentsof Or9(OH)2- taeniolite (Tae) K(MezLi)Si4Oro(OH)2, hydroxyl in lithium micas pertain to lepidolite the range of solid solution is extensive.The most samplescompositionally close to three theoretical Li-rich mica hasthe compositionPhl16-Taees (mole end-members(trilithionite, zinnwaldite, polylithio- 9o) at 400oC (Robert l98l). On this join, lithium nite: Jdrgensen1966), lithian montmorillonites(Cal- micas are systematically accompaniedby a minor to vet&Prost 1971),the raretrioctahedral sodium mica trace amount of quartz (certainly less than 590). ephesiteNa(AlzLiXSizADOr0(OH)2 (Farmer & Thesemicas are characterizedby a variable content Velde 1973),the partial solid solution betweenthe of both the tetrahedral and octahedral layers, but tetrasilicic magnesian mica (TMM) K(Mg2.5 [1 e.5) this lepidolitic series has the simplest octahedral Si4Org(OH)2and taeniolite K(MgzLi)Si4Or0(OH)2 composition,from Mg3in the phlogopiteend-mem- (Robert l98l), and peculiar syntheticlithian micas ber to (Mgzli) in the theoretical taeniolite end-mem- containing Li asan interstitial cation in the interlayer ber. Therefore, the presentationand the discussion space @obert et al. 1983). The salient features of of resultswill start with this join. thesefindings are: (l) the presenceof a high-wave- On the join phlogopite-trilithionite (Tril) number OH-stretching band (z - OH 3750 cm-t), K(Alr.5Lir.5XSi3Al)O1o(OH)2,solid solution extends attributed to OH groups bondedto (Mg2Li), in the to Phla6-Tril66at 500oC(Robert & Volfinger 1979). spectrumof taeniolite-likemicas(Robert 1981),and The peculiar composition Phl, 4-:lrrl24 corresponds (2) the existenceof a low-wavenumber OH-stretch- to the hydroxyl-bearing magnesian equivalent of ing band, around 3600cm-r in the spectrumof ephe- zinnwaldire K(AlMgli)(si3Al)or0(oH)2, bur this site @armer& Velde 1973)and below 3500cm-r in composition yields the assemblagemica * o- the spectrumof high-Li lepidolites(Jdrgensen I 966), eucryptite + kalsilite + sanidine under the ex- attributed to OH groupsbonded to (Alzli). Thus, it perimentalconditions used. The compositionof the is obviousthat the nature ofthe neighboringcations tetrahedral layer remains constant (SirAl) along the and the bulk compositionof the mica influencethe join, but that of the octahedrallayer is complicated OH-stretchingwavenumber, i. e., the O-H bond con- owing to the presenceof the three cations, Al, Mg stant. and Li. The aim of the presentpaper is to identify the On the join phlogopite-polylithionite (Poly) octahedrally coordinated cations bonded to each K(Li2Al)Si4Olo(OlI)2,the range in solid solution type of hydroxyl, with specialattention to lithium, does not exceedthe composition Phlrr-Polyrr at and to analyzethe role of the different crystal-chemi- 500"C and PhlTe-Poly3eat 400oC.The composition cal factors that influence the OH-stretching wave- ofboth tetrahedral and octahedral layersis variable, numbers.For this purpose,four seriesof synthetic and the complexity in the octahedral layer is qualita- LEPIDOLITES 227 tively identical to that along the join phlogopite - proton and apical oxygen atoms of surrounding trilithionite. On the join phlogopite - polylithionite, tetrahedra, controlled by the charge imbalances on the bulk aluminum content remains constant, and the apical oxygenatoms. The OH...Ot interactions only the coordination of aluminum changes from are responsible for the systematically low OH- fourfold in phlogopite and sixfold in polylithionite. stretching wavenumbers in dioctahedral environ- On the join tetrasilicic magnesium mica (TMM) ments,usually below 3630cm-t (Robert & Kodama K(Mg2.5L].5)Si4Oro(OHh- taeniolite, the range in 1988).These OH groups and relatedOH-stretching solid solution is large and can extend to TMM20- bandsbelong to the V-type (V for vacancy),accord- Taq6 at 500'C (Robert l98l). The composition of ing to the nomenclature of Vedder 096/.). the tetrahedrallayer is constant(Si+) along the join. This solid solution exhibits a partial dioctahedral Expr,ntlrapural- METHoD characterowing to the presenceof octahedral vacan- cies. The local relations betweenlithium and vacant spectra were recorded at room octahedral sitesvrill deservespecial attention. Raman scattering temperature,on compactedpowders (average grain- sizeI pm), with a single-channelJobin-Yvon U1000 OH Gnoups rN MrcAs microspectrometerequipped with an argon ion laser (CoherentInnova 90,exciting line \o = 488nm). The The OH group in micas is adjacent to three estimatedaccruacy of measurementsis + I cm-r for cationic octahedral sites. In trioctahedral micas, the high-intensity bands and certainly no better than thesethree sitesare occupiedby a cation (Al, Mg or a 3 cm{ for low-intensity bands and shoulders.In- Li in the lepidolite compositionsinvestigated in this frared (IR) absorption spectra were recorded at work). The O-H vector is nearly perpendicularto room temperaturewith a Perkin-Elmer PE180grat- (001)in most cases,but it can exhibit a higher angle ing spectrometer, using KBr pellets. A high-purity of tilt in lepidolites (Giese 1979,Lin & Guggenheim polystyrenefoil was usedas a standardfor calibra- 1983).Owing to thi$ orientation, the repulsion be- iion. Resolutionwas kept within the range 1-2 cm-r tween the hydroxyl proton and the interlayer cation for all measurements. is strong; the OH...O, interactions between the In every case, OH-stretching wavenumbers ob- hydroxyl proton and the oxygenatoms of surround- tained by Raman and IR are equal within experimen- ing tetrahedra are usually weak (Robert & Kodama tal error, as was previously observedfor triocta- 1988).Therefore, the OH stretchingwavenumber z hedral and dioctahedral Li-free micas (Robert & - OH, which reflects the O-H bond constant fr Kodama 1988).The spectroscopicsource ofdata is (v =kk), is usually high in such environments. not specified in the text, but is indicated in the The bond strength k can be approximated by the figures. bond valence exchanged between the hydroxyl proton and the oxygen of the OH group. Vedder Rrsur-ts eNn INTERPRETATIoN (19@) proposedthe following band nomenclature: N-bands (N fornormal) for OH stretching bandsdue - (Tae) to hydroxyl groups bonded to Mg3, and I-bands (I Join phlogopite (Phl) taeniolite for inpurity) for hydroxyl groupsbonded to MgzAl. The zs-OH wavenumberscommonly lie within the In the phlogopiteend member,a well-known OH range 3735-3700crn-r in trioctahedral magnesium stretching band observedat 37A 3 I cm-r is due to micas @armer 1974,Robert 1973, 1981,Robert & OH groups bonded to three octahedrally coor- Kodama 1988), whereas z1-OH wavenumberslie dinated divalent cations,i,e., the 6 cationic charges within the range3670-3640cm-r in the samemicas. from Mg3. On the join phlogopite - taeniolite a This difference can be interpreted in terms of charge high-wavenumberband (v <374A cm-r) increases balanceonthe oxygenatoms of the hydroxyl groups. progressivelyas the taeniolite (Li) content increases Basedon the observationsand conclusionsdrawn in the solid solution @ig. 1). Consideringthe change in the present work, a new systemof nomenclature in the octahedraloccupancy on this join, from Mg3 suitable for the chemically complex octahedral en- @hl) to Mg2Li (Tae), the most likely sourceof this vironmentsof the OH group is proposed. new high-wavenumberband is the low-chargeca- In dioctahedral environments, the hydroxyl is tionic environmentcaused by Mg2Li (i.e., 5 cationic bonded to two cations and is adjacent to an octa- charges),adjacent to an OH group. In Vedder's hedral vacant site; the OH dipole is tilted toward the (1964)nomenclature, this high-wavenumberband vacancyand is nearly parallel to (001) (seeGiese I 979 can be consideredas a specialcase of an impurity for a reviewof OH dipole orientations).The K+ (or band, since it concerns OH groups that are not Na*) - H+ repulsion is weak in that case, and bonded to three divalent cations in a trioctahedral OH... Q interactionscan occur between the hydroxyl environment. 228 THE CANADIAN MINERALOGIST
Along the Phl-Tae join, the OH-stretchingwave- numbersincrease from Phl to Tae as the Al content decreases.This increaseis in accordvrith theprevious observationson phlogopite solid-solutions(Li-free trioctahedral micas). Note that the OH-stretching wavenumberof the hydroxyl group bondedto Mg2Li in the mica Phlro-Taeesis the highestrecorded for any mica (3755cm-l), and apparentlyfor any com- pound.
Join phlogopite (PhI) - trilithionite (Tril) PhllO - Toegg The outstandingfeature of the Phl-Tril join is the growth of two low-intensity,poorly resolvedsatellite bands, one on eachside of the high-intensityband (Fig. 2). The high-intensity band broadenssimul- Phl29-Toe69 taneously from a half-height width of 17 cm-l in Phl,* to 37 cm-r in Phl5e-Tril5o;for the highest trilithionite contents,a slight splitting of this high- Phl40-Toe69 intensityband is observed(Fig. 2).
Join phlogopite (Phl) - polylithionite (Poly)
Ramanspectra for Phl-Poly in the regionof OH- stretchingwavenumber (Fig. 3) are very similar to those observedfor Phl-Tril. In particular, the two Phl69-Toe49 satellite low-intensity bands are clearly observed, a high wavenumberband at u : 3746cm-r and a low wavenumber band at v = 3686 cm-l. The central high-intensityband also broadensas the Poly com- ponent increasesin the solid solution. The set of data concerning the Phl-Tril and Phl- Poly joins can be readily explainedby considering the change in octahedral compositions of micas Phl75-Toe2g along thesetwo joins. The octahedralposition con- tains Mg3in the Phl end-member,in which eachOH group is bondedto 3Mg2*, the octahedralcompos! tion becomes(Alt.rlir.r) in theoreticalTril end-mem- ber and (LiAl) inthe theoreticalPoly end-member. Apart from the end-members,any mica composition locatedonthe joins Phl-Tril and Phl-Polypossesses - all three cations Al, Mg and Li in the octahedral Phl95 Toe15 layer. Table I givesthe ten combinationsof thesethree cationsaround an OH group in a trioctahedral en- vironment(first column);the correspondingnumber of cationic chargesis given in the secondcolumn. PhllOO The stability ofanyphaserequires local charge-equi librium both on cations and anions. The bond l-..4 -1 valenceexchanged in a M-OH bond, calculatedhere Saoo 3700 36q) cm as the quotient of the cdtionic chargeby the coor- dination number, amounts to the calculated ideal Ftc. 1. Ramanspectra of lepidolitecompositions along the bond-valenceof Donnay & Allmann (1970). The join phlogopite@hl) - taeniolite(Tae) in the region of calculation assumesregular octahedra; for lack of z-OH wavenumbers.TRI-S: OH bonded to Ms2Li; knowledgeof eachactual M-O(H) bond length, we TRI-6: OH bondedto Mg:. mustuse this approximation.The bond-valencesum LEPIDOLITES 229
membersthat exhibit the permitted cationic arrange- ments are given in the last column. Note that in Table 1, the two cationic arrange- ments Li3 and Li2N4gare forbidden becausetheir bulk chargeis much.toolow, 3 and 4 charges,respec- tively, correspondingto the valencesums 0.5 and 0.67v.u., insteadof I v.u. in theideal case. Although these arrangements dre unknown in any trioctahe- dral mica, the four-charge environment (Mg2tr) existsin the tetrasilicicmagnesium mica K(M92.5n6.r) Phlao-Trilno SiaOp(OH)2;there, the OH group belongsto a dioc- tirhedral environment. TRI-6 Phlro-TrilaO I TRr-5i I I 3723 Phl6s-Tril2q
Phlgq-Xrillg
Phlzs - Poly25
Phf6g- Poly26 PhllOO
PhlgO=Polylg 38oO 37oO 3600 Cm-1 Rc. 2. Ramanspectra of lepidolitecompositions along the join phlogopite(Phl) - tdlithionite(Tril), in theregion of z-OH wavenumbers:TRI-S: OH bondedto Mgzli or LizAl;TRI-6: OH bondedto Mgtor elMgli; TRI- 7: OH bondedto Mg2Alor Alzl,i; theoretically received by the hydroxyl oxygen from the threeadjacent octahedrally coordinated cations, expressedin valenceunits (v.u.), is givenin the third column of Table l. The two negativecharges of the hydroxyl oxygenmust be compensatedboth by the three cations and the hydroxyl proton. The higher PhllOO the bond-valence sum received by the hydroxyl oxygenfrom its threeadjacent cations, the lower the bond valenceexchanged within the O-H grdup (i.e., 3A@ 37@ 36q) cm-l the bond constant k), and thus the lower the OH- ftc. 3. Ramanspedra of lepidolitecompositions, along the stretching.wavenumberz. Table 1, adapted from join phlogopite (Ptil) - polylithionite (Pol9, in the Monier (1987),al$o indicates those cationic arrange- regionof z-OH wavenumbers.TRI-S, TRI-6 andTRI- ments that are permitted; examplesof mica end- 7 as in Figure 2. 230 THE CANADIAN MINERALOGIST
TSU 1. PEMITBD m aomIDDf,N MCWmS On A1, tE dd Lt sideringthe changein octahedraloccupancy along ffim A mIoqIM mRonl the joins Phl-Tril and Phl-Poly. Note that the oc- Catiodc tro of Vdenc€ PoF trots Exeplea Li- ar4€b6nt chfg€a a@ 1a bltH bldds tahedraloccupancy is Als.ilIgr.zUo.sin the most HN (and Al-) rich mica stableon the Phl-Tril join. The h& 413 9 1.5 atomic proportion is closet o | / | / l, i.e., closeto the I% 6 1.o pnogoplt€ compositionAlMgLi in the octahedrallayer of this Ll: 3 O.5 mica. [e& g 1.s As in the solid solution Phl-Tae, the OH-stretch- .. 412L1 7 r.r7 spbeglb, bttylb ing wavenumbersalong Phl-Tril are not constanl; a U%41 7 L.17 * esbnlt6' p&lardkl& US2ti 5 O.A bdollb regular negative shift in wavenumber from Phl to L12A 5 O.S Tril is observedfor all the bands.However, no band Lr2!u 4 0.67 shift is observedalong Phl-Poly. Consideringpre- Alilgll 6 1.O .. zlmealdb vious studiesof the effect of the bulk Al contenton OH-stretching wavenumbers(Robert 1973, 1981, &e ten catlonlc @angeB@b of A1, [a ed Ll aead a trlct$6&d- Robert & Kodama 1988), this constancyis quite typ€ by&ryI (ilEt col@)i coespondlng s@ of, posltlve chsg€s (asood co1@) ed valgnce B@ exch4od ln the fu€e tr{H bon& predictable,since the bulk Al content remainscon- (t&trd colte). th€ lst colle glv6a 6@1ea of Seo.etl.€l nlca ond stant along this join. ld€m ruch €dblt Se pemltted arang@dta. pblosoplte KG3(St3A1)o1o(oH)2i €ph61b tra(A12Ll) (S12Al2)Oro(og)2t blttlte ca(Ar2Ll) (sl2AlBelolo(oH)2; €$tonlb K(us2.5Alo.s) (6tz.s[r.s) Join tetrqsilicic magnesiummicq (TMM) - o1o(oF) prolsr€rklte Na([92A1 (s1202 tldout€ K(]lg2lt ) 2 ; ) )o1o(oH)2 ; (Toe) St4o1o(oH)2; polyuthtodte K(L12A1)si4oto(0H)2; zimmldlt€ K(AI4gLt) toeniolite ( s13u o1O( oH) ) 2. In rhe TMM end-member, K(Mgr.r[JojSinOro The group Al3, unknown in any compound, is (OH)2, two types of OH-stretching bands are ob- forbidden becauseits bulk charge is too high (9 served:(1) trioctahedral-typebands, i.e., OH bonded charges);it would correspondto avalencesum of 1.5 to 3Mg, at3735cm-t thigh-intensityband) and 3695 v.u. exchangedbetween the hydroxyl oxygenand the cm-l (low-intensityband), and (2) dioctahedral-type three adjacentcations. Similarly, the group AlrMg band, i.e., OH bonded to 2N.1gand adjacentto an (8 charges)is unknown in micas and is considered octahedral vacancy, at 3595 cm-r. Note that the here as a forbidden group; however, the 8-charge minor-intensity band at 3695 cm-r is unexpected, group Mg2Ti is well known in titanium-bearinp consideringthe octahedralmakeup of TMM. In a phlogopite(Robert 1976, l98l). previous IR study at different anglesof incidence, ConsideringTable l, the evolution of spectraon TMM was found to exhibit the samepleochroism as the joins phlogopite-trilithionite and phlogopite- the high-wavenumber(3735 cm-t) OH-stretching polylithionite can be interpretedeasily. The low-in- band (Kodama et al. 1974);therefore, the band at tensity, high-wavenumberband observed on the 3695 cm-r was consideredas a trioctahedral-type joins Phl-Tril (Fig. 2) and Phl-Poly @ig. 3) is dueto band, perturbedby a secondaryeffect, the peculiar a S-cationic-chargeenvironment of the OH group. distribution of octahedrallycoordinated cations and ExamplesareMgrli, previouslyobservedonthe join vacanciesaround this trioctahedral-typeOH group. Phl-Tae within the samewavenumber range (Fig.1), Along the join TMM-Tae, the behavior of these and Li2Al, known in polylithionite. Similarly, the two main band-types is very different. The salient secondlow-intensity satellite band observedat low feature concerning each trioctahedral-type band is wavenumbers(z-OH < 3700cm-r ) is due to a 7- the growth of a new high-wavenumberband, whose chargeenvironment of the OH group. Examplesare intensity increasesas the Li content increasesalong Al2Li, known in severalmica end-members(ephe- thejoin. Thewavenumbersof thesenewOH-stretch- site, bityite), and Mg2A1,found in phlogopitesolid- ing bands are 3755 a I cm-r (correspondingto the solutions(e.9.,theK-micaeastonite and the Na-mica band at 3735 cm-r in TMM) and 3710 + I cm-r preiswerkite). (correspondingto the band at 3695 cm-t) (Figs. Finally, the broadeningof the centralhigh-inten- 4a,b). This phenomenon,which is the sameas that sity band (a 6-charge band, due to OH groups previouslyobserved on the join Phl - Tae (Fie. l), bonded to 3Mg2+ in the phlogopite end-member), can be interpreted similarly; the two new bands are particularly evident on the join Phl-Tril, can be due to OH groups bonded to Mg2Li, i.e., to a 5- attributedto the existenceofa second6-charge band chargecationic group. The wavenumbersof these whose intensity increasesalong the joins with in- bandsremain constant,within experimentaluncer- creasing Li-contents. The only possible cationic tainties, along the join. The similar behavior of the arrangement compatible with this conclusion is two bandsat 3735cm-r and 3695cm-t supportsthe AIMgLi (Table 1). This seemsvery reasonablecon- proposedassignment. LEPIDOLITES 231
TAAroo
TliAt2g - Taegg
TtlAt\A4g - Tae6g - Tao T,lt/'AOO 20
Tae4O
Tl - T8o r\ 60 40 Trttrltgg - Tao2O
Tr$1640 - Tae 60 rzisl ri tv *; I o Tr'^/tAZO- Tao BO 3650 3550 cmr @ 3750 3050 3550 Cm-l 0 Ftc. 4. Raman(a) and IR spectraO) along the join tetrasilicicmagnesium mica (TMM) - taeniolite(Tae), in the region of z-OH wavenumbers.TRI-5 and TRI-6, as in Figure l. DI-4: OH bondedto Mgz and adjacentto an octahedral vacancy. On the other hand, the high-intensityband at 3595 & Kodama 1988).Both examplesare a consequence cm-r, which is a dioctahedral-typeband (OH bonded oflocal charge-balancerequirements on the oxygen to 2Mg2+and adjacentto an octahedralvacancy), atoms. progressivelyvanishes from TMM to Tae, in re- sponseto the progressivedisappearnnce ofthe octa- hedral vacancies.Here again, no band shift is ob- THE NEw SYSTEMoF NoMENCLATUREoF served along the join, indicating that the initial OH-SrnelcFIINGBANDs environmentof this OH group remainsunchanged - from TMM to Tae, in fact to TMM2. Taes6,the The presentstudy has shown severalambiguities limit of the solid solution. In other words, Li+ is in the use of the previous systemof nomenclature: excluded from the vicinity of an OH goup that (l) the OH-stretchingband due to hydroxyl groups belongs to this dioctahedral environment. The bonded to AlMgLi has almost the samewavenumber hydroxyl oxygen, therefore, would be bonded to as the OH band arising from hydroxyl groups MgLi, i.e., to a 3-chargecationic group; the sum of bondedto Mg3.Acccording to Vedder(1964), (1) the bond valencesreceived by this hydroxyl oxygen first band should be consideredas a specialcase of would be only 0.5 valenceunit, which is much too an impurity band, whereasthe secondis a typical low (compare with the forbidden trioctahedral en- normal band; (2) similarly, the high-wavenumber vironment Li, in Table 1). band of OH groups bonded to Mg2Li should be This is the secondexample of ordering, in fact of consideredas an other impurity band, but on the cationic exclusion,observed for this kind of dioc- commonly acceptedwavenumber scale, the impurity tahedral-type hydroxyl. The first example known bands have low wavenumbers;and (3) the same wasobserved on the join TMM-Phl, and concerned remarksapply to the vacancybands. In the present the exclusionof IvAl from this environment(Roben study, all OH groupsin a dioctahedralenvironment 232 THE CANADIAN MINERALOGIST are bonded 6 zMg?+, but OH groups bonded to band:TRI-5 (OHbondedtoMg2Li orLi2Al), TRI-6 Al3+Mg2+ are known in phengite and OH groups (OH bonded to Mg3 or AlMgLi), and TRI-7 (OH bonded to 2413+ are typical of muscovite, p&ra- bonded to Mg2Al or Al2Li). Similarly, the dioc- gonite and pyrophyllite. Therefore, in order to clari- tahedral-typeband of TMM is named DI-4 (OH fy this nomenclature and to minimize the prolifera- bonded to Mgr; in muscovite,this band would be tion of suffixes, a new systemsuitable for any caseis namedDI-6 (OH bondedto Al). This nomenclature proposed. The results obtained on lepidolite solid is usedin the Figures. solutionsconfirm the fundamentalrole cif the bulk cationic chargesadjacent to the OH group. There- DepsNosNcpor OH-SrnrrcHINc WAvENUMBERS fore, this number of chargesmust appear in the oN CoMPoSITION nomenclature.On the other hand, the only two types groups of OH that needto be consideredare those No band shift is observedon the joins Phl-Poly that belorigto a trioctahedralenvironment and those and TMM-Tae. On the first join, thebulk aluminum that belongto a dioctahedralone. Consequently,the contentis constant;on the secondjoin, all the micas new systemof nomenclature is built as follows : TRI are Al-free. On the joins Phl-Tae and Phl-Tril, a or DI designatetrioctahedral- or dioctahedral+ype systematicband-shift is observedfor all the OH- (we pre- OH-stretching bands use capital letter$ to stretchingbands. In both series,the Al content is vent confusion with the abbreviations such as Tril). variable, and the OH-stretching wavenumbers The numberof cationiccharges bonded to the hydro- decreaseas total aluminum (AltJ increases. The xyl is added to the symbol. sane relationshipis known in Li-free irioctahedral pos- ConsideringTable l, there are only three mica solid-solutions(Robert 1973, 1981,Robert & sibilities for'the nomenclatureof trioctahedral-type Kodama1988). i-on
3740
3710 O O.5 1 1.5 2 thl*ttAl
Fig.5. ChangeoftheOH-stretching_wavenu-mbers (in cm-l) of TRI-5 and TRI-6 bands,inlepidolitecompositions (Raman 'tAl data), as a function of the sum + '"A1. open circle: phlogopiteend-member; solid squares:join Phl-Tae; solid triangles:join Phl-Tril; X: join Phl-Poly. LEPIDOLITES 233
3600 o 4 'hl "'"Ar Fig. Chaggeof the OH-stretchingwavenumbers (in cm-l) of TRI-7 bandsin lepidolite compositionsas a funition of .6. '"A1. "'Al + Solid triangles:join Phl-Tril (Raman);X: join Phl-Poly (Raman);open square: preiswerkite (Raman), after Liu et al. (1987);starred circle: ephesite(IR), after Farmer & Velde (1973);solid circle: Lifree phlogopite solid-solutions(IR), after Robert & Kodama(1988). Lines TRI-5 and TRI-6 are from Figure 5.
TRI-S and TRI-6 OH-stretching bands Al2Li, with the OH-stretchingwavenumber z - OH = 3609cm-r @armer&Velde 1973).Includingthis justified, The trends for TRI-5 and TRI-6 OH-stretching value in the presentstudy is becausethe wavenumbersare given in Figure 5. The data points type of interlayer alkali cation has no detectable plot continuouslyalong two lines whoseequations effectgn OH-stretchingwavenumbers. For example, are: the data for the high-Al trioctahedral sodium mica preiswerkite, (1) line TRI-S: z-OH = -9.33 Albt + 3754(r : NaMezAl)(Si2AlrOlo(OH)2, are in phlogopite 0.96/-) line with thoseobtained in solid-solutions (Robert (2) line TRI-6: y-OH = -16.95Alto, + 3741(r : & Kodama 1988). is known betweenthe low-Al 0.990) No solid solution K-bearing lepidolites and Phl-Pol$ and For the Phl-Poly join, the OH-stretchingvalues @hl-Tril thehigh-Al sodium-bearinglepidolite (ephesite). The of 3746cm-1 for the TRI-5 band and 3723cm-l for gapbetween the correspondingOH-stretching wave- the TRI-6 band wereincluded in the calculationof numbers can be easily filled by considering the thesetwo equations. secondset of data,relative to a 7-chargeenvironment of the hydroxyl, Mg2Al (seeTable 1), known in TRI-7 OH-s t ret ching bond phlogopite solid-solutions(Robert 1981,Robert & Kodama1988). In the lepidolitesolid-solutions investigated in this The equationof the line TRI-7 (Fig. 6), calculated work, the TRI-7 OH-stretchingband is observedon from all the data relative to an OH group in a 7- the joins Phl-Tril (3684 < v < 3689 cm-t) and chargeenvironment, Al2Li or Mg2Al, is: Phl-Poly (z : 3686cm-r). The correspondingdata- (3) z-OH : -29.66Altot +3719 (r = 0.962) points (Fig. 6) pertain to the narrow composition The quality of this fit supportsthe band assign- range 1.0 < Alto! < 1.6. Ephesite Na(Al2Li) ments and emphasizesthat the bulk cationic charge (Si2Alrorg (OlI)2, whosetotal Al content is 4, also adjacent to the hydroxyl fundamentally influences possessesOH groups in a 7-chargeenvironment, the OH-stretching wavenumber. 234 THE CANADIAN MINERALOGIST
CoNcI-uoINcREMARKS observedin the present work. This apparent dis- crepancycan be attributed to the lack of repulsive proton The observationspresented here fit well with the interaction betweenthe hydroxyl and an in- assignmentspreviously proposed for phlogopite terlayer cation; in this Li-bearing montmorillonite, solid-solutions (Robert & Kodama 1988). On the about2/3 ofinterlayer sitesare vacant. This repul- quantified wavenumberscale, the classificationof OH-stretch- sive effect could be by comparing the ing bandsdepends on the bulk chargecarried by the OH-stretchingwavenumber of the TRI-6 band of (3735 threeadjacent cations (first-neighbor effect): TRI-5 TMM, a true mica cm-l), to the OH-stretching (3677 bands, high wavenumbers;TRI-6 bands, medium wavenumberof lalc cm-l), in which the inter- wavenumbers,and TRI-7 bands,low wavenumbers. layer spaceis empty. The correspondingband-shift present Within eachband type, the OH-stretchingwavenum- is 58 cm-I, in satisfactoryagreement with the bersdepend on the intensityof OH. ..O interactions observations. betweenthe hydroxyl proton and O(3), the apical oxygenatom ofthe tetrahedra.Charge imbalances AcKNowLEDGEMENTS on oxygenO(3) control theseweak hydrogen bonds; they increaseas the Al content increases. Thanksare due to Drs. J.L. Jambor, H. Kodama, This study also confirms that the gap betweenthe R.F. Martin and to the refereesfor their constructive different band-typesincreases as Alro, increases @ig. criticism. 6). From equations(l), (2) and (3), it is apparentthat the slopesapproximately double from line TRI-5 to REFERENcES line TRI-6 (ratio 1.82) and approximately double again,from line TRI-6 to line TRI-? (ratio 1.75).In CeLvnr,R. & Pnosr,R. (1971):Cation migration into other words, OH groups in which the hydroxyl properties proton empty octahedral sites and surface of has little interaction with the surrounding clays.Clays Clay Minerals 19, 175-1 86. oxygenatoms are little perturbedby changesin their environment (e.9., TRI-5 bands). In contrast, Dorlwav,G. & AruuarN, R. (1970):How to recognize hydroxyl groupsinvolved in strongOH...O, interac- O'-, Off and HzO in crystal structuresdetermined tions (e.g.,TRI-7 bands)are more stronglyaffected by X-rays.Am. Mineral.55,1003-1015. by thesemodifications. This aspectwill be discussed in a forthcoming paper. FanuEn,!.C, (1974):The layer silicates.Iz The In- The very high z - OH observedat 3755cm-l for frared Spectraof Minerals (V.C. Farmer, ed.). the mica compositions Phlle-Taee6and along the Mineral.Soc. Gr. Britain. London. join TMM-Tae can be explainedreadily. It results from a hydroxyl group in the lowestpossible charge & Veror, B. (1973):Effects of structuralorder in a trioctahedral environment (5 charges); this anddisorder on theinfrared spectra ofbrittle micas. hydroxyl is orientedtoward a ring of 6(SiO) tetra- Mineral. Mag. 39, 282-288. hedra, which is the highest possible tetrahedral Grnsn,R.F., Jn. (1979):Hydroxyl orientationsin 2:1 group. charge around an OH Therefore, this en- phyllosilicates. ClaysClay Minerals 27, 213-223. vironment implies the weakestpossible OH...O interactions and, thus, the maximum value of the J@ncexsrN,P. (1966):Infrared absorptionof O-H bond constantk. bondsin somemicas and other phyllosilicates.C/a1s Compoundsthat exhibit very high OH-stretching Clay Minera ls. 13,263 -27 3. wavenumbersare rare. Such a wavenumber(3749 cm-r) is known in isolated surface silanol groups, Konave,H., Ross,G.J., Lvaue, J.T. & Rossnr,J.-L. Si-OH, free of hydrogenbonding, observedin high- (1974):Effect oflayer chargelocation on potassium temperature amorphous silica (McDonald 1957). exchangeand hydration of micas.Am. Minerol. 59, The very high OH-stretching wavenumber, 3755 491495. cm-I, observed in the present work closely cor- respondsto the r/3stretching wavenumber (IR) of the Lrr, J.-C. & GucosNlrEu\a,S. (1983): The crystalstruc- freewater molecule at 3755.79 cm-I, registeredunder ture of a Li, Be-rich brittle mica: a dioctahedral- trioctahedral intermediate.Am. Mineral. 6t. I 30- very low pressure,i,e,, the conditions of the mini- 142. mum OH...O interactions(Sverdlov er c/. 1970). Calvet & Prost (1971)proposed the values3700, Lru, X.F., Rosrnr,J.-L., BeNv,J.-M. & Henov,M. 3670 and 3640 cm-t for OH groups adjacent to (1987):Raman spectrometry of (OH) groupsin syn- Mg2Li, AlMgLi and Al2Li, respectively,in Libear- thetic trioctahedralsodium micas, comparison with ing montmorillonite. These three OH-stretching infrared andthermogravimetric data. TerraCognita bands are approximately 50 cm-r below the values 7, l, 17(abstr.). LEPIDOLITES 235
McDoNalo, R.S. (1957): Study of the interaction be- bers and composition of micas in the system KzO- tweenhydroxylgroupsofaerosilsilicaandadsorbed MgO-AlzO:-SiOz-HzO: a single model for trioc- non-polar molecules by infrared spectrometry. "/. tahedralanddioctahedralmicas.Am. "I. Sci.2E8-A, Am, Chem.Soc. 79, 850-854. 196-2t2.
MoNrrn, G. (1987): Cristallochimie des micas des & Volrnicrn, M. (1979):Etude expdrimentale leucogranites.Nouvelles donn€es expdrimentales et del€pidoliteshydroxyldes.Ball.Mindral.l02,2l-25. applicationsp€trologiques. In C€ologie et G6o- chimie de I'Uranium. Centrede Recherchessur l^u -' -' BnnnantooN'J'-N' & Besurcu' M' Gdologie (Nancy),Mdm. 14, det'(Jranium (1983):Lithium in the interlayerspace of synthetic trioctahedralmicas'chem'Geol' 40'337-351' RoBERr,J.-L. (1973):Etude Exp,rimentalede Micas dans le Systlme KzO-MeO-TiOz-AlzOt-SiOz- HzO. Application aux Phlogopites Titaniftres. SvERDLov,L.M., Kolrvrn, M.A. & KnarNov,E.P. Thbse3dme cyle, Univ. ParisXI, Paris. (1970): Vibrational Spectra o/ Polyatomic Mole- czlas.Nauka, Moscow. (1976): Titanium solubility in synthetic phlogopitesolidsolulions'c&em'Geol'17'213'227' vEoo*, w. (196a):correlations between infrared compositionor mica'Am' (r98r):Etudes cristauochimiques sur tes Micas. nif#f ffi;*H:t et les Amphiboles: Application d Ia Pdtrographie et d la Gdochimie. Thdsed'Etat, Univ. Paris XI, Paris.
& Konaue, H. (1988): Generalization of the Received March, 21, 1988, revked manuscript accepted correlationsbetweenhydroxyl-sfetchingwavenum- luly 31, 1988.