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<harge de I'environnementde cesgroupements hydroxyles. Pour bands). The latter equation includes all the available values I'ensemble des bandes de type triocta€drique, on observe concerning K-bearing micas (lepidolite and phlogopite un glissement systematiquevers les bassesvaleurs de solid-solutions)and Na-bearing micas (preiswerkiteand nombre d'ondes lorsque la teneur globale en aluminium 225 226 THE CANADIAN MINER{OGIST (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