American Mineralogist, Volume 59, pages 1242-1248' 1974

ZonedLithium-Aluminum Mica Crystalsfrom the PalaPegmatite District

KeNNerH J. Bnocx

D ep ar t ment of G eology, I ndiana U nia ersi ty N or thw est, Gary,Indiana 46408

Abstracl

Mica crystals from the Tourmaline Queen mine have monocrystalline muscovite cores surrounded by pinkish rims of intergrown lepidolite crystallites. The central muscovite has the 2M, structure and is bounded by well formed {110} and {010} faces on which lepidolite has developed through epitafid overgrowth. Lepidolite rims usually consist of lM + 2M - - mixtures, but one sample contained only the lM structure and had a 5.227 r- 0.002,A, 6 9.020 + 0.003A, c - 10.118-+ 0.002A, andF - 10o'46r -r 1'; muscoviteparameters from this crystal were d = 5.182 -f 0.004A, b = 8.997 + 0.004A, c = 20'073 1- 0.007A, and - -+- B 95. 45' 2,. Lepidolite crystallites are twinned about (310) and are attached to muscovite ( 1l0) on a plane in the zone ( 1101,possibly ( I 10). The lepidolite rims contain significantly more Mn, Rb, and polylithionite than the central muscovite; although outer portions of the muscovite core are slightly enriched in polylithionite, the major compositional change occurs abruptly at the interface between micas Closely as- sociated zoned tourmaline, consisting of schorl cores surrounded by alkali tourmaline, suggests that the zoned micas were formed by crystallization from two fluids of different composition; however, polylithionite enrichment in the outermost muscovite implies that the transition be- tween fluids may have been gradual.

fntroduction Occurrence Visibly zoned lithium-bearing mica crystals, con- Samples studied in this investigation were col- sisting of muscovite cores encasedin pink lepidolite, lected from the dump of the Tourmaline Queen mine were described from Haddam Neck, Connecticut, by (located on Queen Mountain ca 3 km north of Bowman (l9OZ), and similar crystals have been re- Pala), and were not observed in place; however, ported by Jahns and Wright (1951, p.34) from the Jahns and Wright (1951, p. 57) report the occur- Pala district in southern California. In each occur- rence of similarly zoned crystals in pocket-bearing rence the zoned crystals appearedin or near pockets portions (in and immediately beneath the core) of within complex pegmatites associated with zoned this pegmatite. tourmaline. Lepidolite ,typically contains 4.O to 7.7 Typical crystals occur in lavender colored books percent LizO and is comprised of the IM, 2M2, averaging about 20 mm in maximum dimension; or 3T polymorphs, whereas muscovite possesses cleaved books reveal that the lavender color is re- the ZMr structure and normally contains less than stricted to the outer L or 2 mm of the crystal and 3.3 percent Li2O, suggesting that the zoning re- that the (001) cleavage plane is continuous across flects both compositional and structural changes both rim and core. Cleavage flakes often exhibit within the mica. diamond-shaped outlines due to well developed are also The presence of zoned lithium-aluminum micas {110} faces on the muscovite; {010} faces indicates variable conditions during crystallization occasionally present truncating acute angles between of an from the parent pegmatitic fluid and raisesthe possi- {110} faces (Fig. 1). Except for the absence bility that a detailed investigation of the zoning might external surface composed of fibrous rose-colored yield information regarding the formation of com- muscovite, these crystals are remarkably similar to plex pegmatites. This paper reports results of a those describedby Bowman (1902). crystal chemical investigation of zoned lithium-alu- Associated minerals include compact aggregates minum mica crystals from the Pala district and dis- of lepidolite flakes, massive quattz, prismatic tourma- cussesgeologic implications of the zoning. line crystals, and albite. The albite generally appears 1242 ZONED LITHIUM-ALUMINUM MICA CRYSTALS 1243

asplaty cleavelanditecrystals less than 10 mm across, but fine-grainedsugary masses of this feldsparwere detec'tedin several,samples. Colored mica rims in- variably appear only along the margins of mica crystalsin direct contactwith cleavelanditeand are absent when the crystal abuts against quartz. A single hand sample may contain pink, green, and black tourmalineprisms, and occasionallyconspicu- ouslyzoned crystals revealing black schorlcores sur- roundedby thin successivezones of greenand pink alkali-tourmalineare observed.

Color Zoning Microscopic examinationindicates that bound- aries betweenthe central muscoviteand the lilac- coloredrim consistof distinctplanes parallel to the {110} and {010} muscoviteforms (seeFig. 1). Each muscovitecore is composedof a singlecrystal which exhibitsuniform extinction,whereas riins are Ftc. 1. Photomicrograph of zoned Li-mica crystal re- vealing a central muscovite core bounded by and comprisedof a mosaic of small (usually ( 1 mm {110} {010} faces surrounded by an external rim composed of across)irregularly shaped crystallites, which rarely lepidolite crystallites. Note the absence of irregularities attain completeextinction and often display highly and embaymentson the muscovite faces. The blemish near distorted interferencefigures. Crystallites near the the center of the muscovite is an abrasion on the cleavaqe rim-muscoviteboundary are generallysmaller and surface. yield more highly distortedinterference figures than those occurring farther from the boundary. Dis- ChemicalVariations Within Mica Crystals torted interferencefigures and the failure to ex- Crystal P-05 was selectedfor chemicalstudy be- tinguish suggestthat most crystallitesconsist of cause it contained a single lepidolite polymorph intergrownmica flakes of differentcrystallographic (lM) and also exhibiteda particularlydeep violet orientation(Bloss, Gibbs, and Cummings,1963), outer rim. Electronmicroprobe traverses along path Optic anglesvary between37 and 50o for the crys- a-a' in Figure 2 were conductedin order to detect tallitesand wereconsistently near 45o in the musco- qualitativechanges in Si, Al, Mn, K, and Rb across vite. the crystal.Approximate SiO2, Al2O3 and MnO con- X-ray powderdata from the colored rims of four centrationsalong the traversewere estimatedby separatemica crystalsindicate the presenceof either a 1M polymorphor a mixtureof 7M and2M micas.l Terr-e l Chemical and Physical Properties of Crystal Two samplescontained IM + 2M1 mixtures and P-05 may have also containedthe 2M2 polymorph which cannotbe detectedin a 2My + 2M2 mixture using Zvll Ntuscovite lM Lepidolite Core Rirn powder data. A third sampleconsisted of a lM -t 2M2 mixture,and the remainingsample (P-05 ) con- Atonic absorption analysis (wt. %) tainedonly the lM polymorph.In all powder cases Si0z 43.7 47.3 data from the muscovitecores were consistentwith A1203 34.9 22.8 the 2Mt structure.Powder patterns of the IM mica Ifti0 .48 3.34 Li20 )6 4.76 are similar to the synthetictrilithionite pattern tabu- latedby Munoz (1968, p. 1498); unit cell param- Ce1l dinensions etersfor sampleP-05 arelisted in Table 1. 5. r8210.004 4 s.227!0.002 b 8.99710.004,$ 9.020r0.003 20.07310.007 A I0.11810.002 'Distinction betweenpolymorphs was based on the pres- r ence or absenceof key strong reflections as outlined by R 9504512' 100'4611 g3r.2t.z Munoz (1968,p. 1947). v R3 468.7!.2L3 1244 KENNETH I. BROCK

(Tl) K2LiBAl3Si6AlzOzo(OH,F)r, and muscovite (Ms ) KzAlrSi6Al2O2o(OH ) 4. Examination of Li-mica analysesfrom the litera- ture reveals a high correlation between Al:Si atomic ratio and mole percent Pl in natural lithium micas.2 55 Li-mica analyses,each containing less than 1.25 percent FezOs * FeO and analysis summations be- tween 99.0 and 101.0 percent, were selected from the literature, and mole percent Pl was regressedon Al:Si atomic ratio (R), yielding the following least squareqquadratic equation

pl : 153.00s- 245.233(R)+ 93.241(R)'t2.3 (l) Variations in Pl content across sample P-05, cal- culated from Eq. ( 1 ) using Si and Al data from 5mm microprobe analyses, are shown in Figure 3 and indicate a gradual increase in Pl content as the mica boundary is approached. This suggeststhat the outer FIc. 2, Sketch showing electron microprobe analytical 1 mm of the muscovite core is in fact enriched in traverse a-a- acrosssample P-05. lithium relative to central portions of the crystal. The major increase in Pl content, however, occurs abruptly at the muscovite-lepidolite boundary indi- comparing the microprobe data with atomic absorp cating a substantial increase in Li content at the in- tion analysesperformed on samples cut from an ad- terface between phases. jacentcleavage flake (seeTable 1). K:Rb ratios were The high correlation between Pl content and Al:Si approximated by comparing microprobe intensities atomic ratio in natural Li-micas can be attributed with an analyzed muscovite. The microprobe data to the fact that lines of equal Al:Si ratio are nearly are not quantitative; consequently, interpretation parallel to the Tl-Ms join on a triangular Pl-Tl-Ms must be basedon chemical variations acrossthe crys- diagram (i.e.,Tl and Ms have similar A1:Si ratios) tal rather than on absolute values. The analytical and that natural micas containing more than 50 mole profile along the traverse was checked by monitoring p€rcent Tl are rare. Rieder (1970, p. 140) reachesa Al on two additional scans parallel to a-a'; in each similar conclusion based on crystal chemical argu- case the results were essentially identical, indicating ments and eliminates Tl entirely, plotting the Pl- that variations in analytical values along the traverse Tl-Ms triangle on a single Pl-Ms join. are reproducible. K, Rb, and MnVariations Si,AI, an'dLi Variations Within the limits of experimental error, variation Figure 3 reveals an abrupt drop in Al concentra- in K:Rb ratio was not detected across either mica, tion at the muscovite-lepidolite interface. The trav- but the Rb concentration was low and subtle varia- erse also shows a gradual decreasein Al content of tions may have been undetected. Figure 3 does re- the muscovite directly adjacent to the mica boundary veal, however, that the K:Rb ratio in the lepidolite suggestingthat the outermost muscovite is enriched rim is about 50 percent below K:Rb values observed in lithium; however, the absenceof Li analyseslimits in the muscovite core. Potassium content was essen- direct substantiation of this contention. Lithium tially constant throughout the traverse; consequently, variations across the mica crystal can be evaluated the abrupt decreasein K:Rb ratio observed at the indirectly providing Si and Al analyses and trans- mica interface is due entirely to higher Rb concen- formed into Li-mica conponents. Foster (1960), tration within the lepidolite rim. Munoz (1968), and Rieder (1970) report that the 'Pl content was determined from the relation Pl = 50 composition of lithium aluminum micas can be (Si-6), where Si representsthe number of Si cations per accurately expressed by three components: poly- mica formula. The cation content of each analysiswas cal- lithionite (Pl) K2l-i{Al]si8or{)(oH,F)1, trilithionite culated assuming24O-',2K*, and 4H* ions per formula. ZONED LITIIIUM-ALUMINUM MICA CRYSTALS r245

MUSCOVITE LEPIDOLITE ------SiOz

zF o lrJ F o \___At2o3 E E lrl (L to F I (9 F - lrJ (9 = lrJ * = K/Rb - MnO

8o

F z. lrJ o E 40 lrl (L

lrl J o = o

-l -2 -3

MILLIMETERS Frc. 3. Schematic presentation of compositional changes calcullated from electron micro- probe traverse along path a-a' shown in Fig. 2. Origin of the millimeter scale is the muscovite- lepidolite interface. PL representspolylithionite component.

Rb enrichment in the outer rim may be in part shows a very slight increasein Mn; however,the attributed to the availability of larger interlayer major changein Mn content occurs abruptly at the cationsites, which more easilyaccommodate Rb ion mica interface. The relatively high Mn content in in the lepidolite structure.Crystal structure analyses lepidolite rims undoubtedly accountsfor their pink of micashave revealeda generalcorrelation between or lavendercolor (sample P-05 possesseda par- the tetrahedralrotation angle (o) and a decreasein ticularly deep violet rim). Correlationbetween the size of the interlayer cation site. The tetrahedral typical lavender,pink, or lilac lepidolite colors and rotation angle of muscoviteis near 13" (Radoslo- Mn content was made by Heinrich and Levinson vich, 1960), whereasTakeda, Haga, and Sadanaga (1953) and has subsequentlybeen confirmed by (1971,) report ,a rotation anglesof 3 and 5.3o re- Faye (1968) throughcomparison of the optical ab- spectivelyfor the 1.M and 2Mz lepidolite structures. sorptionspectra of lepidoliteand Mn(OHz)u'-. Al- lM micas generally exhibit smaller ,c-anglesthan though rose-colored 2M1 muscovite has been re- muscovite (McCauley and Newnham, l97I) and, ported, it is relatively uncommon,suggesting that therefore, possesslarger interlayer cation sites. Par- lepidolite possessesa greatercapacity for incorpo- titioning of the large Rb ion betweenbiotite (typi- . rating Mn into octahedral sites than does the 2M1 cally a lM structure) and muscovite was clearly structure. demonstratedby Gresensand Stensrud(197I) who Hazen and Wones (1972) point out that cations reported Rb contents of.260 and 640 ppm resp€c- with radii greater than 0.78 A are too large for the tively for intergrownmuscovite-biotite portions of a octahedralsites in trioctahedralmicas; they alsonote single mica crystal from the Mitchell Creek peg- an inverserelation between a and R2. radii in stable matite,Georgia. trioctahedralmicas. If the correlation betweenc dnd Muscovitewithin I mm of the lepidoliteboundary R2* can be extrapolatedto the dioctahedralmicas, 1246 KENNETH I. BROCK then the octahedralsites in lepidolite (transitional ingly, the lepidoliteinterplanar angle (001)A(hkI) - between dioctahedraland trioctahedral) must be O must equalthe muscoviteangle (001)n (110) somewhatlarger than the corresponding6-fold sites (Fig. aB). A valueof 85' 05' wascalculated for the in muscovite.This suggeststhat the characteristic muscoviteinterplanar angle (001)A(110) usingre- pink colors typically associatedwith lepidolitemay fined unit cell parametersfrom Table 1. Therefore, reflect a preferential accommodation of Mn ion the lepidolite plane (ftkl) which joins muscovite owing to the availabilityof larger octahedralsites. must satisfythe two relations(010)n (hkr) == 60o and(001) A(hkl) ==85o. Orientationof Crystallites A stereographicprojection of lepidolite reveals The planar nature of the inter-mica boundary that no crystallographicplane with small Miller in- seemsto indicate that the lepidolite fringes were dicese;ractly satisfies the above conditions.Several formed by simple epitaxial' overgrowthrather than planesin the zone1110) fulfill the (010)A(hkl) = throughtopotactic reaction between parent muscovite 60" restrictionwithin one half degree,but indicesof and late pegmatiticsolutions. It is difficult to con- (661) are requiredto closelymeet the C = 85o re- ceive of topotactic lepidolite developrnentwithout quirement;for example,the { valuesof (110), (331), the introductionof irregularitiesand embaymentsin (441), (661), and (77I) are 80o 39',89o 16', the facesof the hostmuscovite crystals. 87o_05',84o 56', and 84o 19'respectively.Although Many crystalliteswithin the lepidoliterim are not (661) satisfactorilyfulfills the 4 condition,the pos- in direct contactwith the muscovitecore; however, sibility existsthat a lepidolite planepossessing smaller in all casesthe optic planesof crystallitesyielding indices,such as (110), actuallyjoins muscovi'teand undistortedinterference figures are related by a rota- the 4' 26'misfit in 6 is accommodatedby local lat- tion of approximately60" aboutI (001). Accord- tice distortions.The latter contentionis supported ing to Deer, Howie, and Zussman(1,962) the optic by the high degreewith which unit cells of muscovite axial plane in lepidoliteis parallelto (010); eerny, and lepidolitefit along their mutual (110) planes. Rieder, and Povondra (1970, p. 324) also report The percentmisfit betweenmica cells joined along that monoclinic micas violating O.A.P. parallel to (110) is lessthan 4.1 percentas calculatedfrom (010) are not observedalong the siderophyllite- 100(OL - OM)/OM, whereOL and OM are re- polylithionite join. Based on the assumptionthat spectivelepidolite and muscoviteunit cell vectors O.A.P. is parallel to (010) in the lepidolite rims, with commonorigin at (0,0,0) and terminationsat (100) directionswere determinedin severalcrystal- (a,b,2c) and (a,b,c). Accordingto Spry Q969, p. lites within the rims of sevendifferent mica flakes 165) misfits up to 15 percent are frequently en- using an optical techniquesimilar to the method counteredbetween epitaxial overgrowths. Additional employedby Blossa aI (1963). Without exception data, includingsingle crystalX-ray photographsof (100) directionsbetween crystallites were relatedby joined muscovite-lepidolitecrystals, are requiredto 120o,indicating the crystallitesare twinnedabout a unambiguouslydetermine the indicesof the lepidolite (310)axis (mica twinlaw). planewhich joins muscovite ( 110). The most extensivelydeveloped muscovite form is Origin of LePidoliteRims {110}; consequently,the majority of lepidolitecrys- talliteswhich attachdirectly to muscovitemust join Jahns and Burnham (1969) proposethat peg- ( 110) muscovitefaces. Microscopie observations in- matites develop through crystallization involving dicatethe optic planesof the crystallitesare invari- both magmatic and supercriticalaqueous phases ably orientedat anglesof 0o, 60o, or 120o relative which differ drasticallyin compositionand viscosity. to muscovite(110). Thus, the lepidolitecrystal- Simultaneouscrystallization may occur from both lographicplane (hkl) which joins muscovite(110) phases,but crystallizationfrom the aqueousphase mustbe located60o from lepidolite(010) (Fig.aA). is believedto continue after completesolidification The uninterrupted(001 ) cleavageplane across both of the silicate melt. Compositionsof both liquid rim and core indicatesthat the crystallographicdi- phases,and the compositionsof their product crys- rection I (001) is parallel in both micas.Accord- tals, tend to changetbroughout crystallization. Ele- ments not readily accommodatedin the lattibesof I The terms "epitaxy" and "topotaxy" are applied in the commonrock-forming silicates (e.g.,F, Li, Rb, erc) strict geneticsense as discussedby Spry (1969, p. 89). are selectivelyenriched in the aqueousphase; con- ZONED LITHIUM.ALUMINUM MICA CRYSTALS 1247 sequently, Li-bearing minerals crystallize primarily from the aqueousphase and the so-called"pockets" develop from the aqueousfluid after the magma is exhausted.Thus, it is apparentlypossible for an indi- vidual pegmatitemineral to crystallizesolely from a magmaor from an aqueousfluid, but many crystals likely develop through a complex history involving =t'- :)-,: crystallizationfrom both magmaand aqueoussolu- --t -t=:: tions. -- It is difficult to determinewith confidencewhether : l:-: the zoned mica crystals grew from a single liquid or were formed by crystallizationfrom two different fluids. As previouslynoted, the enrichmentof Mn and Rb in lepidolite rims may reflect the preferential accommodationof theseelements in the lM struc- B ture rather than growth from fluids of difterent com- position.On the otherhand, closely associated zoned tourmaline, consisting of schorl cores surrounded by alkali tourmaline,seems to support the conten- tion that crystallizationfrom two fluids of different compositionoccurred. That is, muscoviteand schorl FIa. 44. Crystallographic relationship between muscovite crystallizedconcomitantly from the precursory sili- core and lepidolite crystallites. Lepidolite optic axial planes by the short bars and in- cate melt, whereas the rarer pegnatite minerals (parallel to {010}) are indicated variably give anglesof 0, 60, or 120" relative to muscovite lepidoliteand elbaiteformed subsequentlyfrom the (110). Orientations of lepidolite crystallites are related by aqueousfluid. twinning about (310). Prince,Donnay, and Martin (1973) report clear 48. Section perpendicular to muscovite (110) cut along orthoclaseovergrowths (on perthite) from "gem dashedline in Figure 4A. The muscoviteangle (001)A(110) - (001)n (ftkl) where pockets" of the nearby Mesa Grande pegmatites; must equal the lepidolite angle 4 (hkl) are indices of the lepidolite plane which attaches to theseovergrowths are also enrichedin Rb and Cs, muscovite(110). again suggestingthe presenceof late fluids rich in "exotic" elements. Additionally, Munoz (1968, lepidolite which more readily accommodated Rb and 1971) emphasizesthe importanceof HF fugacityin Mn from the fluid phase. the development of lepidolite and was unable to synthesizepure OH end memberlepidolite, whereas Acknowledgments fluorJepidolite was readily produced.Presumably F Inland Steel Corporation Research Laboratory kindly is among the elementsconcentrated in the late provided X-ray diftraction films of mica samples. Chemical aqueousfluids, again implying that the lepidolite analyses were supported by a grant from the Indiana Uni- versity Foundation. rims formed from a phase chemically distinct from the fluid which producedthemuscovite core. References Thus, the zoned tourmaline associatedwith this Bross, F. D., G. V. Grnrs, lNr D. CuvrrrlrNcs (1963) zoned mica, in conjunction with results from other Polymorphism and twinning in synthetic fluorophlogopite. investigations,seems to supportthe contentionthat J. Geol. 71, 537-547. two fluids were involvedin the crystallizationof the Bowr"reN, H. L. (1902) On an occurrence of minerals at Mineral. Mag. 12' 97-121. zonedcrystals; Haddam Neck, Conn., U.S.A. however,Pl enrichmentin the outer- eenNv, F., M. RrroeR, AND P. PovoNone, (1970) Three most muscovitesuggests that the transitionbetween polytypes of lepidolite from Czechoslovakia. Lithos, 3, fluids may have been gradual.As fluid composition 319-325. changed,muscovite crystals adjusted their composi- Desn, W. A., R. A. Howrs, lNo J. Zussum (1962) Rocfr tion, attemptingto maintainliquid-solid equilibrium. Forming Minerals, Vol. 3, Sheet Silicotes. Longmans, Green and Co., Ltd., London. Ultimately the compositionalstability limirs of the Fevr, G. H. (1968) The optical absorption spectraof cer- 2M1 structurewere exceeded,and subsequentcrys- tain transition metal ions in muscovite, lepidolite, and tallizationresulted in the developmentof expitaxial fuchsite.Can. I. Earth Sci. 5. 31-38. 1248 KENNETH T, BROCK

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