Three polytypes of lepidolite from Czechoslovakia

T H R E E P O L Y T Y P E S OF L E P I D O L I T E

FROM

CZECHOSLOVAKIA P. ~ E R N Y , M. R I E D E R , & P. P O V O N D R A

~erng, P., Rieder, M. & Povondra, P. 1970: Three poiytypes of lepidol/te from Czechoslovakia. Lithos 3, 319-325. The micas come from two pegmatites penetrmh~g serpentinite in Western Moravia, Czechoslovakia. T h e lepidglite from Biskupice is themically halfway between polylithionite and trilithionite, and exhibits the 2M, structure which so far was unknown in natural lepidolites. The optical axial plane is perpendicular to (010). ~n lepidolite crystals from Radkovice, the 2M, structure predominates over 1M. The composition i~ close to trilithionite. The optic axial plane is parallel te (010). Refractive indices, densities, and cell volumes are similar, but the IR absorp~.ion spectrum, thermal behaviour, and 2V are different from the corresponding parameters in lepidolite from Biskupice.

P. (~ernS,, Dept. of Earth ,Sciences, Univ. of Manitoba, Winnipeg 19, Man., Canada. M. Rieder, Inst. of Geological ,Sciences, Charles University, Praha, Czechoslovakia. P. Povondra, Inst. fiir Mineralogie, Ruhr-Universitiit Bochum, Germany.

Introduction The lepidolites studied occur ;n lithium-bearing pegmatites that peneuate serpentinites. The imtial purpose of the study was to examine the contamination of late pegmatite stages by components of the ultrabasic wall rock (Mg, Ni, Cr), as reflc,cted in the chemical composition of the micas. The content of these components was found to be low, but the general composition and struc:ural properties of the lepidolites proved to be :~omewhat unusual. The results off a more deta;led study are presented here. Occurrence and paragenesis The lepidolites come from two lithium-bearing pegmatites located in the eastern margha of the Moidanubicum in Western Mora,Aa, Czechoslovahia, approximately 45 km WSW of Brno. Both pegmatite~, named after the adjacent villages and located about 2 km apart, intrude serpentinized gar~letiferous peridotites. The single lepidolite type from the Biskupice I pegmatite (B-41) and t~'o megascopic varieties of lepidolite from the Radkovice pegmatite (R1-43, R2-42) were selected for study. Lack of material did not permit the ,:let~rmination of minor elements in the former mineral. (The numbers 4I, 42, 43 refer to those undl~r which the respective lepidolites are listed by Rieder 1968a). B-41 forms isolated flakes and metaccysts, di.,;seminated with rubelike in the irregularly ~lbitized K-feldspar core of the Biskupice I pegmatite.

320

6ERN';;, RIEDER

& POVONDRA

Lepidolite individuals attain 30 rata in diameter, and display a deep purplish red colour. R1-43 is a dark purplish-red mica in thi~'k flakes 1 to 10 mm long, evenly d~spersed in a fine- to medium-grained a!bite+lepidolite+quartz pegmati~e unit with accessory topaz and rubeilite. This assemblage is located in the inner parts of tb.e pegmatite body, in the neighbourhood of the central lepidolite pods. The sample R2-4S comes from these pods, where it is associated with rubellite and beryl. It is fine-flaked, varying from 0.X to 1.5 mm in size, silvery rosaceous in colour. During late development stages it was locally altered to a boron-be.~ring cookeite (~ern,) et al. 1970). Experimental m e t h o d s and results The coarse B-41 iepidolite was separated 1.manually under a binocular microscope. Specimens from Radkovice were ground to 0.2-1).5 rrtm size and Table i. Chemical composition of two I,:pi"lolit,:s from Radkovice (R) and of one from Biskupice (B). Analyst, P. Povandra 1968

Specimen SiO~ AI.O~ Ga2Oj Cr,O~ Fe~,O ~ FeO Mn() Mg() CaO lfi,O Na20 K20 Rb.,O Cs~,O 'FI,O H.,O -IH:O-F,

R 1-43 wt O,,o

R 2--42 ~vt %

B-41 wt o//,,

51.45 51.40 52.80 22.62 21.31 19.94 0.0098 0.0035 n.d. 0.0005 0.0007 n.d. 0.16 O. 13 0.38 0.036 0.024 0.055 [).51 0.19 0.88 0.53 0.60 0.63 0.20 0.13 0.11 5.42 5.72 5.91 0.26 0.13 0.26 9.00 9.74 9.00 1.69 1.25 1.36 0.94 1.00 0.75 0.0071 O.O0ql n.d 2.36 3.14 | 0.84 0.54 i~ 2.85 7.40 7.08 7..07

--O-2F

103.52 --3.11

102.40 --2.98

101.99 --2.08

Total

'100.41

99..42

99.01

R1-43 R2-42 B-41 Calculated Formulas K Rb Cs Na Ca (H~O)

1.57 0.15 0.05 0.07 0.03 0.13

1.70 0.11 0.06 0.03 0.02 0.08

J .55 0.12 0.04 0.07 0.02 0.20

Li Fe z ~" Mg Mn z+ AI Fe ~ ÷

2.05 0.'!1 0.06 2.56 0.02

3.15 0.12 0.02 2.48 0.01

3.22 0.01 0.13 0.11) 2.34 0.04

1.04 6.96 20.00 3.16

0.96 "~.04 20.00 3.07

0.84 7.16 20.00 3.03

0.84

0.93

0.97

AI Si O F (OH)--

Coefficients lower than 0.005 were omitled in lthe formulas. Oxon;ium was calculated from the analyses to fill up the interlayer pc~sition; there is no direct evidence of its presence. in this way the three analyses give an e~,cess. H , O of 0.4.5, 0.85, and 0,59 ,'e,~pectively that could not be attributed to either (H,O) + or (OH) -. T h e occupancy of the cctahedr:d positions is defective and amounts to 5.70, 5.78, 5.84 respectively in the three ~.icas.

POLYTYP~ 'H(~ve l e n g t h

microns

9

II

I$

15

9

II

I

I

I

I

I

i

IR 2 -

42

RI-

13 '

15 '

9

II ,

-,

43

321

OF LEPIDOLITE

13 ,

15 ,

I00

B-41

8o

'\

,

ta

--J

I j[

!;'-

6o~, z

,l( I-I.i

40 :E O9 Z rY

20

I

:200

J

I

I000

J ._J

I

700 1200

I

~

IO00

j

I~_,

J

"tO0

1200

•

I

I000

I

700

0

,c~

Wove Number cm-I

F:g. I. 1R-absorption spectra of the Radkovice (RI-4.t, R2-42) and Biskupice (B-41) h pidolites; note the close similarity between the Radkovice specimens, and deviations shown by the Biskupice iepidolite in the 11(.',0--860 and 750-69(~ cm -m ranges.

lepidolite was flotated with Katexol. The concentrates so obtained were at least 99% pure, and were further purified by manual separation. Chemical analyses were perfo,:'med by volumetric, complexometric, flamephotometric, colorimetric, al~d spectrographic methods as usually applied to silicate analysis. Because of the high fluorine content, silica was determined volumetrically vir Kz ;"~iF6. The results are listed in Table 1. Single-crystal X-ray data were obtained with a precession instrument calibrated with NaCI and quartz; each photograph was corrected for shrinkage/expansion. F,~r powder data a Guinier-de Wolff camera was used with quartz as internal standard. The results are listed in Tables 2 and 3. Specific gravities quoted in Table 4 were obtained as averages for numer-

Table 2. Unit cell dimensions (precession data) for three polytypes of lepidolite Specimen Structure type a0(A) b0(A) co(A) [3 V(A 3)

B-41 2M~

R1--43 2Mz

R1--43 IM

5.209 -I- 0.006 9.05'3 & 0.008 20.185 4- 0.012 99007.5 ' -;- 2.5' 940 + 4

9.017 -4-0.004 5.208 + 0.003 20.198 -t- 0.010 99°22.5 ' -t- 2.5' 936 -k 2

5.210 :-_0.005 9.026 ± 0.006 i 0.129 -t- 0.006 100040 ' '- 5' 468 4- 1

Other ,;pecimens from the same localities, Biskupice (B) and Radkovice (R) were examined. They yielded identical parameter values within the limits of error.

~EP~, RIEDER

322

:~oo -~

l

600

400 z

& POVONDRA

,i, ~

I

I

i

800 i' '

I

I 0 0 0 oC ,

, I~J

200 ~

, i

f I

400 "

600

!

II "

800 i

, i

|

,

IO00:C ,~

RI-43

DTA DTG R2-42

DTA DTG

_~

B -41

_

_

I

I

I

I

I

I

....... I.......

I

I

!

/qg. 2. Thermal curves of the Radkovice (RI-43, R2-42) and Biskupice (8-41) lepidolites; DTA - - differential thermal analysis, D T G ~ first derivative curve of the T G A record, T G A - - dynamic thermogravimetric curve. The T ( ; A records are shown only as examples illustrating the general weight-loss course; the percer0tage readings are affected considerably by humidi~y in air. A slight shift of the major B--4! endothermic peak to higher temperatures can be observed.

ous flakes allowed to float in a mixture of acetylene tetrabromide and benzene at 20°C. The ,tensity of the liqu;d was 'then measured in pycnometer. Refractive indices were measured in sodium light by immersion techniques, and optic axial angles on universal stage in white light (Table 4). IR-absorption curve:,~ were recorded by the UNICAM SP, 200 IR-spectrophotometer, with 1 mg powdered lepidolite in K Br pellets. Tim spectra covered the range 500I~-650 cm -~ (Fig. 1). Table 3. X-ray ':n ensity data for the new lepidolite polytype 2Mr from Biskupice (B-41). Cell dimensions a:e gi'.,en in Table 2. Gainier-de Wolff camera, Cu/Ni, 7.= 1.5418 A, quartz internai( standard (Fro~Ldel 1962) 1

hkl

i

3 2 4b I 0.5 b 2 2 4 O.5 4 0.~ 5 1 5

002 004 II0 021 022 !!~ I 12 023 114 113 006,0247 lit3 114 025

2 3 0.5 8 8 0.5 1 6 3 0.5 2 0.5 1 4

hki

116 115 026 20.~, 130 13:~,117,200,131 008,13~ 132 134,202 ? 22~,22T ? 220,134 206,22~ 0.42 028 043,13~

I

hkl

1 b 225 0.5 044,118 0.5 223 2 O29,226,0.0.10 1 045 0.5 2Og 0.5 b 047 0.5 b 226 1 138 0.5 2.0.TO 1 b 31T,24~,150,31~ 3 b 315,242,1.3.T'~ 2 153,245,139 1 316

I

hkl

0.5 0.5 049,246 ?,154 ? 1 312? I 31~ 0.5 244,15~ 0.5 247 0.5 b 31g,1.3.10 ?,314 ? 8 060,332,245 1 334,156 1 3 i 0,246 ? 1 24r~ 0.5 333,065 2 15§ ?,334 ? 4 15~i,066

POLYTYPES OF LEPIDOLITE

323

Table 4. Physical properties of three lepidolites from Radkovice (R) and Biskupice (B) Specimen Structure type Colour Na N[3 Ny "Ny-N~ 2V a opt..ax.plane b D meas. D calc.

R 1-43 IM dark purplish red 1.536 1.553 i.555 0.019 35 ° q:0.5 ° //(010) 2.825 2.804

112-42 2M: pinkish-white 1.535 1.551 1.553 0.018 35.3 ° + 0 . 3 ° //(010) 2.83,~ 2.811,

B - ti 21V~t deep redclisim-purple 1.537 ! .552 1.554 0.017 28.3 ° 4- :,. ~o ;22" ± 0 . 7 ° 1 (010) 2.828 2.828

a average value of measurements on 10 flakes or more b none of the m e a s u r e m e n t s listed in this table were c o n d u c t e d on flakes used for the study of the O.A.P. orientation.

The amomatic Derivatograph (ORLON, Budapest) was used to obtain the thermal curves shown in Fig. 2. The charges weighed about 200 mg, and the heating rate was 10°C/min, in air. The d.t.a.curves were corrected by subtracting from them the second d.t.a, runs of the previously analyzed mica samples.

Discussion Chemical analyses of all three lepidolites are sPmilar to each other. The formulae are close to trilithionilte (K2Li~AI~AI~Si6020/F,OH/4) and cationdeficie'.lt. The B-41 lepidolite approaches somewhat the Li/R 3+ ratio of polylithionite (K2Li4AI2SisO20/F,OH/4). This cain be observed if the compositions are plotted in Figs 25-28 of Foster (1960). All three points fall inside the triangular area, cl~.~e to the trilithio~ite corner. In all four diagratns, the B-41 mica is distinctly closer to polyli'thionite than the Radkovice lepidolites. The contents of minor elements do not exceed their usual range in lepidoiite. The low gallium content is noteworth:, as well as the low Rb/Cs ratio (cf. Foster 1960, Heinrich 1967 a, b). The petrological implications of the lepidolite compositions will be discussed elsewhere, in the discussion of the whole assemblages of the parent pegme.'ites. In the lepidolites from Radkovice, the 2M2 structure predominates over IM. Of the 15 mica crystals in 13 flakes from this locality that were e~2:mined by s,ingle-crystal techniques, 10 were found to be 2Mz, 5 were IIV][; 8 powder photographs showed the same relation. However, all X-rayed flakes from Biskupice belong to 2M~; 6 single crystals from 2 flakes, and 4 powdered flakes did not reveal the presence of ,~ny other structure type. This is the first example of 2M! structure in natural lepidolite. Synthetic 2Mr m;.cas close to trilithionite composition, were studied recently by Munoz (1968).

324

~EPa~¢~,RIEDER

& POVONDR&

Discontinuities in the plot of dool~ (N is the number of layers in the',repeat of a pol~ype) against the Li/(Li+R 2+) ratio led to a hypothesis about the structural arrangement of the octahedral sheet of the Li-Fe micas(Rieder 1968b). Values of dooN split far the two lepidolite end members;timyare low for polylithionite and high for trilithionite. The dooN of the B and R lepidolites (used also in Rieder !968b) indicate octahedral disorder; however, the frequent 2M 2 structure and the unusual optical orientation of the 2Mr mica (see,. below) suggest that the structure of trilithionite and rnicas of similar composition is more complex. Another model of trilithionite structure has been proposed by Franzini (1969). Physical properties of the micas examined (Table 4-) are close to one another, as expected from the similar chemical Lomposition of the samples. Differences in colour are striking: the colour varies with total Fe an{I Mn contents. The refractive indices fall within the range given for lepid(,lites, but close to the lowest values, as can be anticipated from the low Fe,Mncontents (cf. Winchell 1951, Deer et al. 1962). The same applies to th,,: low specific gravity. The property that depends clearly on polytypism/polymorphism i~ the orientation of the optical indicatr:x. The. optical and X-ray adjustments of crystals show that in the 1M and 2M2 structures the optic axial plane is parallel to (01(}). In the 2Mr iepidolite B-41, the optic axial plane is normal to (010) as in 2M~ muscovites; its 2V values are low (Table 4). The different orientation ir.6,cates an urmsual behaviour of the 2M~ lepidolite. The 1M, 2M~ or .my other monoclinic polytype of the zinnwaldites (I,i-Fe micas on the sidorophyllite - polylithionite join) have not been observed to violate O..A.P./t(010) (Riede:: 1968a). The IR-record,,s display only minor mutual differences in the 1200650 crn -t range, as, shown in Fig. I. The two Radkovice specimens are particularly .,dmilrr; only slight shifts in itrtensities of some absorption bands (1000 cm -*, 970 cm -*) can be observed. However, the B-41 mica shows di~;tinclt br,sadening of the main Si-O ban,d towards lower wavelengths, a more i,ronc,ur:ted 870 cm -t shoulder on its right side, and a broad extension {}f the 752 cm-t absorption towards higher wavelengths (up to 686 cm-*). The.~e differences seem to reflect the singularities found in the B-41 lepi,'Jolite ~shen examined by other methods. Thermal bchaviour of the iepidolites studied is illustrated in Fig. 2. Besides shallow troughs at about 100°C indicative of loss of adsorbed water, the micas exhibit three closely-spaced endothermic effects at high temperaturc.,,. The second reaction is the most intense, flanked by two subordinate peaks: 820-880-950':'C (R 1-43), 840 - 880 - 950°C (R2-42), and 810 - 895 935':'C (B-41). Generally, these characteristics correspond to those quoted for lepidolite in the literature. Lepidolites undergo a structural breakdown at around 880'~C and thcn melt at slightly higher temperatures, indicated by the last e ndothermic effeclt. However, zinnwaldites and lepidolites related to zinn-,

POLYTYPES OF LEPIDOLITE

325

waldite in composhion decompose at higher temperature~ (Correia Neves 1960, Ivanova 1961, Yakov[eva et al. 1965). As ti,e compo,,;ition of the B-41 mica bears resemblance to that of a polylithionite, which perhaps hab the 'zinnwaldite.-ordered' octahedral sheet, it is tempting to say that this is the cause of the higher temperature of the major endothermic peak of the B-41 sample.

A C K N O W L E D G E M E N T S . The authors express their thanks to Dr. F. Aumento (Dalhousie University, Halifax, N.S.) forcompu~er processing most of the X-ray powder data, and to Dr. W. Schreyer (Ruhr-Universitiit Bochum) for final refinement of the b-,/I unit c,.:ll dimensions based on powder X-raying, using the program written by Dr. C. V'.r. Burnham (Harvard University). Dr. K. Melka (Geological Survey of Czechoslovakia, Praha) took the Guinier-de Wolff photographs, Dr. g. Johan (Geological Institute, Czechoslovak Academy of Sciences, Praha) supplied t:s with the IR-svectra. Thanks are due aJso to Dr. R. B. Ferguson (University. of Manitoba, Winrfipeg) for his critical reading of the man~lscript. Single-crystal X-ray work was done at the Department c,f Earth and Planeta~.~ Sciences, Johns Hopkins University, Baltimore. - -

REFERENCES Cern~,, P., Povondra, P. & Stan6k, J. 1971 : Two cookeites from Czechoslovakia - a boronrich variety al~d a lib polytype. Lithos d. Correia Neves, J. M. 1960: Pegmatitos corn berilo, colun~bite-tantalit,, e.[osfatos da Bendada (Sabugal, Gut,~rda). Coimbra. l)eer,W.A., Howie,R.A., & gussman,J. 1962: Rock-forming Mfiterals, Vol. 3, Sheet Silicate.~. London. Foster,M.D. 1960: Interpretation of the composition of lithium mica~. U.S.G.S. Pro[. P~ p~,r 354-E, 115. Franzini,M. 1969: The A and B mica layers and the crystal structure of sheet silicate,,.. Contr. Mineral. and Petrol. 21,203. Frondel, C. 1962: Dana's System of Mineralogy, 7th ed., Vol. 3, Si/ica Minerals. New York. Giiven,N. 1968a: The crystal strm-mres of 2M~ phengite and 2Mr mascovite. Carnegie lnsl. Washington Year Book 66, 487. Giiven,N. 1968b: A mechani:m of" stacking sequences in dioctahednd micas. Carnegie Inst. Washington Year Book 66, 492. Giiven,N. & Burnham,C.W. 1967: The crystal structure of 3T muscovite. Carnegie Inst. Washington Year Book 65, 290. Heinrich,E.Wm. 1967: Micas of the Brown Derby pegmatites, Gunni=~on County, Colorado, Amer. Mineral. 52, 1100 and 1578. I vanova,V. P. 1961 : Thermograms of minerals. Mere. All- Union Miner. ,~oc. 90, 50 (in Russian). Mur~oz,J.L. 1968: Physical properties of ~,ynthetic lepidolites. Amer. Miner. 53, 1490. Rieder, M. 1968a: A study of natural and synthetic lithium-iron micas. Ph.D.thesis, John=; Hopkins University, Baltimore, Md. Rieder, M. 1968b: Zi~nwaldite" oc~ahedral ordering in lithium-iron micas. Science160, 1338. WinchelI,A.N. & WinehelI,H. 1951 : Elements of Optical Mineralogy, 4th ed., part H., New York. Yakovleva,M.E., lq.azmanova,Z.P., & Smirnova,M.A. 1965: On lepidolite with small op=i.' axial angle. Trans. Miner. Museum dead. Sci. USSR 16, 287 (in Rus,.ian). Accepted for publication April 1970 ,Zl - -

Lithos

3:4.

Printed October 1970