THE AMERICAN MINERALOGIST, VOL. 47, MAY-JUNE, 1962

EUCRYPTITE FROM BIKITA, SOUTHERN RHODESIA1 (Jniaersity, ConNBr-rusS. Hunrsur, ln., Horttard. Cambrid.ge, M ass achus etts.

Assrnacr Eucryptite is an important bearing mineral at Bikita, Southern Rhodesia. Imbedded in a matrix of granular eucryptite are single crystals. From morphological measurements, c:0.6681. Optically uniaxial (*), r:1.572, e:1.586. Unit cell dimen- sions:a:13.48t0.005A,c:9.01 10.005 ft;o,u:9.344,a:107"48'.aic:0.6684;space group, R3. Colorless, transparent. Specific gravity 2.657 (meas.), 2.661 (caIc.). Hardness 6|. Analysis: SiO:49.32, AIzOa40.43, LirO 9.72, NazO 0.24, KzO 0.01, Total99.72'

INtnorucrroN The low temperature, naturally occurring eucryptite2 (a-eucryptite) has been reported from a limited number of localities. It was first de- scribed by Brush and Dana (1380) from a at Branchville, Connecticut.There it occurredintimately intergrown with , formed as an alteration of .Mrose (1953) describedfrom Center Strafiord, New Hampshire, an inter-growth of eucryptite and albite identical with that at Branchville; and a massive granular eucryptite from the Harding Mine, Dixon, New Mexico. In 1955 the writer col- lected specimensof coarsely granular eucryptite from Bikita, Southern Rhodesia.Imbedded in this granular material as a matrix werecrystals of eucryptite. Bikita is believedto be the only locality where eucryptite is found other than in fine-grainedaggregates, and the data presentedin this paper wereobtained from a study of theseunique crystals.

OccunneNcB The geologyof the Bikita district, which lies about forty miles east of Fort Victoria, SouthernRhodesia, has beendescribed by Tyndale-Biscoe (1952).His map showsa seriesof pegmatiteswhich extend for two and one-halfmiles in a north-south direction. The principal dike, which is in the southern portion of the area, is about one mile long and has a N-S strike and easterly dip. This dike representsone of the world's great con- centrations of lithium and at present is being actively mined over nearly its entire length by Bikita Minerals, Ltd. The chief lithium minerals are and , but spodumeneand are recoveredin lesser amounts. Bikitaite, a new lithium mineral found only at this Iocality, is also present. In recent years eucryptite has taken its place among the major Iithium bearing minerals at Bikita. It is remarkable that

1 Mineralogical Contribution No. t103, Harvard University. 2 Unless otherwise stated eucryptila in this paper refers to the low temperature a- eucryptite.

557 558 CORNELIASS. HURLBAT, IR.

this mineral, which elsewhereis extremely minor, is found here in thou- sands of tons. Current mining produces 200 to 300 tons of eucryptite a monthl The eucryptite is mostly in granular aggregatesassociated with large massesof petalite. By inspection it is extremely difficult to distinguish it from , but under the ultra-violet light it can be identified easily be- causeof its pink fluorescence.

Puysrclr exo Oprrcar PnopBnrrps Many specimenscomposed largely of anhedral granular material con- tain clear, transparent single crystals of eucryptite, some of which show bounding faces.The largest of thesecrystals is 3X2X2 cm. By carefully

Tlnlr 1. Eucnyprrre Axcrn Tanrn Hexagonal; rhombohedral -3 a i c : I i0.668l q : l07o 48' po:ro:O.7715 :l tr: 63"54'

Ar Az

c 0001 0000' 90000' 90"00' m 1010 30000' 90 00 30 00 90 00 a 1120 000 90 00 60 00 60 00 r 10T1 30 00 3739 58 03 90 00 S O1T2 -30 00 2r 06 90 00 7150 t 1014 30 00 1045 80 34 90 00 y 1232 -10 34 45 35 7615 62 t9 breaking away the matrix of granular eucryptite, three crystals were iso- lated that could be measured on the reflecting goniometer. The most prominentforms are [0001], {1010} and {1120}; the othersare present as only small truncating faces.Faces of {01121 were present on all the crystalsand gave consistantp valuesof 21o06'.It is this value that has beenused in calculating the axial ratio and anglesgiven in Table 1. The coarser crystalline eucryptite is colorless and transparent and breaks with a conchoidalfracture. There is poor {1011} .The {0001} cleavagedescribed by Brush and Dana (1880)was not observed. The hardnessis 6f . The specificgravity measuredon the Berman balance is 2.657;the calculatedspecific gravity is 2.661. Larsen (1921)reported eucryptite as uniaxial negative,co:1.545 and gives a U. S. National Museum specimenfrom Branchville, Connecticut as the sourceof his material. This was the only reported refractive index of eucryptite until Roy et aI. (1950) gave refractive indices for synthetic a-eucryptite. These authors also re-examined the Branchville, Connecti- EUCRYPTITE 559

Ttst-n 2. Oprrclr- Pnoprntns or d-EucRyprrrE

o 1.572+0.001 1.573 1.572+0.002 1.572+0001 e 1.586+0.001 1.583 1.587+0.002 1.587+0.001 Opt.+T-.r+

l. Bikita, Southern Rhodesia. 2. Branchville, Connecticut and synthetic. P.oy e, al. (1950). .1. Harding Mine, New Mexico; Center Strafiord, New Hampshire; and synthetic. Mrose (1953). 4. Synthetic. Stewart (1960). cut material and found its optical properties coincided exactly with the synthetic. In like manner Mrose (1953)reported that a-eucryptite syn- thesized by Kennedy had optical properties identical with eucryptite from the Harding Mine. A comparisonof theserecently reported optical propertiesin Table 2 showsthem to be all closelysimilar.

CuBursrnv In Table 3, column 2, is given the chemical analysis of a transparent crystal of Bikita eucryptite and in column 3 the analysis of eucryptite from the Harding Mine. Rotation and Weissenbergphotographs of the Bikita eucryptite were were taken with the crystal rotating about the a and c axes. From these the unit cell dimensionsgiven in column 1, Table 4 weredetermined. The determination of the spacegroup as R3 agreed with Winkler's (1953).

Taer-n 3. Eucxvpu:rn Cnrurcar- Ar,ql,vsrs

SiOz 47.69 49.32 54.64 AlzOa 40.46 40.43 35.76 Lt:O 11.85 9.72 8.36 Naro o.24 o.62 KrO 0.01 0.38 CaO 0.19

100.00 99.72 99.95

1. Compositionof LiAlSiOr. 2. Eucryptite, Bikita, SouthernRhodesia. J. Ito, ana.lyst. 3. Eucryptite,Harding, Mine, New Mexico.F. Gonyer,analystin Mrose (1953). CORNELIUS S. HURLBUT, JR.

Tenr-r 4. Eucnvprrrn UNrr Crr,r, Drurnsrows

a 13.48+0.005A 13.53A 13.s4A 13.476+0.003A c 9.01+0.005 9.04 9.01 9.003+0.001 0.6684 0. 668 0.6654 0.6681 G (meas.) 2.657 2.67 2.64 G (calc.) 2.661 2.63

1. Bikita eucryptite. 2. Synthetic eucryptite, Winkler (1953). 3. Harding Mine eucryptite, Mrose (1953), 4. Synthetic eucryptite, Skinner and Evans in Stewart (1960). a,n:8.342 A, o: 107"48'.There are six formula weights per rhombo- hedral unit cell. Considering eucryptite as a stuffed derivative of SiOz, or Si12O2,as suggestedby Buerger (1954), the general formula can be written as (Silr-"AI") (Li"-"Aly7s)O2a.This, as pointed out by Mrose (1953), indi- cates an interesting coupled substitution in which valence compensation for Al:Si is provided by 3Li*Al rather than by Li alone. For Bikita eucryptite (Anal. 2, Table 3) r:5.77 and y:0.78 and the measured rhombohedral cell contents are (Sio zaLIs.zz)(Lin sgAlo.ze)Oz+or Li.r.ggAlo.oa

Tasle 5 X-R,q.v Pownrn D.trl ton Eucnvprtrn Cu Ka R.lorarroN f:154184

d obs. A d calc. A d obs.A d calc.A

lt20 J 6.736 6.740 4133 1.943 Ir.o+s 0112 n 4.220 4.202 6066 f1.e4s 2r3l 9 3.965 3.962 437r ' 1.873 )r.877 3030 3 3 88s 3.890 5270 \r.soo 2022 2 .t.555 3.566 3363 .) | 797 1.799 2240 10 3 362 3.370 4044 <1 r. /6r t.782 J 1 1i7'' 1 3.148 3.152 3472 [1.781 r34l I 3 .038 3.046 6063 1 1.632 I. OJJ tt23 7 2.733 2.74s 7180 2 t.5M I . 54.) 4150 6 2.545 2.547 2682 <1 1.523 t.523 0442 <1 2.450 2.M9 0006 t.499 1.500 3033 5 2.373 2.377 6390 <1 r.47r 1.471 3360 (z.z+z +;B3 1 t.469 1.469 2243 2.2M 12.242 2246 2 r.372 1.371 0551 12.240 9090 1 t.297 1.297 246r <1 2.141 2.143 0884 <1 t.229 | 225 0224 <1 2.091 2.101 6066 <1 t.182 1.188 4262 <1 1.978 1.981 EUCRYPTITE 561

Sio.zsOza.This approachesmore closelythe theoretical composition of 6(LiAlSiO4) than the Harding Mine eucryptite or the material synthe- sizedby Kennedy (in Mrose, 1953). The a axial dimension of the Bikita eucryptite is 0.06 A lessthan that given by Mrose (1953)for Harding Mine eucryptite, whereasit is only 0.004 A greater than that given by Stewart (1960) for pure synthetic eucryptite.The specificgravity of eucryptite from Bikita is 0.017greater than that from the Harding Mine. Although the data are insufficient for generalizations, it would appear that the nearer the compound ap- proaches the theoretical composition the smaller the o axial dimension and the greater the specific gravity. The d spacingsof Bikita eucryptite determined from a powder photo- graph taken with a camera of 57.3 millimeters radius are given in Table 5 and agreefor the most part with thosegiven by Isaacsand Roy (1958).

Rnrunrucns

Bnusn, G. J. mn E. S. DeNe (1880), On the mineral locality at Branchville, Connecticut. Am. Iour. Sca.(3rd ser.),20, 257-285. Bunncnn, M. J., (1954), Stuffed derivatives of the silica structures. Am. Mineral, 33, 600-614. Isucs, T. ,tNn R. Rov (1958), The a-p inversions of eucryptite and spodumene.Geochim. et Cosmochim.Acta, 15,213-217. LARsEN, E. S. (1921), The microscopic determination of the non-opaque minerals. [/. S. Geol,.Surt:ey Bul,l,.679. Mnost, M. E. (1953),a-eucryptite problem (abs.), Am. Mineral 38,353. Rov, R., D. M. Rov anr E. F. OssonN (1950), Compositional and stability relationships among the lithium aluminosilicates: eucryptite, spodumene, and petalite. Jou,r. Am. Ceram. Soc. 33, 152-159. Srnwrnr, D. B. (1960), The system LiAlSiOrNaAlSi:Oa-HzO at 2,000 bars. Rept. XXI Inter. Geol. Congress, Norden. Part XV[, 15-30. TvNnr.r-Brscon, R. (1952), The geology of the Bikita tin-field, Southern Rhodesia. GeoI.Soc. South AJrica Trans.,54, ll-25. Wrxrcrnn, H. G. F. (1953) Tief LiAlSiO4 @ukryptit). Acto Cryst.6,99. fuIanuscript rcceiued.,July 24, 1961.