Canadian Mineralogist Vol. 18, pp. X97-200(1980)

GAITITE, HgGagZn(AsOn),(OH),,A NEW MINERAL FROM TSUMEB, NAMIBIA (sourH wEsTAFRIGA)

B. DARKO STURMAN Departrnerrt of Mineralogy and Geology, Royal Ontario Museum, 100 Queen'sPark, Toronto, Ontario MSS 2C6

PETE J. DUNN Department of Mineral Sciences,National Museum of Natural History, SmithsonianInstitution, lilashington,D.C. 20560, U.SA.

ABSTRACT b 7.61(l), c 5.57(1).4" a 111040(5)',p 70o50(5)', v 119"256)'i Doarn= 3.80 Pur Z = l, D'"" 3 Gaitite occurs as coatings and as small on 3.81(5). ks neuf raies les plus intenses prosperite, and on specimens from du clich6de poudresont (4 I, hkl): 5.05(40)(100)' Tsumeb, Namibia (South West Africa). Gaitite is 3.5SI (40) ( 121), 3.350(50)( 1 10. 3.208(50) (020), white to colorless with vitreous ; white ; 3.0s0(80)(Tll), 2.7sL0oqQh,1I1), 2.7s|(7D',) hardness (Mohs) 5. is good on {010}, (11D, 2.r6s(4u1l2) et r.72r(6o) 4,. and Gaitite HgCazZn(Aso4),(oH), {001} {0I1}. la R6daction) and HzCagMg(AsOn)r(OH), are isostruc- Cfraduit Par tural and form a solid-solution series. The indices gaitite, Namibie, ars6niate,nou- of refraction of the zone with the composition H:.oo Mots-cl6s: Tsumeb, Ca1.rr (Zno.s0Mgo.reFeo.orMno.ot)(AsOn)r.oo (OH) r.oo velle espdcemin6rale. are a 1.713(2), P 1.730(2). r 1.748(2'); 2l/'l X (6 - 28", 88"; the orientation of the indicatrix is INnoPucrtox p - 32o), Y (6 : 185o, p = 60") ao.dZ (6 - - pl -79"_, p 80o). Gaitite is triclinic, During the study of prosperite (Gait et al. -roselite or Pl. Cell dimensions. similar to those of F 1,979), another phase found on the type speci- and talmessite,are a 5.90(1), b 7.61(l), c 5.57(l') men from Tsumeb, Namibia (South West Afri- A; c 111o40', = 70"50', v ll9o25', ! 5'; F atl as talmessite, D."t. is 3.80 g,/cms for Z = 1, D."* is 3.81(5) sa) . was tentatively identified g/cm8. The strongest nine lines in the X-ray HzCazMg(AsOo)z(OH)a.However, an examina- powder-diffraction pattern are: 5.05(40)(100). tion of the optical properties indicated that a 3.581(40)(121),3.350(50) (1lQ, 3.208(50)(020). large proportion of Mg must be replaced by 3.080(80)(l 1 1), 2.78r(1AO)Qzr,rrr), 2.7sO(70) another element, later identified by X-ray fluo- (TD,2.r6s(4c)(o.5Dand 1.721(60) A. rescenceas Zn. Subsequentmicroprobe analysis Keywords: Gaitite, Tsumeb, Namibia, arsenate, new confirmed this observation and also showed that mineral description. the grains are strongly zoned with large varia- tions in the Mg:Zn ratios. Sot"lNrerns We take pleasure in naming this new mineral speciesin honor of Dr. Robert I. Gait, Curator On trouve la gaitite en enduits et en petits cristaux of Mineralogy at the Royal Ontario Museum. prosperite, des sp€ci- sur adamite et austinite sur We do so in recognition of his contributions to mens Tsumeb, Namibie (Afrique Sud-Ouest). de du (in particular his curatorial excel- C'est un min6ral blanc i incolore, I l'6clat vitreux, mineralogy rayure blanche, duret6 Mohs de 5 et bons clivages lence) and his many exhibits explaining the {010}, {001} et {011}. La gaitite HzCa,Zn(Asoa)g concepts of mineralogY. (OH)z et le talmessiteHgCarMg(AsOa)g(OH)n for- The name and the mineral have been ap- ment une solution solide compldte et sont donc iso- proved by the Commission on New Minerals and structurelles. Les indices de r6fraction de la zone Mineral Names, I.M.A. The name is pionounced de composition Hg.ooCar.ge(Zns.66Mgs.1*Feo.ntMrlo.ot ) GET'AIT. A type specimen, containing about (AsOa)z.oo(OH)e.oo p sooti a L.713(2), 1.730(2),'{ 50 mg of gaitite, is preserved in the Royal 1.748(2),2Vt 88': l'indicatrice a l'orientation X (6 Ontario Museum, Toronto (M35388). A pol- 28", p 32"\, Y (6 185o, p 60o), Z (6 :79", of the grain analyzed by p 80o). Triclinic: le groupe spatial reste ind6ter- ished thin section min6. Les dimensions de la maille ressemblent i microprobe has been registered at the United celles de la F-rosdlite et de la talmessite: a 5.90(1), States National Museum of Natural History, I97 198 THE CANADIAN MINERALOGIST

Smithsonian Institution, Washington, D.C. CRYSTALLOGRAPHY (NMNH t44799). The strong Mg-Zn zoning indicates complete Gaitite is developedmostly as a polycrystalline solid-solution between gaitite HzCaeZn(AsOo), coating; only a few small crystals were fo rnd, (oH), and talmessiteHzcaeMg(Ason),(oH),. with the largest dimension of about 1 mm. The The name gaitite is to be used for members in facesof onlylour forms were observed.The edges which Zn predominates. between the well-developedfaces t010), t001) There is some confusion about the name for and {0T1} enable one to find the a axis on the the Mg end-member.Two names,talmessite and crystals or fragments; the face of {100} is usually arsenate-belovite,have been used. The first, and verv small. unfortunately incomplete, description of the ihe crystals were first examined with the two' phase H,CazMg(Asoe):(OH), was given by circle opiical goniometer. Then, the unit- cell, Nefedov (1954), who proposedthe name belov- similar io thai given for talmessite, was found ite. However, the name belovite was also given on the projections of the 0 and 1 levels of the to the Sr analogue of hydroxyl-apatite, de- reciproc-al lattice, prepared from Weissenberg scribed by Borodin & Kazakova (1954). Yakhontova & Sidorenko (1956) proposed TABLE2. X-MY POI.IDER-DIFFRACTIONDATAFOR GAITITE AND TALMESSITE for the that the name arsenate-belovite be used TALMESSITE, OAITITE mineral with the compositionHrCaeMg(AsOa), JCPDScard fl7-164 (OH),, and the name phosphate-belovite for I dt"u, I dr"a, dcalc wL Sr5(Po4)s(OH). The data were still incomplete, o.qJ 20 6.40 6.4'14 010 r0 5.08 5.167 lI0 and it had not been determined whether arse- OU (no I t40 5.05 5.054 'rIlt00 nate-belovite belonged to the monoclinic roselite 40 4.62 20 4.64 4.640 't0l l0 5 3.958 3.964 gtoup or to the triclinic B-roselite group. 60 40 3.581 3.583 rZl (1960) describeda triclinis 30 3.570 3.572 0ll Bariand & Herpin 6 3.44 modification of HaCa,Mg(AsOo)a(OH)", from 40 3.34 50 3.350 3.343 lt0 2 ?n4 t ?nn Tol Talmessi in Iran, that is isostructural with B- 80 3.21 50 3.208 3.207 020 used I30 3.098 3.088 oZt roselite, HrCazCo(AsOe)r(OH)2,and they 100 3.07 t80 3.080 3.077 Irl the name talmessite for it. Later, Pierrot (1964), 2.838 2.842 2Il 2.815 2.810 lll the material of both talmes' I 2.782 221 after studying Epe r r00 z't6t 't00 I I z.zgo lll site and arsenate-belovite,showed that they are 2.77 [70 2.750 2.746 r12 the same species. The name talmessite is now 5 130 2.607 2,602 0i2 used by most mineralogists; in 1976 the Com- 20 2.58 tl0 2.582 2.583 220 a Ed [ 2.527 200 mission on New Minerals and Mineral Names, 6 2.51 dl0 t 2.516 r3r its 20 2.44 10 2.445 2.447 2Ql I.M.A.. officially approved use. 2.356 2.357 I30 'I. | 2.336 2tr1 TABLE CRYSTALLOGMPH1CANDOPTICAL DATA FOR GAITITE 5 | 2.320 222 120 HZCaZ(zn.. ) (As0n)r(0H)2 5 2.303 2.3U B4MsO.l4Feo. o,,ltlno. ot 40 2.16 40 2.165 z. too I32 5 2.135 UNITCELL: Spacegroup P] or PIi Z E 1 5 2.1? 5 2.118 10 2.07 30 2.07'l a . 5.90(l)A drnns.05(l)A 2.O4 l0 2.039 , 7.61(1)A aniie.ertr)A l0 2.022 " 20 2.010 - 20 2.01 o 5.57(l)A aii, s.osttlA 20 l.B9 30 I.892 20 1.86 ]0 t.870 o lll'40(5)' o* 75"20(5)' 20 1.829 10 1.829 l0 1.801 B 70"50(5)' Br 100.30(5)' 20 1.782 15 1.782 1.721 r ll9'25(5)' rr 65'05(5)' 60 1.717 60 20 1.712 40 | .697 30 1.700 COORDIIIATES: Op 5 1.688 r { I00.} 65"05', 90"00' '10 1.608 5 |.601 1.584 20 I.581 roro: 0'00, 90'00' l0 1.555 Foms I toort - 5f'29' 23"57', 5 t.l4a I 1.5J6 10 1.538 I tol]) 211'0r' 33'59' 5 I .521 5 1.492 Princl pal 28" 32" l0 1.461 Vibratlon lrJ: 185' 60' 5 1.432 Dirstlons l0 1.407 Iz - 79' 80" 5 I.390

OPTICALDATA: R lt?o o 1.713(2) 2V, = 06121'tt"u"'' 5 1.242

cmera, Cul.€radlatloni intensltl€s estimated = (calc.) Gulnier-de Colff r 1.148(21 2v" 89' vlsually; d. double. GAITITE. A NEq/ MINERAL FROM TSUMEB 199

photographs. The rotation- and precession-sam- era methods were used to complete the single- study reported in Table 1. The X-ray powderdiffraction pattern was in- dexed using the unit-cell dimensions determined by the single-crystal methods, and the intensities observed on films prepared by the single-crystal methods. In Table 2 the indexed pattern for gaitite is compared to the pattern of talmessite. The indexing for gaitite does not agree entirely with that of talmessite(c1., PDF 17-164).

Puvsrcel euo Oprlcel Pnopnntras

The small grain size and zoning made an accurate determination of density nearly impos- sible. Density was determined by heavy-liquid methods on a small crystal fragment that was later analyzedwith the microprobe (Table 3). Examination of the indices of refraction showed Flc. l. Stereographicprojection of optical and that the major part of the fragment is homo- crystallographicelements of gaitite. geneous.The measureddensity, 3.81(5) g/cms, compares well with the calculated density of 3.80 g/cms for gaitite with the composition given It was impossible to determine accurately the in Table 3. The Mohs hardnessis approximately relationship betweenthe Zn:Mg ratio and optical 5. properties because of the strong zoning and Cleavage is not easily observed, but there are small grain size. A zone with Zn:Mg - 1:1 is probably three equally good cleavages, {010}, optically negative, 2Vn = 85(5)', and q, {001} and {0T1}. Most gaitite fragments have 1.689(4),B 1.707(4) andy 1.727(4).Replace- an irregular outline with only one or two cleav- ment of Zn with Mg seemsto lower the density, ages visible. decrease the indices of refraction and change Gaitite is white to colorless; the lustre is the optic axial angle to a negative value. The vitreous and the streak is white. The optical orientation of the indicatrix in the zones with properties were determined on the fragments different Zn:Mg ratios is nearly identical, as is that were analyzed with the microprobe. Optical indicated by nearly identical extinction for all data for the zone with the composition of zones. HrCaz(Zno.arMgo.raFeo.orMno.or)(AsOa) r(OH)r are Gaitite does not fluoresce in either long- or shown in Table 1 and Figure l. short-wave ultraviolet radiation. The Gladstone- Indices of refraction and indicatrix orientation Dale constantKc (Mandarino 1976), calculated were determined with the spindle stage in Na from the chemical composition given in Table 3, light on an analyzed grain that was oriented is 0.188, which compares well with the value by X-ray single-crystal methods. The optic axial Ke - O.192, calculated from the measured den- angle was determined on the universal stage. sity and indices of refraction

TABLE3. MICROPROBEANALYSIS OF GAITITE CneMrsrRY Analysls Theory using an Mgo 1.2 0.0 Crystals of gaitite were analyzed Cao 24.2 24,42 ARL-SEMQ electron microprobe utilizing an zno 15.3 17.71 Feo 0.2 0.0 operating voltage of 15 kV and a sample current llno 0.2 0.0 As-o. 5O.2 50.04 of 0.15 p,A. The data were corrected using aa HzO 7,8 7,83 Bence-Albee factors. The standards used for analysiswere fluor-apatite for Ca, synthetic ZnO Total ,r-l I 00.00 for Zn, synthetic olivenite for As, manganite for Accuracy of data: :39 of the amountpresent. Mn and hornblende for Mg and Fe. A wave- 'HrO detemined by DTA/IGA length-dispersive scan indicated the absence of Enplrlcal fomula calculated on the basis of 10 oxygenatms: any elements with an atomic number above 9, Hz.ooet .gg(zno.goMgo.r4F"0. orho. or )xr .02(As04)2.0o(0H)2.00 except those reported here. The analysis (Table 200 THE CANADIAN MINERALOGIST

3) is in good agreement with the theoretical ved. Gaitite is followed in the sequenceof for' composition. mation by euhedral crystals of light green austin- DTA/TGA were performed with a Mettler ite, yellow barite and colorless to light pink Thermoanalyzer on 16 mg of material. The qaartz, in that order. accurate Mg;Zn ratio of the grains used for analysesis not known, but it is probably similar ACKNowLEDcEMENTS to that of the grain analyzedwith the microprobe (Table 3). The grains used for DTA/TGA The authors are grateful to Mr. Prosper J. were separated with heavy liquids and later Williams for bringing the mineral to their atten- examined on the microscope in the immersion tion and for donating the type specimen to the liquids to ensure that major parts of the grains Royal Ontario Museum. We thank Dr. Paul have density and indices of refraction similar to Keiler for allowing us to describe his specimen those of the grain analyzedwith the microprobe. of gaitite. We also thank the following people A total weight loss of 7.8% is related to two at the Royal Ontario Museum: Dr. J.A. Manda- endothermicpeaks, at 470 and 513oC, indicating rino for discussionsabout the use and the cal- that the water content can be expressedas hydrox- culations of the Gladstone-Dale constants, Mr. yl in the formula. We do not haveany addilional R.A. Ramik for performing the DTA/TGA' evidencethat hydroxyl and hydrogen are present, Mrs. Cynthia Peat for preparing the X-ray but propose to write the formula in this way powder-diffraction patterns, Miss Helen Driver until the structure is solved or IR analysis is for typing-for the manuscript, and Mrs. Josephine made. At the moment, however, it is impossible Galt iyping the tables. We thank Associate to select enough material for IR analysiswithout Editor Dr. J.L. Jambor and the referees for a destroyingthe type specimen. critical reading of the manuscript and for many The empirical formula calculated on the basis useful comments. of O = 10 from the composition given in Table 3 is H:.ooCar.re(Zno.ruM&.rnFeo.orMno.or):=r.o:(AsOr) REFERENCES ,.oo(OH)r.ooor, ideally, HzCar(Zno.aeMgo.raFoo.or Mno.or)(AsOn)r(OH),. BARTAND,P. & P. Hrnrrw, P. (1960): Un ars6niate de la The gaitite crystals examined in the present de et de magn6sium, isomorphe g-ros6lite. Mindral. Crist. Bull. 83, study are strongly zoned. The solid.solution Soc. frang. l 18-121. series between gaitite and talmessite extends at (1954): Belovite - least from Zn:,Mg 86:14 to 36:64. In general, Bonoorx, L.S. & Kazerove, M.E. from an alkaline pegmatite. Dokl, the Zn-rich zones occur in the centre of the a new mineral Akad. Nauk ^S.S.S.R.96, 613-616. Abstt.: Amer. crystals. Mineral. 40, 367 (1955). Gerr, R.I., SrunulN, B.D. & DUNN, P.J. (L979): AssocrerBo Mlxenars Prosperite,HCaZnz(AsOn)r(OH), a new mineral from Tsumeb, South West Africa (Namibia). Gaitite was found on the specimen from Can. Mineral. L7' 87'92. Tsumeb, Namibia (South West Africa) on which MeNoenrNo, J.A. (L976): The Gladstone-Dale re- prosperite first identified. The associated was lationship. I. Derivation of new constants. Can. minerals on this specimenare described in detail Mineral. 14, 498-502. by Gait et aL (1979). Ghitite is one of the last Nrreoov, E.I. (1954): Report on nelrt minerals is developedas a very thin, minerals formed and discovered by him. In The Scientific Session of polycrystalline coating on prosperite, adamite the Federov Institute together with the All- and . The matrix of the type speci- Union Mineralogical Society (V.A. Mokievski, men of gaitite is strongly altered chalcocite. ed.), Zap. Vses. Mineral. Obshchest. 82(4), 3lI' Subsequentto its characterization as a species, 317. Abstr.: Amer. Mineral. 40' 55 (1955). gaitite was found independently by Dr. Paul Ptrnnol R. (1964): Contribution b la mindralogie Keller of the University of Stuttgart, Federal Re- des ars6niates calciques et calcomagn6siensnatu- public of Germany. Through the courtesy of Dr. rels. ,Soc.frang. Mindral. Crist. BulI.87, 169'2L1. Keller, we describe here the paragenesison his YAKHoNrova, L.K. & SmoneNro, G.A. (1956): A specimen,which is different tlan that on the type new mineral - arsenate*belovite. Zap. Yses. specimen: the matrix is a severelyaltered chalco- M ineral. O bshchest. 85(3), 297-302. Abstr,: Amer. cite, coated liberally with the alteration product, Mineral. 42, 583 (1957). conichalcite. The conichalcite is covered by mas- sive, polycrystalline gaitite with a maximum grain Received December 1979, revised manuscript ac- size of 0.5 mm. No euhedral crvstals were obser- cepted February 1980.