Canadion Mineralogist Yol.27, pp. 199-203(1989)


JOEL D. GRICE AND GEORGE W. ROBINSON SciencesDivision, National Museumof Natural Scienca,Ottawa, Ontario KIP 6P4

AssrRAC"r i (Cae.62Naj3slq;or)rr.rzGe?j3Mg1.21Tie.2elvlno.or)e:.on (Ala.7 2Mgs.s2Fere-3aFe'ei )s6.00(BO3)3 (Si5. gAlo. ro) Feruvite, ideally CaFq(A1,Mg)e@O:)lSieOls(OH)a,is a rs.srOrr(OH)a.D6"3.21, D-*3.207(9) g/cm3.La struc- new species that is associated with , ture, qui estcelle de$ membres de la famille de la tourma- , chlorapatite and ir a tourmalinized rock line,a €t6affind jusqu'l un r6sidu.R de l.6Vo.la r6parti- at Cuvier Island, New Zealand. Feruvite forms dark brown- tion descations des sites fet Z a aussi&6 affin€e(r€sultats black, equant grains up to 2 mm in diameter. The mineral citdsci-haut). Cette espOce est rare i causede. sa parage- is briille, H about 7, conchoidal , optically uniax- nbseunique. Son nom indiquequ'elle est l'€quivalent fer- ial negativewi& ar1.687(l), e 1.669(l). [t is trigonal, space rifdre de I'uvite (ou l'6quivalentcalcique du schorl). group .R3rn, a 16.012{2),c 7.245(4 A. ard Z=3. TlLe stroqgest X-ray-diffraction lines in the powder pattern [d (Iraduit par la R€daction) in A (, (hkt)l are: 6.43(40xl0l), 4.24(60)QrD, 4.00(60)(220), 3.50(60X012), 2.979(80)(t22), Mots-cl&:feruvite, nouvelle esplce min{mls, groupede la 2.586(l00X0s1), 2.05r(s0x152), r.928(40)(342), tourmaline,lle deCuvier, Nouvelle-Zdlande, affinernent 1.600(40X550)and 1,439(N)Q40).Results from an elechon- de la structure,tourmalinisation. microprobe analysisgave: SiO2 33.33, TiO2 2.19, B2O3 (calc.) 9.93, Al2O3 23.38, FeO 11.26,Fe2O3 (calc.) 2.56, INTRoDUcTIoN MgO 7.80, MnO 0.07, CaO 3.30, Na2O 1.16, K2O 0.05, H2O (calc.) 3.8, sum 98.45 wt.Vo, corresponding to In a systematicsearch for the iron analog of uvite, (Cae.62Na6.3eKp.p1)p1.p2(Fe'1-53Mg1.21Ti9.2eMno.or)lr.oathe authors found severaltourmalines whosechemi- (Ala.72Mg6.32FedjaFe6 jdre.o@O3b(Si5.83Alo. to)rs.s:Ors cal composition suggestsan end member in which (OH)4. Dek 3.21, DM 3.2U7(9)g/cm3. The structure, Ca and Fe dominate, but subsequentcrystal-structure which is that of the tourmaline group, refined to an R of determinations proved that not to be the case, as l.6a7o.Y and Z site occupanciesrefined to those in the for- there axemultiple site-occupanciesby iron, magne- mula above. The rarity of the ipecies is attributed to its unique paragenesis.Feruvite is named as the Fe-analog of sium and aluminum atoms. Tourmaline composi- uvite (Ca-analog of schorl). tions may be expressed by the general formula X\26(BO)3Si6O'8(O,OH,F)0, where X is com- Keywords: feruvite, new mineral species,tourmaline group, monly Na,Ca or K; ris Al, Fd*, Fe3*, Li, Mg or Cuvier Island, New Zealand, structure refinement, tour- Mn2+, and Z is N, Mg, Cr3+, Fd+ or V3*. In feru- malinization. vite, X, Y and Z must be dominated by Ca, Fe and Al, respectively. Black (1971)reported the occurrenceof a Ca-Fe Sotrluernn tourmaline from Cuvier Island, New Zealand. Sub- sequentinvestigation of this material has shown that La feruvite, de formule id6ale CaFe3(Al,Mg)o@Ot): it contains the new speciesferuvite, the iron analog pOle Sr6O1s(OII)a,est un nouveau de la famille de la tour- of uvite (calcium analog of schorl). Feruvite seems maline. Elle quartz, est associ6ee microcline, chlorapatite glo- et pyrite dans une roche tourmalinisde pr6lev6esur I'lle de to be a relatively scarcemineral in spite of the Cuvier, en Nouvell+Z61ande.Les cristaux brun foncd i noir bal abundance of iron-bearing uvite @unn et al. et 6quidimensionnelsatteignent 2 mm en diam&tre. C'est 1977). un min6ral cassant, ayant une duret€ de 7 et une fracture Both the new mineral and its name have been conchoidale. La feruvite est uniaxe n6gative, co1.687(1), approved by the IMA Commissionon New e 1.669(l); elle estrhognbo6drique, groupe spatial R3n, a and Mineral Names. The name was chosenbecause l6.0l2p), c 7 .2.45Q)A, Z : 3 . L6 dix raies les plus inten- of the mineral's crystal-chemisal similarity 1o uvite. sesdu clichdde poudre ld en A (D (hkt)l sont: 6.43(,10)(10l), The holotype specimenis deposited in the National 4.24(ffi)QrD,4.W@)Q2O),3.50(60X012),2.n9(f,0)(r22), Mineral Collection at the National Museum of Natu- 2.586(100X051), 2.051(50)(152), r.928(40\(342), (53776). 1.600(40X550)et 1.439(&)Q&). Une analysei la micro- ral Sciences,Ottawa sondedlectronique a donn€ SiO233.33, TiO22.l9, B2O3 (calc.) 9.93, Al2O3 23.38, FeO 11.26,Fe2O3 (calc.) 2.56, OccunnrNcr AND PARAGENESTS MgO 7.80, MnO 0.07, CaO 3.30, Na2O 1.16, K2O 0.05, H2O (calc.) 3.8, total 98.4590(en poids), ce qui correspond Feruvite occursin a pegmalitic, tourmalinized rock 199 2N THE CANADIAN MINERALOGIST

lated density of 3.21 g/cm3 basedon the observed empirical formula and unit-cell volume. Optically, feruvite is strongly pleochroic, with O light brown and E very dark brown. It is uniaxial negative,with &, 1.687(l) and e 1.669(1),measured with sodium light (590 nm). No variation in optical properties consistentwith the chemicalzoning noted in the following section was observed within individual grains. Although variation in the ratio Fe:Mg is significant, it doesnot affect color except in the polar overgrowths as noted above.

Crmurcer CoupostrroN

Severalferuvite-dravite crystals were mounted in epoxy, polished flat, and coated with a vacuum- evaporated layer of carbon for microprobe analysis with a JEOL Superprobe733. Becauseof strong composilional zoning, baclscattered*electronimages coupled with qualitative energy-dispersionanalyses were used to locate zonesof interest for subsequent wavelength-dispersion analyses, rvhich were done using an operating voltage of 15 kV, a beam cur- rent of 25 nA measured on a Faraday cup, and a Ftc. l. Backscattered-electronimage of a rypical zoned beam diameterof l0 pm to minimize volatilization. quartz. grey feruvite-dravitecrystal in Theligbter core The following standardswere used: Mg, con- is feruvite, and the darkerrim and overgrowths Fe, Si), (Ca,Al), (K), (Na), The white grainsare chlorapatite.Bar sist of dravite. (Mn) (Ti). The data were scale:l@ pm. and reducedutilizing the Tracor Northern 56(F computer program TASK, which employsa conventional ZAF correction routine. Chlorine and fluorine also were from Cuvier Island, New Zealand. Theserocks were sought, but neither was detected. describedby Black (1971),who attributed their ori- Boron also was determined by wavelength- gin to hydrothermal replacement of the aluminous dispersionanalysis with operating conditions of 5 kV and ferromagnesian host-rock silicates by tourma- and 100 nA, using as a standard. Data for line and tourmaline-quartz vein sequences.The som- boron were reduced with the Tracor Northern 5600 positions of the tourmaline are thought to be at least program 6pZ. An averageof five analysesdone on partly derived from the host rock by such tourmali- the crystal usedfor the structure determination gave: nization. The actual processprobably was complex, sio2 33.33(18), TiO2 2.t9(3), B2O3 11.25(60), as demonstratedby the compqsitieaal 26ning v/ithin Al2O3 23.38(13), total iron expressedas FeO individual feruvite-dravite crystals and later over- 13.56(13),MgO 7.80(5),MnO 0.0?(2),CaO 3.30(3), growths of schorl-dravite upon them @ig. 1). Asso- NarO 1.16(2),K2O 0.05(l), H2O(calc.)3.48, sum ciated speciesinclude quartz, microcline, chlorapa- 99.57. The strongly zoned crystalsdid not permit tite and pyrite. meaningful determinations of the ratio of ferrous to ferric iron, but bond-valence calculations based on PHYSICAL AND OPTICAL PROPERTIES the crystal-structure data indicate the presenceof both ferrous and ferric iron. There is, however, no Feruvite occurs as dark brown-black, subhedral structural evidencesupporting an excessof boron; to anhedral, equant grains that attwn 2 mm in although the microprobe-determinedvalue is within diameter. The mjneral is aggregated into dark two standard deviations of the calculated value, it patchesthat composeapproximately 3090 of the fine- is consideredto be too high. grained tourmalinized rock. Someindividual grains If the aboveconsiderations are taken into account, have an acicular, lighter-colored dravite overgrowth and the proportion of FeO, Fe2O3,B2O3 and H2O on one pole. Feruvite has a grey , vitreous to calculated by stoichiometry assuming 19.0 cations dull luster, and showsno fluorescence.It has a Mohs and 31.0 anions, which is consistentwith the struc- hardnessof -7, is brittle, and has a conchoidal frac- ture refinement, the analytical resultsmay be rewrit- ture. The densi8, measuredin methyleneiodide, is ten as: SiO233.33, TiO2 2.19, B2O39.93, Al2O3 3.n7Q\ g/cm3, which agreeswell with the calcu- 23.38, FqO3 2.56, FeO 11.26,MgO 7.80, MnO THE STRUCTUREOF FERUVITE 20r

.I. 0.07, CaO 3.30, Na2O 1.16,K2O 0.05, H2O 3.43, TABLE FERUVITE:X-RAY-DIFFRACTION EATA sum 98.45.With the siteoccupancies for Mg and Fe determined by the structure refinement, the result- dcalc dobs .robs dcalc dobs -robs ing formula is: (Cao.urNae.3eKo.or)",.0r(Fe2,*r,'It0 8.00 8.00 r 342 1,929 1.928 '|

X-ray precessionphotographs show feruvite to be Mn-flltered Fe{q radlationi a 16.012(2), e 7.245(2') trigonalwith possiblespace-group choices Rim, R3m and R32. Resultsof the crystal-structurerefinement, presented below, confirm the noncentrosymmetric TABLE2. FERUVI'IE:STRUCTURE-REFINEI4ENT DATA space-gtoup found in all , R3n. Unit- Jdeal Fomula: caFesAl6(803)3si60b(0H)s a(E):ts.ooo(z) cell parameters were refined from X-ray powder- SpaceGroup: xli o([): 7.24811l, diffraction data obtained with a 114.6-mm-diameter zr3 v(X3):1606.8(3) Radlation/l'1ono.: Mo/ No. of Fo: 1161 Gandolfi camera with FeKcu(Mn-filtered) radiation ' u: 23.9m No. oi Fo>z.so(t): 1154 (Table 1). The refine! unit-cell parametersare: a Mln. transnisslon: 0.652 Flnal a: I .60U t6.0t2?), c7.A5Q) A, V t606.6(4)N, anLdZ:3. ilax. transnisslon: 0.676 Final .Rw: 1.63fl R. r(lFol-lrcl)/tlFol

RSFINEN{ENToF TIIE CRysrAL Srnucrunn Rw=[rw( I Fol- lFcI )27rwlro121tr, u=o-z(Fo)


The grain chosen for the crystal-structure deter- atomic coordinates of buergerite (Barton 1969)were mination was that analyzedby electron microprobe. used.Barton (1969)did not haveany H atomicposi- The grain was ground to a sphere, 0.27 mm in tions in his structure. Refinement of positional, diameter, which not only facilitated the absorption isotropic tlermal parametersand the occupancyfac- correction but also removed an outer zone of the tors of Fe and Mg in the Y and Z sitesgave a residual crystal that is poorer in Fe and Ca. Intensity data indexR = 2.80/0.Withinthe Ysite, Tiwas assigned were collected on a fully automated Nicolet R3m (Povondra 1981), Mn was assigned (Nuber & four-circle diffractometer using the method of Grice Schmetzer1984), Fe refined to 1.54(3)atoms, and & Ercit (l9SO. The data relevant io the structure Mg refined to 1.17(3)atoms. Within the Z site, Al refinement are given in Table 2. was assigned,Mg refined to 0.80(3)atoms, and Fe refined to 0.48(3) atoms. Two H atoms were located Structure refinement with a AF synthesis and added to the refinement, with anisotropic thermal parameters for all other The structure refinement of feruvite was essential atoms. The final residualindex R was 1.6090;with for the definition of the species.From the chemical a weighting scheme incorporating an isotropic, data given above, the number of Fe and Mg atoms primary-extinction correction, an R* of 1.6390was is almostequal,1.99 and2.03, respectively.The dis- obtained. The H3 atomic site agreeswithin I o of the tribution of these cations in the Y and Z sites deter- one hydrogen atom found in the neutron-diffraction mines the species.Simply stated, if all the Mg is in study of buergerite(Tippe & Hamilton l97l). I then the species would be uvite, but with a Bond-valencesums for Ol and 03 are 1.00 and predominanceof Fe in Yit is the new speciesferuvite. l.l2 v.u., respectively,which support bur proton For the structural refinement of feruvite, the assignments,yet the Ol-Hl bond length of 0.41(6) 202 THE CANADIAN MINERALOGIST

]NTEMTOIIICDISTNCE5 (8) EIO MOT.SS(O) A is too short. The O3-H3 bond length of 0.74(3) TABLE5. FERWIIE:SELECTED positional A is - reasonable. The final lla (X-slt€) polyhedrcn parameters and equivalent isotropic temperature- Ca-02 2.496{2)13 factors are given in Table 3, and the anisotropic Ca-04 2.775(2\ \3 Ca-05 2.692(2)t3 temperature-factor coefficients are given in Table 4. nean 2.654- Bond lengths and angles are given in Table 5. The Fe (Y-site) octahedron observedand calculated structure-factors have been Fe-01 2.00212) 0l-Fe-02 86.5(r) x2 submittedto the Depository of UnpublishedData, Fe-02 2.055(ttx2 0l-Fe-06 98.9(1)x2 Fe-03 z.tttizi o2-Fe-06 9o.o(l) x2 CISTI, National Research Council of Canada, Fe-06 2.0210\ x2 o2-Fe-02 90.4(1) Ottawa, Canada KlA 0S2. rean Zngf* 02-Fe-03 98.6(1) x2 03-Fe-06 76.1(1)x2 05-Fe-06 89.2(1) D$cussroN nean 9On- Al (z-slte) octbhedron Dietrich (1985) alluded to the probable existence A1-03 r.990(r) 03-Al-06 A1-06 r .9r5(r) 03-Al-07b of the "Ca-analog of schorl" (feruvite) and showed A1-07b 1.923(1 ) 03-41-08 Al-07c 1.e82(1 ) 03-A1-08 a few compositionson a plot of Mg/(Mg+F*+) A1-08 1.e42(2) 06-A1-07c versusCa"/(Ca+ Na) in X, that fall within the quad- Al-08 r.el3(2) 06-Al-08 mean 06-41-08 rant of Fe>Mg and Ca>Na. Neitherthe provenance 07-A1-07 07-A1-08 of these analytical data nor the designation of sites 07-41-08 0?-A1-08 for Mg and Fe is given, but the rarity of such a phase 07-Al-08 is evident.Mittwede (1984)reported a Ca-Fe tour- rean maline, which he found to be calcian shorl, from a Borcntrlangle pegmatite dyke containing calcic in B-02 1.380(3) O2-B-08 120.2\1)t2 B-08 t.ttziz\ o8-B-08 119.6(2) CherokeeCounty, South Carolina. Brown &Ayuso rean T3f rean 120.0 (1985)reported on the presenceof a Ca-Fe tourma- slllcon tetrah€dron line from 4 lsslmalinite breccia in St. Lawrence 't.632(1 'r01 sl-04 ) 04-5t-05 .3(1) 111 Efll si-05 r.54e(r) 04-st-06 sl-06 1.607(1) 04-st-07 TABLE3. FERUVI'rE:PoSITIoNAL AND THERI'IAL PARAT'IETERS si-07 I .598(r ) 05-st-06 'ilr.r(r)1r0.4(l) mean 1.7 05-sr-07 05-si-07 1ro. e(1 ) rl(eq)xlo{E'z nean Toe-;r ca(X)000.2222(2)177G\ County, New York, but the specieswas not deter- Fe(Y) 0.12299(4) 0.0614s(2) 0.53661(9) gs(z\ Af(z) 0.29832(3) 0.25167(4t 0.61132(10) 65(2) mined. B 0.r1001(t1) 0.22003(2't) 0.4s175(38) (3) 84(8) Consideringthe Ca-Fe-rich skarn assemblagesand sr 0.re15r 0.18e85(3)^ _^.9,_. _c!!1) 0l00u.roreqrlryu\y, pegmatitesof tle Grenville Province a likely environ- 02 0.0607(r) 0.1214(t) 0.4764(3) rrO(e) 03 0.2660(2) 0.1330(l) 0.5125(3) l4l (6) ment to host such a tourmaline, the authors began 04 0.09190) 0.1838(r) 0.0710(3) 1t4(6) 05 0.]8r4(l) 0.0907(l) 0.0902(3) lll(6) a systematicsearch for the Ca-Fe-Al member of the 06 0.r953(r) 0.t862(1) 0.7789(2) 98(5) group in theserocks, but dozensof analysesyielded 07 0.2842(1) 0.2835(l) 0.0789(2) l0s(5) 08 0.2088(1) 0.2695(t) 0.4411(2) 123(5) mostly dravite and uvite in the nephelinesyenites and Hl000.725(9)lo0(0) pegoatites. H3 0.258(3) 0.r2e(l) 0.412(4) roo(o) skarn assemblages,and schorl in the The apparentpresence of such a member of the group

X.( CaoeNao.e Ko.or) only in the tourmalinized rocks of Cuvier Island, Y- ( Fe1 .sl,lg 1.2 Tl q.a l'{n6 .61 ) New Zealand, attests to the unique geochemical Z=(Al q.zMgo.eFeoa ) environment qf this locality. Severalauthors have proposed various distortion 'TEnPERATURE-FACTORS TABLF 4. FIRUVITE: ANIS0'IR0PIC (XlOq Rr) parameters to explain adjustments within tle tour- maline structure to compensate for the variety of uzz aat azz 0rz tJtz t(eg) cation substitutions in theX, Iand Zsites. The fun- damental topological unit within tourmaline is the ca(x) 167(4) 167(4) 197(6) 0 0 84(2\ 177(41 Barton (1969) Fe(Y) 102(2) 73(2) lre(2) -22(r\ -44(2) 51(1) e5(2) hexagonalring of SiO4 tetrahedra. Ar(z) 63(2) 66(2) 65(2) 8(2) 1(2) 31(2) 65(2) proposedthe ditrigonal distortion index, o, and Gor- 83(8) es(r2) 7e(10) -6(e) -3(4) 47(6) 84(8) si E7lt\ 5s(2) 67(21 -5(2) -3(2) 26(2) 60(2) skayaet al. (1982)proposed the Ae index to numer- 0l 201'r (1r ) 201(1r) 16e(7) 0 0 100(5) 190(9) 02 r9(6) 65(8) 147(e) 20(6) l0(3) 32(4) tr6(6) ically describethe distortion of this ring. Schmetzer 03 2r5(lo)r66(7) sB(7) e(4) l9(7) 108(5) l4l(6) et al. (1979)and Nuber & Schmetzer(1984) tabulated 04 'rs8(e)88(6) r68(e) il4(8) -r2(7\ -6(3) 84(s) ll4(6) 05 'r 83(6) il8(8) 8(3) l5(7) 7e(s) lll(6) mean cation- bond lengths for the Y alld Z 06 14(6) r07(6) 80(5) -16(4) -7(5) 6l(5) eB(5) (1982) ry', 07 er(6J 76(6) 117(6) -7(4) -2e(5) l8(5) 105(5) sites. Gorskaya et ol. calculated the dis- @ 69(6) r 10(6) 184(6) 20(5) 6(5) 4l (5) 123(5) tortion ofthe Y- and Z-site octahedra.No satisfac- tory correlation betweenany of the aboveparameters TIIE STRUCTURE OF FERUVITE 203 explains the distortion of the tetrahedral or octa- Blecr, P.M. (1971):Tourmalines from CuvierIsland, hedral rings in terms of cation substitutions. New Zealand.Mineral. Mag. 38, 37+376, In feruvite, notable distortions in its topology can always be related to the 03 site (an OH species).In Bnowr.r,C.E. & Ayuso, R.A. (1985):Signifrcance of the SiOa tetrahedron the bond angle OGSi-O5 is tourmaline-rich rocks in the Grenville complex of Geol. Sum. 101.3o,which.is considerably smaller than the ideal St. LawrenceCounty, New York. U.S. Bull.l6Xi-C. tetrahedral angle of L09.4'. This angular change occursin responseto elongationof the Si-Os bond BuERcER,M.J., Bunmrau, C.W. & PBacon,D.R. length due too the hydrogen bogd with 03 (1962):Assessment of the sweralstructures proposed (O3-O5 : 3.2SA and H3-O5 = 2.56A). The effect for tourmaline.Acta Crystallogr.15, 583-590. of this hydrogen bond influences both the I- and Z-site octahedra by extending one bond length in Drrrnrcn, R.V. (1985):The TourmalineGroup. Yan each, as shown in Table 5. NostrandReinhold Co., New York. Although the above discussion of distortions in the feruvite structure is helpful in explaining the Dur.nq,P.J., ArelrueN, D., NnLeN,J.A. & Nonurnc, J. (1977):Uvite, a new (old) coulmon memberof structural responseto the large Ca and Fd* cation the tourmaline gxoup and its implications for col- substitutions, it doesnot answerthe question of rar- lectors.Mineral. Rec. E, 100-108. ity of the species.In general, it would seemthat Ca- dominant tourmalines are much rarer than Na- Gonsre.ve,M.G., FneNr-KarvmNsrsKAYA,O.V., RozH- dominant tourmalines eying to early fractionation DEsTvENSKAvA,I.V. & FnaNr-KaMeNE'IsrIt,V.A. of calcium minerals in magrnatic processes.Thus (1982): Refinementof the crystal structue of Al- Ca-Fe silicatesprobably crystallize in igneousrocks rich elbaite,and someaspects of the crystalchemis- long before a Ca-Fe tourmaline would form. The try of tourmalines.Sov. Phys. Cryst. t|,63-66, most likely geological process that would produce Gnrce,J.D. & Encrr,T.S. (198O:The crystalstructure such a unique chemical speciesis that exemplified of moydite. Cqn. Mineral. U, 675-678. on Cuvier Island: tourmalinization of rocks rich in Ca, Fe and Al. Mrrnvron, S.K. (1984):Signifrcance of tourmaline compositionsfrom the inner Piedmontgeologic belt AcKNowLEDGEIVIENTS of Soutl Carolina. SoutheasternGeol. A, 2fi 2rc,

The authors thank PhiliFpa Black, Auckland Nussn, B. & ScutvIrrzpn,K. (1984):Structural refine- University, who generously gave us the specimens. ment of tsilaisite (manganesetourmaline). Neaas Frank Hawthorne, University of Manitoba, kindly Jahrb. Mineral., Monatsh,, 301-304. four-circle for made availabletle diffractometer data PovoNDRA,P. (1981):The crystalchemistry of tourma- collection. Dorian Smith, University of Alberta, lines of the schorl-dravite seies. Acta Univ. Caro- David Kempson,Queen's Universify, and Bob Gault, linae Geol. 3,223-264. National Museum of Natural Sciences, provided many of the earlier electron-microprobeanalyses. We Scrnarrzn, K., Nusen, B. & ASRAHAM'K. (1979): also are indebted to T.S. Ercit, National Museum Crystal chemistryof magnesium-richtourmalines. of Natural Sciences, for assistance with the NeuesJahrb. Mineral., Abh. 136,93'112. microprobe analyses,and to R.V. Dietrich for his W.C. (1971): A neutron- helpful dissussions. Trpps, A. & Hauu.ron, diffraction studyofthe ferric tourmaline,buergerite. Am. Mineral.56, l0l-113. REFERENcES

BanroN,R. (1969):Refinement of the of buergeriteand the absoluteorientation of tour- ReceivedJune 16, 1988,revised manuscript accepted malines.Acta Crystallogr. 825, LS2/-1533. October11, 1988.