903

Thz CanadianMineralo gist Vol. 32,pp. 903-907(1,994)

THECRYSTAL STRUCTURE OF TUSIONITE, Mn2+Sn4+(BOg)2, A DOLOMITE.STRUCTUREBORATE

MARK COOPER. FRANK C. HAWTHORNE ANDMILAN NOVAK* Deparxnentof GeologicalSciences, University of Manitoba" Winnipeg,Manitoba R3T2N2

MATTTIEWC. TAYLOR Departrnentof Eanh Sciences,University of Califomia. Riverside,Califomia 92521, U.S,A.

Arsrnecr

.. Tusionite, Mn2+Sn4*(BO3)2,has been found at two new localities: ThomasMountain, Riverside County, California and R.edice,Moravia, Czech Republic. At both localities, tusionite occursin granitic pegmatitesof the elbaite subtype,together with tourmaline,hambergite, danburite, hellandite and boromuscovite.Tusionite occursas small tabular crystalsin miarolitic cavities,and as thin flakes and rosettesin massivepegmatite; in the latlgr occurence,it is commonlyreplaced by fine-grained cassiterite.The crystalstructue, a 4.781(l), c tS.jA(Z) A, V ZU.SQ) N, fr, Z = 3, hasbeen refined to an i? ndex of 2.4Vo for 204 observedreflections measured with MoKa X-radiation.Tusionite is isostructuralwith dolomite, CaMg(CO3)2. Electroneutrality constraintsshow Sn to be teftavalent and Mn to be divalen! and the observedmean bond-lenglhsare in accordwith this: = 2.055, =2,224 A. The variation in as a function of cation radius is significantly nonlinearfor the calcite-typestructures, the values for M =Zn,Fe2+, Mn2+being -0.01 A less*ran predictedby linear inter- polation betweenmagnesite and calcite. For tle dolomite-typestructures, variations in (A representingCa Mn) and (B representingMg, Feh, Mn) as a function of cation radius are linear, but the two octahedrashow very "different behavior.The distanceshows a responsesimilar to^that of the calcite-typestructures, except that it is -0.025 A shorter for a given cation-radius;the distanceis -0.025 A longer than the correspondingdistance in calcite, but decreasesin sizewith decreasingcation-radius more rapidly than in the calcite-typestructures.

Keywords:tusionite, crystal structure,dolomite, granitic pegmatite,Thomas Mountain, California, F.edice,Czech Republic.

Sotrdlaans

Nous avonstrouv6 la tusionite,Mn2+Sn4f(BO3)2; tu rtrontThomas, comt6 de Riverside,en Californie, et i Redice, en Moravie, RdpubliqueTchbque, localit6s nouvellespour cette espdce.Aux deux endroits,la tusionite se trouve dansune pepmtite granitique du sous-gpe dit 'd elbat'te', avec tourmaline, hambergite,danburite, hellandite et boromuscovite.La tusionite se presenteen plaquettesdans les miaroles,et en nince cristaux ou en rosettesdans la pegoatite massive.Dans cette dernidre,la tusionite se voit remplac6epar une cassit6ritemassive. La structue cristalline de la tusionite,a 4.781(l), c 15.381(7)A,,VZU.SQ) N,n3,2=3,;eldffi1n1u- jusqu'lunr6siduft de2.4Vopow2M r6flexions observ6es etmesur6es avec nyonnement MoKcr. La tusionite est isostructuale avec la dolomite, CaMg(CO3)r.D'aprbs les contraintesd'6lectro- neuftalit6, le Sn est t6travalent, et !e Mn, bivalent; les longueurs moyennesdes liaisons concordentavec cette assertion: = 2.055, =2.224 A. La variation dansla longueur en fonction du rayon du cation est fortementnon lin6aire pour les structurestle type calcite, les valeurspow M = Zn,Fe*, Mn2+6tant inf6rieuresde ,-- 0.01 A I ce que pr6dit une interpolation lin6aire entre magndsiteet calcite. Dans les structuresde type dolomite, les variations dans les longueurs (A repr6sentantCq lvln) et (B repr6sentantMg, Feh, Mn; en fonction de rayon cationiquesont lindaires,mais les deux octabdresse comlnrtetrt tres diff6remment.La distance montre une r6ponsesemblable i celle des structures aletype calcite, sauf qu'elle est environ 0.025 A plus courtepour un rayon cationiquedonn6. Par contre,la distance est environ 0.025 A plus longue que la distancecorrespondante dans la calcite, mais diminue b mesureque diminue le rayon cationiqueplus rapidementque dansles structuresde type calcite. (fraduit par la R6daction)

Mots-cl6s: tusionite, strucfure cristalline, dolomite, pegmatite granitique, mont Thomas, Californie, i.edice, Rdpublique Tchboue.

* Presentaddress: Departrnent of Mineralogy andPetrography, Moravian Museum,Zelnf trh 6, 659 37 Bmo, CzechRepublic,

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INTRoDUCTIoN forrnly distributedwith regardto 20 were measuredat l0o intervals of ty (the azimuthalangle corresponding Tusionite is a borate mineral, (Mn,Fe)Sn(BOr)r, to rotation of the crystal about its diffraction vector). tiat was fust repor0edby Konovalenko et al. (1983) Thesedata were used to calculatean absorptioncor- from the soutlwesternPamirs, Tajikistan. It occursin rection, modeling the crystal as a plate; this reduced a granitic pegmatite that cuts Precambrian the azimuthalR index from 6.2 to 3.4Vo.This correc- garnet-biotitegneiss, both as lamellar intergrowths tion was then applied to the O-20 intensity data. The with tetrawickmaniteand as small tabular crystals data were then correctedfor Lorentz and polarization associatedwith tourmaline, danburite,hambergite, effects, averaged,and reducedto structurefactors, A qtJartz,orthoclase and albite in miarolitic cavities. reflection was consideredas observedif its magnitude Konovalenkoet al. (1983) showedfrom powder data exceededthat of 5 standarddeviations on the basis of that tusionite is isostructuralwith dolomite and wrote counting statistics.Miscellaneous information perti- the ideal end-memberformula as MnSn(BO3)2; nent to data measurementand structurerefinement is electroneutralityrequires that Mn be divalent and Sn given in Table L be tetravalent. Electron-microprobeanalysis was done in the New localities of tusionite were recently found in wavelength-dispersionmode on a CamecaCamebax the granitic pegmatitesBelo HorizonteNo. 1 and New SX50 instrument,with a beam diameterof l-2 pm Columbia No. 1, ThomasMountain" Riverside and an acceleratingpotential of 15 kV. A samplecur- County, soutlern California, and at i.edice, Moravia, rent of 20 nA measuredon a Faradaycup and a count- CzechRepublic. All occurrencesare in B-rich com- ing time of 20 secondswere usedfor Mn, Fe, C4 Sn, plex pegmatitesof the elbaite subtype(Nov6k & Nb, andTi. For Mg, Sb, As, Y, Sc, 7r, andW, 40 nA Povondra1994) characterized by the presenceofcom- and 40 secondswere used.The standardsMnNb2O6 mon tourmalineand other B-rich minerals.such as (MnKa, Nbl,cr), FeM2O6 (FeKc), CaNb2O6(CaKcl), hambergite,danburite, hellandite and boromuscovite. SnO2(Snlo), rutile (TiKa), MgNb2O6(MgKcr), Tusioniteoccurs as thin bladesand fan-like rosettesup stibiotantalite(Sblct), mimetite (AsZcl), YAG (Yl,CI), to 3 cm acrossenclosed in massivepegmatite, or as NaScSi2O6(ScKcr), ZrO2QtLa) and W metal (WMp) small tabular crystalsin miarolitic pockets.It is asso- were used. The data were reduced usine the PAP ciated with K-feldspar, albite, quartz and tourmaline. Tusionite agglegatesfrom massivepegmatite are com- monly replacedby fine-grainedcassiterite. TABLE 2. ELECTRON-MICROPROBEDATAT AND UNIT FORMULAE FOBTUSIONITE E)cERnGr.lrAL

Wo" o.46 o.10 0.40 A tabular fragmentof a single crystal from Belo SnO2 61,62 61.87 Horizonte No. 1, ThomasMountain. Riverside 62.44 62.46 ZtO. o.10 0.o8 County, California, 0.00 was mountedon a Nicolet R3n ScrO. 0.06 o.o7 0.07 automatedfour-circle diffractometer equippedwith a Mso o.00 o.@ o.80 Mo X-ray tube. Twenty-five reflections were mea- 1.71 o.14 o", suredon a random-orientationphotograph and aligned Fso 2.51 2.27 o.77 3.67 automatically on the diffractometer.From the result- MnO 14.72 14,78 20.47 20.44 ing settingangles, least-squares refinement gave the 23,72 23,70 24.O4 24.14 cell dimensionslisted in Table 1, togetherwith the Total 98.o' ,8.54 100.o3 iol -o5- orientationmatrix, Intensity data were measured o.oo6 o.ool o.oo5 0.oo0 accordingto the procedureof Hawthorne & Groat (1985). Sna" 1.006 1.Ol1 1.O14 1.@3 A total of 1052reflections was measuredto a o.oo2 o.oo2 0.000 maximum 20 angle of 60o.Ten strongreflections uni- Sc'n 0.003 o.003 0.003 o.ooo Mg:+ o.000 o.000 0.062 o.ooo 0.090 o.087 o.007 o.o20 Fs" o.o94 o.o93 o.o31 0.r4rl TABLE 1. MISCELLANEOUS INFORMATION FOR TUSIONM Mn2* o.776 o,774 0.851 0,831 a (A) Crystalsize (mml 0.23 x O.19 x O.O0 SUM 2.OOO 2.OOO c 15.38117) Radistion Mor(olcraphhe ' Nb, Ti, As, Y, Sb nol detected '1052 v tA"l 304.6(2) No. of lntereidos 'r B2o3 calculated by slolchiometry - = spacegroup R5 No.ot lF.l > sdlFl) 204 not detemined R(azimrnhall % 4.2 * 3.4 R(obsl o/ 1 - B€lo Horlzonte No. l, Thomss Mdmdn. RlvsGidgCounty, Callfomla - wB(obsl Vo 2 New ColumbiaNo. 1, Thomao Mountain, RlvorsldoCounty, Calltomia 3 - Falica, Morsvla, Czsch Rspublic Cellcontsnts 3lMn,rsn.+(Bos).1 4 - southw$tem Pemirs, Taiiklstan (Konovalsnko et al. 19831

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TABLE3. F1NALATOMIC PARAMETERS FOR TUSIONITE Urr u2 u$ ua Ut" Ut,

Mn0 0 0 91(3) 72(41 72(4t128(5)0036(2) Sn0 o 1t2 62(2t 36(2) 36(2) 114(3) o o 18(1) BO o o.2444t3't S2t12l 62(15\ 62t151124t211 0 0 31(8) o 0.2800(3) -0.0155(4) 0.2453(1)105(6) 64t71 80(7) 183(9) -17(6) -10(5) 46(61

'Ul-Ulxl0ra

routine @ouchou& Pichoir 1985).Ten or more points There have beenmany studiesof the stereo- were analyzedfor eachsample; compositions and unit chemicalvariations in carbonateminerals. Zemann formulae aregiven in Table 2. (1981)has shownthat most carbonategroups are planar or nearly planar, but also showedthat small Srnucrr,ns Rnrnmanl'r deviations from planar geometry do occur. Zematn et al. (1982)and Effenberger &Zemann (1986)noted Scatteringcurves for neutral atoms, together with that (BOr) groups can show significant pyramidal coefficientsof anomalousdispersion, were taken from distortion, particularly in nordenskirhodochrosite,siderite and smithsonitehas a TABLE 4. SELECTED INTEMTOMIC DISTANCES (A} AND -0.98, -0.01 ANGLES (O) IN TUSIONITE slopeof and magnesitelies A above this line. We note that the value for maEnesite, B-O x3 1.377121 O-B-Oa x3 120.0(1) = 2.102 A, is confirmed by three deLrmina- tions (Oh et al. 1973, Effenbergeret al. 1981, Sn-O x6 2,O55(1) Ob-Sn-Oc x6 83.9(1) Markgraf & Reeder1985), and we suggestthat the Ob-Sn-Od x6 91.1(1) deviationsfrom linearity in thesestructwes are real. 9O.O The behavior of the dolomite-type structuresis Mn-O x6 2.225111 Os-Mn-Of 86.8(1) distinctly different (Fig. 2). Following Reeder& Dollase(1989), we label the large octahedronand its Os-Mn-Og 93.2{1} central site o'A", and the smaller octahedronand its centralsite '?". For the A site, thereis a linear rela- (Fig. a: -y, x-y, zr bi -x, -1h+y, -yt-t, ct1/s-y, -y3+x-V, -Ua.+z; tionship between and cation radius 2), -Vs th-x r1u-2, -1.22. -0.02 dt + Y, + y, at -lh + x, ti + V, y3+ zi tt -\/E-y, with a slopeof The distanceis -?A+x-V,1h+zi gt th+V, ,/e-x+V, ?h-2. A longer in pure dolomite than in pure calcite, but the

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/ aatr'

o< ,/" o € I u2fi o I t0 )za o I

/e o"70 ,',/- (4, Ftc. l. Variation in as a function of cation radius in the calcite-typestructures. Symbols: M: magnesite,Sm: o.70 o.ao 1.@ smithsonite, Si: siderite, R: rhodochrosite,HMC: high- (A) Mg calcite, C: calcite, O: NiCO3. Nole that CoCO3over- laos smithsonite. Frc. 2. Variation in <4-O> (O) and (O) as a function of cation radius in the dolomite-typestructues. Symbols: D: dolomite, DAS: dolomite-ankerite solid-solution, A: ankerite, K: kutnahorite, DD: disordereddolomite. The borate structures(tusionite and nordenskidldine)are sizes of the octahedrain the two structuretypes con- shown as I and n, and the broken line denofesthe line vergewith decreasingcation radius to becomeequal at tbrougfi calcite and magnesitefor the calcite-typestruc- overlaps a cation radius of -0.93 A. There is a linear relation- tures.The value for the A site in nordenskidldine that for dolomite (r - 1.0 A, = 2.377 A); several ship between and cation radius @g. 2), with a valuesfor also overlap. slopeof -0.95. The distanceis -0.025 A shorter in pure dolomite than in pure magnesite,and unlike the distance,it does not linearly con- verge significantly with the calcite-typestructures as r Natural Sciences and Engineering Research Council of increases.It is possibletlrat at intennediatevalues of Canada in the form of Operating, Equipment and cationradius, 0.85--0.95 A, theA andthe B curvesjoin Infrastructure Grants to FCH and an International in a smooth fashion, but many more data are needed Post-Doctoral Fellowship to MN. on disordereddolomite-type structuresbefore it is clear whetheror not this is tlte case. REFEREI.{CF.s The distancein tusionite Grg. 2) lies -0.03 A above the trend for the other dolomite-type struc- ALTHoFF,P.L, (1977): Structuralrefinements of dolomite and tures. In this region of cation radius, the and a magnesiancalcite and implications for dolomite curves are defined by disorderedkutnahorite formation in the marine environment.Am. Mineral, 62, (Peacoret al. 1987); whetheror not this differencein 772-783. stereochemistry is the result of differences in the Bm.ar, A. & Znuam, J. (1977):Refinement and comparison degreeof order will only be resolvedby further work of the crystal structuresof a dolomite and of an Fe-rich on this sftucturetype. ankerite. TschertnaksMineral. Petrogr, Mitt, 24, The nonlinearvariations in meanbond-lengths as a 279-286. function of cation radius in both the calcite- and the dolomite-typestructures seem to be significant(i.e., EFFENBERGER,H., Krnrrl, A. & WIll, G. (1983): not due to random experimentalerror), and a more Untersuchungenzur Elektronendichteverteilung im extensiveexperimental examination of thesestructures Dolomit CaMg(CO3)2.Tschertnaks Mineral. Petrogr. seemswarranted. MitL3l.l5l-1,U. Mrnrrrr,n, K. & Znuaml, J. (1981): Crystal struc- Acro{owI-EDGErvIErr.lTs ture refinementsof magnesite,calcite, rhodochrosite, siderite,smithsonite, and dolomite,with discussionof We thank Rick Abbott and an anonymousreviewer some aspectsof the stereochemistryof calcite type for their comments.This work was supportedby the carbonates.Z Kristallogr. 156, 233-243.

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& ZEMANN,J. (1986): The detailed crystal struc- PEAcoR,D.R., EssENE,E.J. & GAINES,A.M. (1987): ture of nordenskitildine, CaSn(BO3)2.Neaes Jahrb. Petrologic and crystal-chemical implications of cation Mineral.Monatsh. 1I l-1 14. order-disorder in kutnahorite [CaMn(COr)z].An. Minzral. 72.319-328. HAwrroRNE,F.C. & Gnoer, L.A. (1985): The crystal struc- ture of wroewolfeite, a mineral with [Cu4(SO4) Poucuou,J.L. & PIcHoR,F. (1985):"PAP" procedurefor (OIDoGzO)l sheets.Ala Mineral. 70, 1050-1055. improved quantitative microanalysis. Microbeam Anal. 20,L0+r0s. IBmS, J.A. & HaunroN, W.C. (1974): Internatiarnl Tables for X-ray Crystallography IlI. Kynoch Press, REEDER"R.J. & Dotl-ess, W.A. (1989): Structural variation Birmingham,England. in the dolomite - ankeritesolid-solution series: an X-ray, Miissbauer,and TEM study.Am- MiruraL74,ll'59-1167. KoNovALENKo,S.I., Vorosrmt, A.V., PArcroMovsro,YA.A., Aner,q'rv,S.A., PERLTNA,G.A., Rocecssv, D.L. & & WEtt<, H.-R. (1983): Structure refinementsof KuzNrrsov, V.Ya. (1983): TusioniteMnSn(BO3)2 - a some thermally disordereddolomites. Am Mineral. 68, new borate from granitic pegmatites of the southwest 769-776. Pamlrs.Dokl. Acad. Sci. USSR,Earth Sci, Sect.272, t$9-t453. ZEMANN,J. (1981): Zur Stereochemieder Karbonate. Fortschr. Mineral. 59, 95-l 16. Memcnar, S.A. & ReBnen,R.I. (1985): Higb-temperature structure refinements of calcite and masnesite.A/2. EFFENBBRGER.H. & Ppnrltr, F. (1982): Mineral.70,590-600. Verfeinerung der Kristallstruktur des Kotoits, Mg3(BO3)2:ein Beitrag zur Aplanaritiitvon BOr- NovAr, M. & PovoNDRA,P. (1994): Elbaite pegmatitesin Gruppen.Osterr. Akad. Wiss.,Math. natarwiss.Kl., the Moldanubicum; a ne\il subtype of the rare-element Anzeiger1982,61-62. class.Mineral. Perrol. (in press).

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