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:
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:
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, 904 TTIE CANADIAN MINERALOGIST
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$tem Pemirs, Taiiklstan (Konovalsnko et al. 19831 TIIE STRUCTUREOF TUSIONITE 905
TABLE3. F1NALATOMIC PARAMETERS FOR TUSIONITE Urr u2 u$ 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 nordenski
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