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The Crystal Structure of Braunite with Reference to Its Solid-Solution Behavior

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The Crystal Structure of Braunite with Reference to Its Solid-Solution Behavior American Mineralogist, Volume 60, pages 1098-l104, 1975 The Crystal Structureof Braunitewith Referenceto Its Solid-solutionBehavior JoHeu P. R. pn VrLLrnRsl NIM/RAU Applied Mineralogy ResearchGroup, Departmentof Geology,Rand Afrikaans Uniuersity, P.O. Box 524, Johannesburg,2000, South Africa Abstract The crystalstructure of braunite(Mn'z+Mn!+sio,r) has been determined by single-crystal X-ray diffractometry.The crystal from Langban, Sweden,has a composition of CaoorMgo ,rAlo orFeo ,rMnu rrSio.ruO, and is tetragonal, space Eroup I4r/acd, with cell dimensionsa : 9.a32Q)4,c : 18.703(7)A,z = 8. The structurewas refined by leastsquares usinganisotropic thermal parameters to a conventionalR = 0.025,and consists of unitsof six cationsarranged octahedrally around vacant sites.The Mn2+ ion has distortedcubic coordination,the Sia+has tetrahedralcoordination, and the threenon-equivalent Mn3+ ions havedistorted octahedral coordinations. The mechanismof silicasubstitution in the structureis discussed. Introduction cent silica.This compositionalfield encompassesthe a-MnrOs structureas well as the braunitestructure. Braunite (Mn'+Mn!+SiOrr) is one of the more Sincea-MnzOs is orthorhombicwith spacegroup common manganeseore minerals that usually occur Pcab (Geller, l97l) and braunite is tetragonal,this in association with bixbyite, hausmannite, and cannotbe a true single-phasefield. pyrolusite in deposits such as Langban and The presentstudy was initiated to determinethe Jakobsberg in Sweden,as well as in the Postmasburg crystal structureof braunite and its relationshipto and Kalahari manganesedeposits of South Africa. In the a-Mn2Osstructure. The occurrenceof an ap- the latter deposits it occurs as the principal parently stablephase field with varying amountsof manganesemineral. SiO2must alsobe explained. The first structure analysis of braunite was made by Bystriim and Mason (1943),who determinedthe Experimental basic structure and established its correct formula. The structure analysis was based on space group Difficulty was encounteredin obtaining single 14c2.However, P. R. de Villiers and Herbstein(1967) crystals of suitable quality. Finally, good-quality braunite crystals determined the spacegroup to be I4r/acd, so that the showingno lineagestructure were above structure cannot be entirely correct. chosenfrom the Langban locality in Sweden.The was The structure of the compound CaMnrSiO, was sample obtained from the U.S. National Museum Natural determinedby Damon, Permingeat,and Protas (1966) of History,labelled U.S.N.M. No. given on the basis of space group I4r/acd. Unfortunately, 95307.The composition in Table I was deter- minedby microprobe this structure was not fully refined, but it conforms analysisand calculatedon the closely to that of braunite. basisof l2 oxygenatoms. Weight percentagesof the In a study of the phase equilibria in the system elementsare 0.08Ca, 0.54Mg, 0.09Al, 61.30Mn, manganeseoxide-silica in air, Muan (1959) deter- 1.64Fe, and 4.48Si. mined the existenceof a stable silica-containing phase X-ray measurementswere carried out on a ground (see with the a-Mn2Os structure. The SiOz content of this sphere Table I for dimensions)mounted on a phasecan vary from zero to approximately 40 wt per- Philips P.W. I100 computer-controlledfour circle diffractometer using graphite-monochromated 1 MoKa radiation.The cell dimensionswere Publication authorized by the Director General of the obtained National Institute for Metallurgy, private Bag 7, Auckland park from least squaresanalysis of the angularmeasure- 2000, Transvaal mentsof 25 reflectionswith 20 in the region of 40o. 1098 CRYSTAL STRUCTURE OF BRAUNITE AND ITS SOLID-SOLUTION BEHAVIOR 1099 Trsle l CrystalData of Braunite minimizedis R(4 : ) c.r(lFol-lf.l)'2. Scattering factors of neutral atoms were obtained from Inter- Cornposition,c"0.oIMBO, t3A10.02r"0.t7Mo6.7zsio.960tz national Tables,vol. IIL The conventionalR factor decreasedto R : 0.193,and the temperaturefactors o (8) 9.432(2)" immediatelybecame negative. The c (R) 18.703(7) of all the atoms standarddeviations of thepositional parameters were Space group 14.tACd I also excessivelyhigh. Examinationof the reflection high-intensityreflec- L 8 data showedthat the low-angle, tions are consistentlymeasured too low. 4 .80 tcalc *6 "* Refinementthen proceededwith a correctionfor obs anomalousdispersion, the valuesbeing taken from u ("r-') 108.2 Cromerand Liberman (1970), and with refinementof crystal diameter (cm) 0.035 ! 0.002 the extinction parameterg (Larson, 1967).Refine- ment with isotropicthermal parameters rapidly con- Nunber of reflections vergedto an R factor R : 0.041,with acceptable 0bserved 486 Unobserved 108 positional standard deviationsand reasonable temperaturefactors. Refinementof positional and well as the *Figures in parenthesis represent estimated sEandard anisotropic thermal parameters,as deviations in terrrs of Ehe last decimal-. isotropicextinction parameter, was terminatedat R : 0.025. The final positional standard deviations have valuesone-tenth of those determinedwithout isotropicextinction refinement.The valueof the ex- given in Table l. The intensities of a The cell data are tinction parameteris givenin Table2. Becauseof the up to 20 : 54.8o were col- unique set of diffractions similarity of the Mn and Fe scatteringfactors, and by the c't/20 scan technique, and the lected becauseof the small amountsof Ca, Mg, and Al at the ends of each scan background was measured present,no attempt wasmade to refinecation occu- absentreflections are as fol- range. The systematically pancies.Final positionalparameters with estimated lows: standarddeviations are given in Table2. The aniso- tropicthermal parameters are given in Table3, inter- hkl reflections : ft * k * I * 2n atomicdistances and anglesin Table 4, andobserved ftkOreflections'.hI2n and calculatedstructure factors are comparedin Oklreflections:.kl2n Table5. hhl reflections : 2h -t I I 4n This uniquely definesspace group I4r/acd sincethe Tesr-E2. PositionalCoordinates of the Atoms in Braunite 550 reflection,mentioned by Bystrdm and Mason Atoni Site (1943)as being presentin braunite,could not be slm- detected. meEry nll Correctionsfor sphericalabsorption and Lp cor- M(l) 222 cuo.otMEo,t3Mto.ao E rectionswere applied. The significancelevel of 5 per- M(2) I r"o.o3ho.97 000 M(3 2 tto. 0,03365(8)++ 0 ) o3ho .97 I centwas taken as the criterionfor the determination I M(4) 2 r"0.03"10.97 0.23250(5) 0.48250(*) of unobservedreflections (1 < 1.65o(1) where / is the E observedintensity.) si4si 3 programsused were those of The crystallographic o(l) l 0.1494(2) 0.3549(2) 0.0s44(l) the X-ray systemof programs(Stewart et al., 1972). o(2) l 0.l4sl(2) 0,0730(2) 0.0s68(l) The drawingswere produced by the programOnrrp o(3) r 0.4219(3) 0.1340(2) 0'0747(l) (Johnson,1970). RefineDent based on extinction paraEeter g = 4.0(l) x l0-3 The structureproposed by Bystrdmand Mason (1943) was verified as correct by Pattersonand *0rigin on cenEer of sltnetry f+Figures in parenthesis rePresent the estimated standard Fourier methods,and refinementwas initiatedusing deviations Gsd) in tems of the lovest numbet of units spacegroup I4r/acd, and variation of positionaland cited for the value to their imdiate left. *A paraDeter indicated is related by s)@etry. isotropic thermal parameters. The quantity so I 100 JOHAN P. R. DE VILLIERS Tlnlr 3. Thermal Parametersin A, (x lCtn)for the Atoms in C oordination P olyhedra Braunite As can be seen from Table 4, the Mn'+ cation is u(l r)"* u(22) u(33) u(r3) u(23) situated in a distorted cubic coordination M(r) t24(4) t24(*) 76(5) -63(4) o o polyhedron, with a large difference between the M(2) 5s(3) 54(3) 48(4) -r 7(3) - 8(3) I (2) M(3) 52(4) 6t (3) 57(4) 0 0 l0(3) M(l)-O(l) and M(l)-O(2) bond lengths.The Mn3+ M(4) se(3) se(.) 47(4) - - 6(2) 4(2) 4(*) atoms are situated in three non-equivalent distorted si s2(s) s2(.) s2(8) 000 octahedra, that around M(3) being the most dis- 0(l) il3(10) 82(t0) 76(9) - 8(8) 6(9) -13(8) torted. The distortions were calculated from the for- o(2) 90(l0) 74(l0) 66(9) - e(8) - 7(e) 7(8) 0(3) 104(il) 89(|r) 8r(9) 5(8) -l | (9) -14(8) mula for the variance of polyhedral angles given by *A paraneter so indicated is related by symetry Robinson, Gibbs, and Ribbe (1971)and are shown as **The anisotropic tenperature factor fom is = -2n2 ) 2 g 2 2 o2 in Table 4. r erp | \a* h2u r r+b* k2 r, *e+ 1 u 33+2a, b" hkun The reason for the distortion of the M(3) oc- +2a"c* hLU| 3+2b*c* kLUz e) l tahedron in braunite lies in the fact that it contributes four bonds to one (Mn3*) cation arrangement,and only two bonds to the adjacent(Mn3*) octahedron. Descriptionof the Structure Structurql Arrangement TlaI-e 4 lnteratomic Distances in A and Angles in Degrees- Braunite The underlyingfeatirres of the braunite structure are two three-dimensionaloctahedral arrangements M-0 distance* 0-0 distance O-M-O of cationsaround vacant sites. These cation arrange- M(l)-o(l) z.t7o[4)** 0(t)-o(2) z.aool 68.87 mentsmust not be confusedwith the coordinationof 0(2) 2.s08[4] 0(I )-0(2) z.8sg" 7s.98 oxygenatoms around each these o(r)-o(r) z.totl 7j.2o of cations.One ar- o(2)-o(2)2.588" bz.t4 rangementis shownin Figurel(a). The Si andM(l) Average 2,34 atoms are situatedat the oppositeends of the dis- torted M(2) octahedroD octahedralcation arrangementdefined by the M(2)-o(l) z.ztzlzl o(l)-o(2) z . essl 88.70 M(l), M(2), M(3), M(4), and Si atoms,and referred o(2) r.eorIz] o(2)-o(3)2,602' 83.9i o(3) 2.o221il o(l)-o(2) 2.924 9 | .30 to as the (Mn'+Mnl+Si) arrangement.This occurs 0(2)-0(3) 2.890 96.03 0(| )-0(3) 3.230 99.35 aroundthe vacantsites defined by theposition 0,1/2, 0(t)-0(3) 2.7L4" 80.65 l/8.
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