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 determinedby 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 distorted 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. The other arrangement,shown in Figure l(c), Average 2.03 consistsof an octahedralarrangement of the M(2), M(3) Octahedron M(3)-o(r) r.goslzl o(l)-o(l) 2 ,782 93.s7 M(3), andM(4) atoms.This arrangement,referred to o(2) r.ge4[z] o(t)-o(3) 3.330 105.t3 o(s) z.zttlzl o(t)-o(2) 2.660" 86, r9 as the (Mn!+) arrangement,occurs around the vacant 0(l)-0(3) 3,258 r01.89 sitesdefined by 0, l/4, l/4 and l/4,0, 0(2)-0(3) 2.695" 78,l6 l/4, and is 0(2) -0( 3) 2,bo2t 74.g4 identicalto the cation arrangementin a-MnrO, (Fig. o(2)-o(2) 2.902 94.05 1b). Six of the (Mn'z+Mn!+Si)octahedral cation ar- Average 2.06 o2 = 137.8 rangementsare grouped around each of the con- M(4) Octahedron stituentM(l) andSi atoms,whereas only four arear- 11(4)-0(l) l.9sr [2] o(r)-o(:) z.zaal 8r.28 0(2) |.92tl2l o(l)-o(r) 2.7o7' 87.95 rangedaround the M(2), M(3), andM(4) atoms,the 0(3) 2.248[2] o(l)-0(3) 3.t7s 98,01 0(r)-0(2) 2.827 93.78 other two arrangementsbeing the (Mn!+) ones.The 0(2)-0(3) 3.2r4^ 100.s9 groupings (Mn'z+Mn!+Si) (Mn!+) o(2)-o(3) 2.695" 80.ls of the and oc- 0(2)-0(2) 2.s88 84.68 tahedraaround the Si and M(3) atomsare shownin Average 2.O4 o2 = 69.1 Figure 2. Thesearrangements have interestingim- plications Si-0 Tetrehedron on the substitutionof silica in the struc- si-0 (3) l .620[4] 0(3)-0(3) 2,649 109.73 ture, and will be discussedlater. 0(3)-0(3) 2.637 108.95 The arrangementof oxygenatoms around eachof a2 = 0.2 thecations is asfollows. The M(1) atom hasdistorted "Esti@Eed standard deviation for M-0 distances are 0.0028. for Si-o dietaoces O.OOZ8, ana for O-O digEances 0,0038. Esiinated cubic coordination.The three non-equivalentM(2), standErd deviations for 0-M-0 and 0-Si-0 angles are 0.1 M(3), andM(4) atomicpositions each have distorted degrees **The octahedral coordination, whereasthe Si atom is bond oultiplicitiee are given in square blackets aThe tetrahedrallycoordinated with surroundingoxygen superscript @aug that thi6 distance lepresents an edge atoms. thared between tvo polyhedra. CRYSTAL STRIJCTURE OF BRAUNITE AND ITS SOLID-SOLUTION BEHAVIOR I l0l

Tlst-r 5. Observedand CalculatedStructure Factors for Braunite

t3 FF -r1r -5 I 573 000 u6 66 r17 45 06 39 u9 09 r2r 36 L27 61 6 A9 -41 000 000 000 U5 75 -120 4t -t 5l 45 59 4l 16 07 r 95 000 000 000 L 601 154 -091 r25 r27 31 2! 26 L r14 000 000 000 r052 5Il 71 419 73 679 94 15 9t 16 t1 -13 0 55 157 000 000 000 99 57 100 t2 100 17 9 31 205 92 207 G 214 04 r0 05 l3 93 33 19 {4 74 23 7l 95 -74 0r -6 39 206 99 2L! 59 22L 61 1 14 57r 000 000 000 3791 3326 1326 rl3 4At 95 47] 92 544 61 27 51 r50 1621 932 000 -000 000 171,61 16940 17r 44 r0 3l 331 693 695 053 ia t4 11 1J -77 59 -7 61 154 62 156 25 r53 U ll rl 6A2 000 000 000 r-15 249 1L Ztt !1 259 92 16 E9 13,26 L6.14 -t6-54 -2-61 15 31 14 93 -!! A\ -l 77 9569 9530 9601 403 -0m -000 632 55 652 69 352 62 36 33 411 000 13.13 1.36 -rr 2303 2674 2675 030 12 aL 7L 44 -7t 45 -5 6! 7 42 000 -000 -000 33 -r, 35 -r !' 15 02 1r 25 19 62 116 5a 1r7 r7 9 63 tl 59 r, I -(7 -t 33 117 4(3 5l 443 04 494 31 34 U 1091 000 -000 -ooo 27-15 45 09 47 52 55 15 21 15 t3 -r5 r3 -0 .7 -16 -6 -000 -000 4165 4335 4339 095 15 15 11 34 15 16 493 000 -034 r14 94 !74 91 r77 0o l0 5l 4a 6t \5 59 t6a AZ 25 24 zza 000 -000 162 162 0rt r49 r23 :00o 125 21 04 LO 49 -20 3r -2 12 15 41 31 25 -t1 26 0 06 000 000 -000 151 79 150 22 151 49 9 19 42 Ar 41 30 11 37 -721 -A$ 163 20 16641 170 5t 11 69 LO47 -9 9? r 5$ 123 4331 4250 4239 433 102 04 r00 03 -lo0 33 -6 63 773 363 060 & 66 6! 37 -63 40 -i 57 1426 1057 1057 011 rrr4 r43 227 9655 9339 9905 670 r0 35 r15 !5 L20 77 r22 14 r 612 9t9 929 -O06 lr4 oo r33 il 131 12 -000 -0@ 5615 5550 5553 37r r7r0 1719 \119 r19 16 5r 16 35 -16 27 _r 70 756 lt11 r306 r06 L 717 000 1r4 43 116 r0 -rr7 05 -1 93 3160 tza2 \279 164 DA 65 rl7 n \14 91 9 11 rt4 15 lr2 E4 rr3 33 6 0l J250 tZ61 3Z26 436 I019 324 091 31r 1634 1176 1776 0r0 -333 290 -140 -9 56 33 60 22 -60,06 -t 37 r 356 44r r43 l5 r]9 42 2l 33 96 55 93 16 93 43 rO l5 r049 3027 179 000 000 17 99 74 24 74 ZL 40 95 73 32 70 73 -70 36 -3 22 126 000 000 9951 9642 9635 523 64 r0 65 91 66 09 42 o! 40 96 2156 2724 449 [3 90 rr9 13 192 33 r0 52 13 99 13 32 -r3 24 -1 76 11lO r055 r056 014 LZA 000 000 000 -43 -0 14 14 10 31 -10 3r -0 14 000 000 51)9 43r3 4311 453 15 4 4t 75 73 6r 219 200 r71 643 000 -29 03 -0 64 26 19 19 161 15 3 70 L 692 000 517 000 000 26 12 29 06 A23 -244 159 2106 1936 1905 M 1113 359 91t 012 a72 361 463 30 475 37 5)4 24 3n 95 64? 76 65A \2 954 39 3l 73 r52 73 14965 151 t6 -7 t7 -75 -6 40 24 4r t2 -4r 52 -4 44 -* -4 lZ 14799 ra7 t7 146 30 3 43 74 90 15 16 06 52 tl 2! 33 30 6r zz 11 z4 2s 23 94 3l 3 57 t99 7i 299 16 !10 23 22 37 03 435 30 24 36 r35 14 rr3 96 134 196 1! 3S 5500 012 691 A4t 112 1341 Ut6 13l7 -00r 45 t2 41 1r -47 11 -5 5l 349 1255 -r265 002 -000 -000 -000 -0m -96 -O r 465 000 460 000 93 94 96 26 92 2? 451 L4 119 'L 65' 72 LO AZ 19 27 13 33 -1r 17 Z 13 614 000 -000 -000 1205 r063 1063 004 59 12 55 24 -t6 20 -5 tr 000 -o00 -000 71 20 74 a7 15 5L 1!Z 36 703 95 rom 67 16 39 3S,36 33 55 612 000 -000 -000 4t2 000 -000 -000 1r0 51 r09 03 -109 79 91 L2 9L 9A 95 62 26 A1 56 11 r 531 000 -000 J6 02 35 07 -35 04 644 93 64A 22 rr32 64 19 r0 411 42 407 30 447 62 56 94 66 -66 24 -1 59 lr3 6 rlr 32 -112 63 -0 53 -0 00 -0 0o L4 14 44 la -41 79 -r 61 9! 61 92 1L -91 69 -5 45 65 23 5t -000 -000 142 341 -003 r50 000 r764 156 -1602 116 557{2 539 16 630 41 31 75 -93 -7 -000 -67 -6 23 00 26 30 99 12 93 0t ?? 01 L 63r 396 000 61 AA 12 Sa -006 3 953 1251 1256 047 3r0 000 000 000 543 543 LL92 r7 -3r2 003 49 10 51 4\ 5r 4S 5l zr 49 97 -49 57 -6 E7 r99 69 2& 36 2A1 5A L6 17 312 -rA -O 13 7l -r3 73 -0 06 619 t24 -521 25 41 24 69 10 46 1634 1650 1650 002 -000 -000 tr \2 -3L 44 -O t2 165 05 L6l 23 -164 37 -10 11 11 42 74 53 -74 36 -0 13 !31 000 30 66 419 000 000 17 41 32 33 -32 .1 -0 03 la 93 19 16 L 294 1031 1073 037 5r. -23 Aa -O 000 533 000 -000 -000 L6 62 537 53 593 9S 19266 31 43 z3 a9 71 000 000 -22 -5 L1 97 13 {6 13 {5 611 000 -000 -000 136 r21 165 24 lO Z! 27 65 19 -9r -3 71 411 04 t39 25 10 4t 11693 133 37 133 000 -000 -000 10 -53 24 C 91 91 90 96 32 41451 51 59 56 l9 1319 119 t1 13 7L 51 -14 46 -6 36 r 535 714 -771 -059 000 -000 -000 428 560 -50' 245 1516 $4r -41 -9 -32 11 16 261 66 26701 214 73 2A rA 3r 51 33 72 41 2! )3 44 12 29 1A O 62 r09 72 109 33 -109 32 -6 10 -004 -11 -0 35 26 90 6r -91 It 1369 909 Lr r5 35 12 45 56 41 L2 -43 \4 -5 2L 451 -45r 46al 4762 4763 020 2A 42 -23 44 -A 60 1919 003 r35 36 117 66 13 54 6 16 -233 -11 a95 139 739 -005 5 94! 1006 1006 -006 349 -094 211 t6 Z\0 95 94 l4 -000 -000 L 617 354 12 56 35 0r 34 93 000 000 000 331 000 25 16 rr2 11 rr7 L3 -117 30 9 43 40032 !r5 24 45963 4t 57L66 1Z 61 196 350 162 )11 311 S52 -3rt -251 465 13 479 L4L7 46rt 4616 1.4 jZ 1545 143r r419 014 ! 000 2r5 9L Z2OZO 274 16 L7 73 2l 30 1256 251 133t Dr0 -16 33 -O 04 154 60 15t 24 173 L 1?A 13 16 16 33 tA LZ -\7 15 -5 41 11 11 -0(l a42 -a!1 0s7 545 0@ -000 -000 11 90 L 000 t69 -61 -0 1543 000 20 55 -20 55 -0 12 92 29 66 90 2r 94 LL 96 -rL 95 -16 -0 l17 -lrr 06r t6 94 16 22 22 03 1A4 000 000 ru 90 11140 rD 36 5 50 -2 r33 3o r91 3r -r99 9t -1 2S t33 9r r33 91 )5r 16 2! 24 15 45 \6 21 16 00 95 30 10 23 !5 -lA 26 -A 63 20 93 917 -2t9 6! 61 62 16 -62 42 -4 44 r3 91 21 rr L401 r011 -10 -3 lro 94 r70 77 -r7l 55 -r0 42 000 000 000 61 37 -61 97 -1 t3 000 -000 -000 r0 37 65 32 000 000 67 30 -423 4r16 1952 l951 177 000 162 165 L 76r r073 992 000 _000 -000 41 93 41 59 11 4L -t r1 -000 -000 oo0 o0r 001 000 26r !1 49J4 4496 4461 590 000 -000 257 1A 4r4 0@ 000 4024 3990 1937 21r -rrl -5 100 l3 96 656 000 -000 -000 391 000 000 000 4199 4149 1741 l15 22604 Z\r 64 242 52 -61 -000 -000 29 a6 t0 76 63 17 66 33 tO L 02 000 -15 13 3r ll 32 625 703 704 -073 395 000 -000 s41 000 -000 -000 r7 or t6 95 S4 I 91 -102 35 -r0 33 r32 55 131 07 -r3r & -9 36 547 0oC 000 -000 1615 519 -24 z\ 6L 6t A 1r 310 339 -333 041 -0 00 -0 00 92 69 9{ 65 -95 24 -3 90 L 000 001 -00r -000 -r6s -r0 -31 19 Lo -79 29 161 30 16163 s3 t9 Jr 12 31 76 73 0 30 4234 519 000 -000 000 -rr9 90 -9 36 22 t2 21 36 -20 50 -6 0l 20a6 r50 6)1t 6411 6116 290 -00r -000 9\ AL 94 36 -94 9r -3 24 -4A 22 -l 11 169 001 2533 2564 2564 056 4A 23 -0 -127 -10 353 -357 037 46 LA 4A A2 -41 9A 3 33 !6 51 15 2t -15 10 6r 126 16 126a9 l3 01 113 66 r17 Lr r!3 01 (r -4r -l -z5t 63 -1 6t -Zr -Z 19 64 91 33 t2 -O 2L 99 2l L6 aL 55 L 46\ 000 -000 -000 254 10 155 54 215 56 4 7a 14 16 12 4r 1Z 4r 41 2t -000 -r2A -rO 19 113 75 11342 rr9 000 000 z4 54 21 19 -2L 1A -\ 05 r13 95 120 17 61 A7 91 -33 l? -3 45 13 2l a6 1A L 713 000 1r3 20 114 32 -135 34 9 33 ro 9! 25 66 -25 4\ 1 10 L 154 000 -12 06 -2r -O 052 1 7Z 1? 06 299 r6l 2! )4 2L 9A 96 96 16 64 t6 59 1000 21zE 29 r4l 19 -r44 41 -10 55 -r -195 -9 -9\5 !\r9 1441 r9r 146 t9 0r 16 5t -36 55 l0 r92 49 1S1 94 66 60 L 617 917 022 n4 l? rr4 03 r15 39 35 30 34 79 -14 71 -2 6t 25 l3 23 r3 23 13 632 \211 r55 99 rs7 4r r53 91 9 l4 tol t0 10r 34 102 16 lt 6r l5 37 15 34 13 57 2A t2 2I 6L r3 6l 16 61 13 20 0) 21 {1 04 -!0 9l LA 46 lz 5r 42 26 993 996 161 042 3 3113 rr3 31 65 33 47 -3r 66 60 36 r2r 69 r23 60 r24 03 97 -A a44 510 559 rl0 !11 -2tt t79 71 r31 37 r35 6r -3 66 rr 4l -2 5r 62 -94 _3 t9 \5 24 00 Z\ 99 051 526 -171 -r0 91 56 93 94 02 04 1244 121 -414 354 -013 12 r0 -12 39 -0 25 3345 3365 3351 902 14 36 L2 90 12 2l 67 61 69 16 -69 92 -1 64 33 rs -33 02 -3 73 906 7O3O 71!L 322 tL 16 r27 75 rrl 5l -116 07 -5 (7 -1r -6 60 r20 26 5 97 -4 11 21 \! \9 r2o 75 1r9 11 t9 24 6A t1 67 ro1 73 10r 12 -r03 53 -5 75 30 -7t 0( -5 rl r73 33 r71 66 33 73 M1 49 9r L5 46 6344 6394 6391 531 -61 -r 3a1 00r 000 -4!0 60 29 51 31 46 l0 1226 06r r07l 6i9 462 452 000 000 3091 !224 947 16 53 16 57 1$ 55 r90 2l 43 63 L7 96 4t 99 L 310 J7 24 37 20 29 A7 29 99 75 03 r0 50 3996 l90r $37 365 45 -Z3Z 4340 563 r 599 5LO r29 9r r29 L3 rr0 40 6 r9 41 17 39 -r7 39 0 43 1 4L A71 49r2 4A61 911 319 9! 16 94 11 94 37 SL M9 r56 1913 1954 440 (4 26 a8 ?6 ro0 77 r0r r01 47 9 19 71 7B 95 99 96 53 -96 6r -t 92 i921 3A9t 1335 301 452 4L 46! 12 5r4 13 l1 43 50 21 19 39 rza 3t rza oa rzq 24 6 07 zo 1t 63 67 -63 5A -5 56 19 62 r9 39 -r9 37 1539l 1tr 7S 155 33 7 30 5307 5337 5333 137 r35 35 r54 4r 311 LL 21 96 za6t 19s6 65r r03 -220 2t5 -000 -000 rz2 )5 l2t 4J rz6 34 L 53( r36 rl5 -0rl 651 216 L 000 000 r032 52 36 5L 16 54 64 24 15 24 16 339 000 -000 -ooo L 111 000 20921 2r3 r3 Zl2 l9 6 25 z6 25 3r0 6351 6134 133 20 02 ra 25 35 46 31 1) lr 55 952 -000 000 45 16 1937 315 46 73 {6 35 46 30 !o5 2l r05 27 106 00 5 6? 170 63 U4 03 r3r 21 4 69 56!t 5614 510 r01 76 Lol 39 raz ?o 6 27 66 61 z014 524 r7r 50 r71 0t r7l r3 7 90 -226 r3 33 r53 73 16133 r55 71 9 4r -0 00 -0 00 L 903 51\ -\26 L 337 -52 -4 21 2t 4r ?5 42 1S 42 19 36 3r 36 51 3? 17 52 66 52 63 56 50 16 6A -16 !7 -Z L2 -0 00 -0 00 95 55 94 19 -94 Z1 -1 9t ll4 12 t9 -l 3l -L44 -5 4t 16 -45 61 -0 6303 6216 6246 1r9 34 r4r 14 93 51 56 34 56 Z5 56 16 17 1946 7796 7199 6t5 r433 L4r4 1121 490 -15 -7 -54 -4 35 99 l0 16 r04 29 101 16 r0r 61 7 2\ 5{ 30 53 56 59 41 rr3 22 rLJ 64 -I3 0r -3 6l -1r3 -t 03 r00 r0 r0r 67 102 05 -15 -l -52 -6 rr0 97 11693 53 15 47 1r 01 65! 53 00 73 02 63I 639L 74 36 r00 3r r04 02 104 20 9 19 00r 36 12 36 94 zs 60 27 60 L 653 36 6l 252 -740 -O11 6160 6a17 6416 410 11662 1r7 9r -113 30 -6 57 46 a2 47 19 47 Z2 Jl 69 4A A1 -tA 4\ -2 lr 1413 alz 198 -269 4353 5030 5073 ll2 r05 04 r04 04 10{ 27 3 53 -5 a122 33rl 3l2r 624 000 -000 -000 11 4t 66 62 66 10 1Z -1 000 -000 514 000 r03 45 107 l9 r07 70 1 26 66 91 62 7A 62 4A AL 7606 5t2 1L S5 21 20 -21 2A A 1\ r04a 2ao -271 062 45 1A L6 11 95 9a 96 16 1173 r66 9az0 91?a 909 ra t2 000 -000 -000 426 69 -000 000 65rr 6rz0 6031 176 tL66 1561 1452 512 313 66 63 ZO 63 26 360 000 -?2 LO 23 19 25 -19 r0 I 3r 000 -000 000 2d 33 ZZ 90 9L A l7 L0 49 167 10t 2r rro s3 rrr 55 1 25 r09 64 16 51 109 rr s 56 000 L L71 000 000 1609 74ll 74Zo !O) ta 96 39 51 t9 41 431 000 163 9t 161 70 165 60 7005 1025 393 49 9r 23 62 -A 12 !r9 53 r40 26 146 5l a 14 4116 1621 4603 397 55 53 53 77 5t l2 -7 49 ilr 72 lrr 71 1r4 33 7 31 lr 63 31 04 61 48 69 4S 651 644 -19 -A 2215 1922 1922 014 23343 213 99 1A 6) 19 4A 29 39 r2r rt \22 t4 -123 75 -1 00 r4l 35 rr4 7A 12 15 -!2 14 -O 41 000 000 000 -315 -119 r0 6r 740 t. 673 000 114 41 15 2! u5 57 15 17 -20 46 4A 46 21 -(6 t2 -l 71 20 !! 4\ A tZ L rls 569 043 613 tll 000 -000 669 2lt -264 051 {9 43 50 19 -50 39 -2 3l 000 000 SZZr 7144 1714 637 -0oo -000 174 A0 17544 r30 44 000 233 051 16r 04 162 71 164 15 r0 16 4r5 25 r4 l9 L r32 tt02 JOHAN P, R, DE VILLIERS

This results in a distorted configuration around M(3 ) since the four contributing bonds must now have angles considerably less than 90 degrees to each other. This is shown very clearlyin Figure l(c), and is analogous to the arrangement around Mn(4) in MnrOr. It can also be seen that the O-O interatomic distances along shared polyhedral edges are considerably shorter than the unshared ones.

ValenceState of the Mn Atoms It has been stated by many authors (Bystrcim and Mason, 1943;Muan, 1959,among others) that the structural formula of braunite must be Mn2+ (Mn2+14nr+)6SiO,, instead of Mn2+Mn3+SiO,r. This conclusion was used to support a proposed mechanism by which silica was incorporated in the structure. Crystallographically it is not possible to have an ordered version of the first formula above since there are three non-equivalent octahedral sites. The distribution of Mn2+ and Mna+ over sitesM(2), M(3), and M(4) would therefore have to be disordered. To explain the replacementof manganeseby silica, it is not necessaryto invoke the first model, and the formula Mn2+Mn3+5iO,, will be used here. Average bond lengths M(l)-O, M(2)-O, M(3)-O, and M(4)-O havevalues of 2.234,2.03A. 2.064. and 2.04A respectively,which compare very well with the high spin valuesof Mn2+-O : 2.224 and Mn3+-O : 2.05A given by Shannon and Prewitt (1969).

Comparison with CaM nusiO rz The structuredetermined by Damon et al (1966) for CaMn.Siol2has the samespace group and essen- tiallythe sameatomic coordinates as braunite. It can thereforebe regardedas the Ca end member of a solid solution series between CaMn.SiOl2 and Mn2+Mn.SiO12.

Comparisonof the Brauniteand a-MnzOsStructures Flc. l. Atomic arrangements around vacant sites. (a) the (Mn'+Mnl+Si) arrangement in braunite; (b) the (Mni+) arrange- Description ment in a-Mn2Or. (c) the (Mn!+) arrangementin braunite. In this discussion,the nomenclatureof Geller ( 197I ) will beused for thecell dimensions and atomic braunite correspond closely to those of MnrOr, and positionsof MnrOr. that the M(l) atomic positions having cubic coor- It has long been surmisedthat the braunite and dination correspond to half the Mn(4) octahedral MnrO, structuresare similar. and the a cell dimen- atomic positions in MnrO, in an ordered fashion. The sion of braunite(9.423 A) is comparedwith the value Si in tetrahedral coordination in braunite corresponds of 9.416A of MnrOr. The braunitec dimensionof to the other half of the Mn(4) atomic positions in 18.703A is slightlyless than twice that of Mn2O3(c : Mn2Os.The braunite M(2) atoms correspondto half 9.405A). From the presentinvestigation of the two of the Mn(l), Mn(2), and Mn(3) atomic positions, structures,it is apparentthat the cation positionsin and the M(3) atoms correspond to the other half of CRYSTAL STRUCTURE OF BRAUNITE AND ITS SOLID-SOLUTION BEHAVIOR I 103

Frc. 2. The arrangements of cations around Si and M(3) in braunite. The largest black circles are Mn'+ atoms, the intermediate white ones Mn'+, and the smallest black atoms Sin+

valueof at thesepositions. The braunite M(4) atoms correspond (1959).This valueis comparedwith the by to the Mn(5) atoms in the MnzOs structure. least 40 wt percent determinedexperimentally The mechanismof the formation of braunite from him. MnrOe can now be describedas replacementof one Any crystal-chemicalexplanation of silicareplace- the half of the octahedral Mn3+(4) in MnrO' with ment in the MnzO, structuremust also explain for a tetrahedral Si'+, in which the other half of the fact that an apparentlystable phase field exists Mn3+(4) changesto an eight-coordinatedMn2+. wide rangeof silica contents.It must be mentioned (1959) The octahedral cationic arrangement described that the phaseMn2O3(ss) described by Muan and he also earlier for the braunite structure is therefore also wasidentified by powderX-ray methods, presentin the MnrO, structure.Figure 1(b) showsthe mentionsthe existenceof faint additionaldiffraction ordering' arrangementin the MnrO3 structure,and Figures l(a) lines in someruns, which he ascribedto a and l(c) the arrangements in the braunite structure. De Villiersand Herbstein(1967) also described hasone These show unequivocally that, despite changes in naturallyoccurring phase, braunite II, which in braunite' coordination from octahedral to cubic plus half the amount of silica that is present which is tetrahedral, the cation arrangement is still the same, This phasehas a c cellparamet er of 37.76 A, MnrOt' It and that small shifts in the oxygen atomic positions twicrJthat of brauniteor four timesthat of of a are responsible for the change in coordination. It is is therefore likely that this phase consists regularstack- clear that these structures are very closely related. braunitecell and two MnzOscells in a ing arrangement.This postulatewill be testedwith a structureanalysis of brauniteII. Silica Substitution The phasefield as describedby Muan probably I propose that silica substitution in the MnzOa consistsof MnzOs plus similar, but well-defined, structure occurs by replacement of an Mn3+ ion with phasesin which the manganeseatoms are replaced by an Sia+ion. To regain charge balance,the opposite one, two, or three silicon atoms.The intermediate Mn3+ in the octahedral cation arrangement reduces phaseswill probably be regular stacking arrange- to Mn2+. Further substitution of silica will replace ments consistingof mixtures of these four basic two more Mn3+ atoms with Mn2+ and Sia+.Finally, phases. silica substitution will reach a maximum when all the Thereis alsothe possibilitythat the experimentally Mn3+ ions are replacedwith Mn2+ and Sin+.This oc- determined phase field consists of a disordered curs at a theoretical SiO, content of 44.05 wt percent MngOg structure with the silicon randomly dis- as depicted on the SiOz-MnaOn system of Muan tributed among the cationic sites in the structure. I 104 JOHAN P, R, DE VILLIERS

This distribution could become ordered after a or Vrlr-rBns,P. R., eNo F. H. HEnssrBlN 0967) Distinction suitable annealing process. between two members of the braunite group. Am. Mineral. 52, 20-30. Acknowledgments GELLER,S. (1971) Structuresof a-MnrO", (Mno"".Feoo,r)rOr,and (Mno,"Feo ur)rO, and relation to magnetic ordering. Acta The author gratefully acknowledges the assistanceof the follow- Crystallogr. B.27, 821-828. ing people: Dr. R. E. Robinson, Director General of the National JouNsoN,ClnnoLL K. (1970)Onrpp: A Fortran thermal ellipsoid Institute for Metallurgy, for the opportunity to conduct this plot program for crystal structure illustrations. Ll. S. Natl. Tech. research and for permission to publish this paper; Dr. G. Gafner I{orm. Seru. OnNr- 3794, 2nd rev. and Dr G. J Kruger, of the South African Council for Scientific Llnsou, A. C. (1967) Inclusion of secondaryextinction in least- and Industrial Research, for collecting the data and for helpful dis- squares calculations. A cta Crystallogr 23, 664-665. cussions; Dr. Elvira Gasparrini, of the Nationbl Institute for MUAN, A. (1959) Phaseequilibria in the systemmanganese ox- Metallurgy, for the microprobe analyses;Dr. J. C. A. Boeyens,of ide-SiO, in air. Am. J Sci. 257,297-315. the National Institute for Metallurgy, for helpful discussions;and RostNsoN, K., G. V. Grnns, eNo P. H. Rrsnr (1971) euadratic Dr. John S. White, Jr., of the SmithsonianInstitution, for the sam- elongation: A quantitative measureofdistortion in coordination ples of braunite polyhedra. Science, 112, 561-570. References SHINNoN, R. D., lNo C. T. Pnrwrrr (1969) Effective ionic radii in oxides and fluorides. Acta Crystallogr. B;25,925-946. Bvsrndu,A., eNo B. MlsoN (1943)The crystalstructure of Srrwenr, J. M., G. J. Knucrn, H. L. AlrlroN, C. DrcrrNsoN, lNo S. R. Hell, (1972) braunite, 3MnrOr. MnSiOr. Arkit: fdr Kemi, Mineral. och Geol Eds. The X-ray systemof crystallographic 168, t-8. programs. Tech. Rep. 192, Computer Science Center, Uniuersitv Cnourn, D. T., eNo D. LrsrnMnN (1970) Relativisticcalculation of Marvland of anomalous phys. scattering factors for X-rays. "/. Cher1. 53,, 189l-l 898. DnproN, J., F. PEnlrrNcEAT,AND J. pRorAS 0966) Etude struc- turale du composd CaMn"SiOrr. C.R Acad. Sci. paris. 262. Manuscript receiued, December 20, 1974, accepted t67l-1674. for publication, June 10, 1975.