American Mineralogist, Volume 59, pages 79-85, 1974 The GrystalStructure of Danburite:A Gomparison with Anorthite,Albite, and Reedmergnerite MrcHnBrW. Pnrrrrps,G.V.Glnns, eNn P. H. RInnp Department of GeologicalSciences, Virginia PolytechnicInstitute and Stote Uniuersity,Blacksburg, Virginia 24061 Abstract The crystal structure of danburite (CaB,Si.O"; a - 8.038 A; b - 8.752 A; c - 7.730 A; spacegroup Pnam) has been refined to an unweightedresidual of R - 0.021 using 1076 non- zero structure amplitudes. It consists of a tetrahedral framework of ordered B"O" and Si"Or groups which accommodatesCa in irregular coordination ((Ca'*-O) - 2.585 A; (CavII-O) = 2.461 A). A 7-fold coordination model appearsto be more consistentwith structurally similar anorthite, reedmergrrerite, and albite as evinced by the linear relationship between the mean Na/Ca-O bond lengths and the mean isotropic temperature factors of Na and Ca in these structures.The mean T-O bond length is 1.474 A for the B-containing tetrahedron and 1.617 A for the Si-containing tetrahedron, both of which are somewhat longer than those in re- edmergnerite(NaBSLOo). The trends observed between tetrahedral bond lengths, coordination number and Mulliken bond overlap populations for Si-O-+Al and Al-O-+Si bonds in anorthite are similar to those observed for Si-O+B and B-O"+Si bonds in danburite; shorter bonds with larger overlap populations and smaller coordination numbers are involved in wider tetrahedral angles, Introduction tion of bondingin thesestructures directed toward a The crystalstructure of danburite,CaBzSizOs, was rationalizationof their topologiesand the stericde- (cl determinedby Dunbar andMachatschki ( 1931) who tails and distribution of the tetrahedralatoms describedit as a frameworkof corner-sharingSi2O7 Ribbe, Phillips, and Gibbs, 1973; Gibbs, Louisna- and B2O7groups with the Ca atomscoordinated by than,Ribbe, and Phillips,1973). eight oxygens.Because they had beenunable to de- ExperimentalProcedure termine the positions of the boron ato,msprecisely, Johansson(1959) refinedthe structureusing 538 A smallcleavage fragment (0.10 x 0.t7 x 0.23 intensitiesmeasured by film methodsin a partial mm) was taken frorn a large danburitecrystal from three-dimensionalleast-squares synthesis. His refine- SanLuis Potosi,Mexico. The specimenis part of the ment confirmed the structure and yielded bond C. A. Michael Collectionof Gems and Minerals at lengthswith e.s.d.'s< 0.02A. ConcurrentlyBakakin, V.P.I.S.U. Single crystal photographswere consis- Kravchenko,and Belov (1959) completeda similar tent with Bakakin,Kravchenko and Belov's(1959) analysiswith poorer results.A study of the nuclear choiceof spacegroup Pnam. They statethat a sta- magneticresonance spectrum of 118established that tistical test of intensitiesindicated that danburiteis B-Si order persistsin danburiteup to its decom- centrosymmetric.Counter diffractometermeasure- positiontemperature (Brun and Ghose,1964). mentsof 40 alongthe axial zoneswere used to de- This studywas undertaken to determinemore pre- terminethe unit celldimensions: a = 8.038(3); b = ciselythe bondlengths and anglesof danburite(Phil- 8.752(5) ; c = 7.730(3) 4.. lips, Ribbe, and Gibbs, l97l) for comparisonwith Intensity data were collectedon a Picker auto- topologicallysimilar paracelsian(BaAlzSi:Oa) and mated four-circle diffractometer,using Nb-filtered hurlbutite (CaBe2PpO6)and with structurallysimi- Mo radiation and a scintillationcounter. A com- lar feldspars:anorthite (CaAl:SigOs)and its hexa- puter programwritten by C. T. Prewitt was usedto gonal polymorph; celsian (BaAl2Si2Os);synthetic correct the intensity data for backgroundand Lo- SrAl2SisOs;albite (NaAlSi3Os); and reedmergnerite rentz-polarizationeffects and to convert to lRr,"l. (NaBSi3Os).This is part of an extensiveinvestiga- Absorption and extinctioncorrections were consid- 79 80 PHILLIPS, GIBBS, AND RIBBE TAILE 1. Observed and Calculated Structure Factors for Danburite* F(bb3) Ftcalc) h L k.tobs) rtca)c) k \ F(abg) rlcatc) F(abs) Ftcatc) 1 5 2111 2312 e B 2799 257r r0 5 2313 2315 2 4 2A6! 2654 I 16 29ro ?81) r0 r 29rr 2979 rz t 2956 29,1e 9 ro 2935 2796 4 rr r0(l rr24 |2 \ )o41 26ol r 9 r05r zass 7 5 r0t6 2916 2 6 1|45 2J\5 rt I 6 tr 3ir9 235? 50 9 I 3 3rl5 2S97 50 s 6 1 t\t1 iz12 14 5 i I l17r rrrT 27 r 6 6 lrss ,3a4 5 r355 15r9 4 4 1 1192 21\J 65 r0 6 5 r?05 Irsl 66 12 0 r 1216 1056 2 1966 r7 33 1 1 | t26r 1215 6L 6 | 9 316s 212A 14 6 1 7 1Jr5 i2S1 l1 73 )4 s5 13 5 0 rr 135? lr01 rr 5 r 12 lr9r r1e7 31 79 30 33 a5 rr I 3 t4r5 lllr s! 3 1 ro 1164 J625 13 ? 12 5 31 35 lr.6 1 2062 ?2 1t I 1 3 1te| J146 rr s 4 I 1501 1612 29 I r? I 1501 rr4{ 50 , 3 6 l514 ]57A ! 109r r1 16 t a rt l561 3291 * The k and / columns are not in their usual order. CRYSTAL STRUCTURE OF DANBURITE 8I ered unnecessary.An anisotropicleast-squares re- Tesrn 3. Interatomic Distancesand Angles and Bond finementwas calculatedwith 1076 structureampli- Overlap Populations (n) in Danburite* tudes, (lF"o"l > 4c-),using the programby Busing, B-o discances (l) n(B-o) o-o dlstances (fr) o-B-o ansles (') Martin, and Levy (1962). The lF"5"lwere weighted T1-01 1.479 0.534 or-o2 2.32r 102.5 accordingto the schemeproposed by Hanson ( 1965) 02 r.498 0.513 01-03 2.419 I10, 7 03 I. 461 0.545 02-o3 2.429 110,4 and the variation of (waF'?)over the entire range of 05 r. 456 0.539 o2-o5 2.372 106.9 iiean 1.474 03-o5 2.406 11r.2 lF"o"l was minimized as suggestedby Cruickshank q.-o n (si-0) o-o distdnces (;.) (1965). The valuesof lF"o"land F*L are given in 4+tqreee=(4) .o:.!:9:_!al_99(') T2 0t 1,617 0.507 or 02 2.612 rlt,l Table 1. Atornic scatteringfactors for neutral atoms D2 r,624 0 549 0r-o3 2.650 110.4 were taken from Cromer (1965). 03 1. 6r.t 0.492 oL-04 2.66tt ltt. I and Waber The 04 1.614 0.514 o2-o3 2.574 105.4 positionalparameters and temperaturefactors (Ta- ileatr I. 6l 7 o2-o4 2.636 109.0 o3-o4 2.63a r09.7 ble 2), interatomicdistances and angles(Table 3), qcrg-{r.9Erres G) --l!ci|l L!:f-l!eleq-() and thermal ellipsoiddata (Table 4) were obtained ca 01- 2.496 t2l Tl-0r-T2 132.4 from the final cycle of the anisotropic refinement 02 2.452 I2) TI-O2-T2 t26.3 03 2.467 l2l T1-O3-T2 128. r 'r2-o4-'r2 which yieldedan unweightedresidual of R = 0.021. 05 2.399 trl 136.a 11-O5-Tl 130.6 02 3.020 t2) Description of the Structure * Estinated standart! errors fot aff distances are O.00lA and aff anqles 0.L". The asymmetricunit of danburite containstwo tetrahedrallycoordinated (7) cations (B and Si), one calcium, and five oxygen atoms. Of the latter BzOz and Si:1O7groups with the Ca atomsin either O(1), O(2), andO(3) arebonded to both Si and 9-fold coordination((Ca-O) = 2.585 A) or 7-fold B, whereasO(4) and O(5) arethe bridgingoxygens coordination((Ca-O) = 2.461 A). The 7-fold co- of the SizOz and B2O7 groups, respectively. The ordination model appearsto be more consistentwith mirror planesnormal ta c at heightsof 0.25 and structurally similar anorthite, reedmergnerite,and 0.75 contain the calcium atoms and the bridging albite as evincedby the strong correlationbetween oxygens,O(4) and O(5). The structurecan be thought of as a continuousframework of alternating Tesrs 4. Thermal Ellipsoid Data for Danburitet' Tnsrn 2. Positional Parametersand Temperature R.l'1.s. Factors for Danburite's Atom Ellipsoid displacement Angles to crystal axes (degrees) axis tll x y-z I'osicional parameters 0.068(1) 82(3) 8(3) 90 Atom jJ(82) xgz . 073(1) 90 90 180 .077(1) 8 (3) 98(3) 90 Ca 0.38s4(1) 0.0765(1) r/4 0.42(r) TI .060(3) r14(39) 24(39) 91(r 4) TI .2s90(1) .4r92(r) .4zoL(r) .31(1) .063(3) 155(38) 114(39) 81(19) T2 .0s33(r) .L924(r) -.0ss8(1) .25(1) .069(3) 82(18) 85(14) 9(19) T2 .0s2(r) 89(6) 19(7) 7r(7) ol .re30(1) ,.0680(r) -.0032(1) .s6 (1) (1) -.0433 .0s7 48(23) i6(13) 135(19) o2 .r263 (r) .36s0(1) (r) .4e(1) .060(1) 41( 21) 103(7) s1(19) o3 .3998(r) .3r3s(1) .078r(1) .47(r) o4 .sr36 (1) .6636(1) r/4 .60(2) .066(2) 1r2(5) 23(4) 82(4) 05 .1818(1) . 4282(L) r/4 .54(2) .082(2) 137(4) 101(s) 131(4) , ro2 (2) 12s(3) 1r0(2) 42(3) Anisotropic temPerature factors (x 10q) .o62(2) 63(s) Atom ^ s6(4) 46(7) 611 6zz B:: Btz Br: Bzz 67(5) r35(7) 54(5) .094(2) 43(3) 98(3) 132(3) Ca 18(1) 12(1) 18(r) -r (r) 0 0 r1 12(t) e(1) 16(1) o3 .070(2) 42(rr) 101(1i) 130(18) 0(1) 0(1) 0(1) .074(2) 7r(17) 138(9) 54(17) 12 r0(r) 7(r) 1r(1) 0(1) 0(1) .1(r) .086(2) s4(5) 50(6) 6r(s) or 23(1) 13(1) 29(r) 4(1).-s(1) 4(1) .066(3) 90 900 02 2r(1) 12(1) 22(r) -3(r) -7(1) -r(1) .086(2) 46(5) 136(5) 90 (2) 03 18(r) 16(r) re(1) 2(1) 3(1) 2(r) .104 44(5) 46(5) 90 04 28(r) 23(r) 14(1) s(1) o 0 o5 .066(3) 90 90 0 os r8(r) 26(L) 14(1) 3(1) 0 0 .016 (2) 167(4) t7 (4) 90 , ro2(2) 17(4) 13(4) 90 * Estiilated standard errors are in parentheses and * Escinated standard eirors are in parenfheses and refer to the Iast decimal pLace.