321
Cannlian Mineralngist Vol. 31, pp.321-329(1993)
EDGE-SHARINGMn2*Oo TETRAHEDRA lN THESTRUCTURE oF AKATORETTE,M n3*Al2Si8O24(OH)8
PETERC. BTIRNSAND FRANK C. HAWTHORNE Departmentof GeolagicalSciences, University of Manitoba"Winnipeg, Manitoba R3T2N2
ABSTRACT
The crystalstrudure of akatoreite,Mn!*Alrsiroza(oH)s, a s.337Q),b 10.367(2)"c 7.629(l) A, a 1M.46(1),0 93.81(2),y 104.18(l);,V 613.2(2)L3, fr, zol, has6een solved'by direct methds andrefined to an R indexof 2.9Vofor 2691observed reflectionsmeasured with MoKo radiation.There are five independentMn positions.Four ofthese are octahedrallycoordinated by O2- and OH- anions,and the meanbond-Jengths at rhesesites show the Mn to be entirely in the divalent state.The fifth Mn positionis tetrahedrallycoordinated by four 02- anions,and the mean bondJength shows the Mn to bein the divalentstate. Adjacent 6AnO; teuahedrashare an edgeto form an [Mn2O6]dimer. Thereis one Al position,coordinated by six anionsin an octahedral arrangiment,and both site-scatteringrefinement and meanbond-length show no substitutionof Fe'* or Mn'* for Al at fiis site. Thereare four distinctSi positions,all ofwhich aretetrahedrally coordinated; one ofthe silicateteuzhedra is anacid silicategroup: SiO3(OH).The four Si tetrahedraform a linear [Si4O12(OH)]cluster, and akatoreiteis thus a sorosilicate.The (MnQ) and (A106) octahedra(Q: unspecifiedailon) form edge-sharingstrips of octahedrathree ocahedrawide, that extendalong the a direction. Thesestrips are cross-linkedinto sheetsby the fMn2O6ltetrahe&al dimers, which shareedges with the peripieral ctahedra of adjacentstrips. The resultantsheets arc linked into a complexheteropolyhedral framework by the sorosilicatefragments and by a hydrogen-bondnetwork. Relationships to other sorosilicatesare reviewed.
Keywords:akatoreite, crystal structure, silicate, sorosilicate, tetrahedrally coordinated divalent manganese.
SoMMAIRE
Nous avons affind la structurecristalline de l'akatoreite, Mne2+A12SisO24(OH)e,a 5.337(2), b 10.367(2),c 7.6290) A, u 104.46(l),F 93.31(2),y 104:18(l)",V613.2(\ L3, fl,Z= l, parm6thodes directes jusqu'b un r€siduRde2.9voen utilisant 2691 r6flexionsobserv6es, mesurdes avec rayonnement Mo/(cr. ta structurecontient cinq sitesind6pendants de Mn. Quatred'entre eux sont en coordinenc€octaddrique avec des anions02* et OH-; danschaque cas, la longueurmoyenne des liaisons montre que le Mn est entibrementbivalent. Le Mn de la cinquidmeposition possbdeune coordinencetdtra6drique avec quatreanions O'-; la longueurmoyenne des liaisons montre que le Mn est bivalenl Les tdtrabdres(MnOa) adjacentspartagent une a€te, pour former un dimbre[Mn2O6]. fl y a uneposition occupdepar l'Al, entour6ede six anionsformant un octaCdre.Un affinementdes pouvoirs de dispersiondtette position,aussi bien que la longueurmoyenne des liaisons, montrent qu'il n'y a aucunremplacement de I'Al par Fel* ou Mnh. [a structurecomporte quatre positions distinctes occup€es pax Si, toutesi coordinencet6tra6drique. Dans un ias, il s'agit d'un groupesilicat6 acide: SiO;(OH). Les quatret6trabdret 51 lottnsnl un alignement[Si4Ol2(OH)]; I'akatordite serait donc un sorosilicate.Les octabdres(MnOd et (At$) forment des rubansd'octabdres h a€tes parag€es,d'une largeur de trois octaidres, orient6sle long de a. Ces rubanssont li6s transversalementen feuillets par les dimbresde tdtrabdresMnzOel, qui partagentune a€te avecI'ocaddre Sriph6rique desrubans adjacents. I€s feuilletsqui en r6sultentsont 1i6s en une uame complexe h6t6ropoly6driquepar les fragmentssorosilicat6s et un r6seaude liaisonshydrogbne. Nous examinons la relationde cettestructure aveccelle d'autressorosilicates. (Iraduit par la R6daction)
Mots-clls: akator6ite,structure cristalline, sitcate, sorosilicate,mangandse bivdLlent h coordinencet6tra6drique.
carbonatelens on the southeasternmargin of the Haast SchistGroup, South Island, New Zealand.Akatoreite is INTRODUCNON associatedwith rhodochrosite,rhodonite, spessartine, quartz, hiibnerile, alabandite,pyroxmangite, tinzenite, Akatoreite is a hydrous manganesealuminosilicale apatite,psilomelane, pyrolusite and todorokite. first describedby Read & Reay (1971). It occurs as Results of chemical analysesreported by Read & vitreous, orange-brown,massive to radiating twinned Reay (1971) indicate that akatoreitehas the formula colurnnaraggregates in a manganiferousmetachert and (Mn6.61Fq.1el\4g0.o8ca0.orAlzoesi7.75oa.r7(OH)8.83.This 322 TIIE CANADIAN MINERALOGIST structural study results in revision of the akatoreite mn 2. Amrrc PitA}lE'mm FoR mR'EIT8 formula, which may be given ideally as MnnAlrSirOro(OH)r.
h(1) 0 L/2 1/2 87(2) h(2) o.508s(7) 0.42848(6) 0.36083(8) 85(2) EXPERIMENTAL h(3) 0.8x62t-<7) 0.29882(6) 0.08032(8) 7912) h(4) 0.56979(7) 0.77338(6) 0.05242(8) 77(2) h(5) 0.8!542(8) 0.94289(6) 0.3793?(8) LO2(2' A1(r) 0.221"0(1.) 0,3686(:.) o.2x32(L' %(3) The material used in this work is from the type sr(1) 0.1155(1) 0.3580(1) "0.1861(1) 54(3) s1(2) -0.4317(1) 0.1019(t) .0.7056(1) 57(3) Iocality, 3 km south of the mouth of Akatore Creek, sr(3) -0.2559(1) 0.2987(1) "0.3211(1) s6(3) easternOtago, s1(4) 0.1910(1) 0.0480(1) 0.1988(1) 60(3) New Zealand.An optical examinationof 0(r) 0.1671(3) 0.4991(3) -0.252614) 79(8) theakatoreite crystals indicated that it is usually 0(2) 0.0953(3) 0.3845(3) 0.0294(3) 81(3) nvinned, 0(3) -0.2330(3) 0.4429(3') -0.3805(4) 75(8) asreported by Read& Reay(1971). Several crystals of o(4) -0.3370(3) 0.1047(3) "0.1358(4) 92(8) 0(5) o.2433<4) o.2643(3) -0.2440(4) 93(8) akatoreitewere crushed and examined optically; a small o(5) -0.3956(3) 0.2212(2) "0.8120(4) 94(S) 0(7) -o.w9<4, .0.0198(3) -0.7731(4) 108(8) untwinned cleavagefragment was eventually selected 0(8) -0.3682(4) 0. r.741(3) "0.4894(4) 118(8) 0(9) -0.0716(3) 0,2701(3) -0.3002(4) 92(8) for study. The crystal was mountedon a Nicolet R3n 0(10) 0.0?s(3) 0.0573(3) 0.35X1(4) 92(8' o(11) o.3678(3' 0.0348(3) 0.2767(4) 115(9) automatedfour-circle diffractometer,and twenty-four o(12) 0.2138(3) 0.1800(3) 0.1161(4) 78(8) of,(13) 0.0168(3) 0.312s(3) 0,3242(4) 73(8) reflections were centered using graphite monochro- os(14) 0.3625(3) 0.3680(3) 0.4465(4) 80(8) mated 0s(15) 0.1094(4) -0.0957(3) 0.0348(4) 110(8) MoKd X-radiation.The unit-cell dimensions on(16) o.4247<1' 0.4265(3) 0.1448(4) 86(8) (Table E(1) 0.035(7) o.2t5(4) 0.366(7) 15O* l) were derived from the setting anglesof the H(2) 0.330(7) 0.291(4) 0.501(7) 150 twenty-four automaticallyaligned reflectionsby least- tr(3) -0.0!.2(1) -0.r20(6) 0.004(7) 150 lr(4) 0.424
frBrl 4. SEITCTED INTEUTOI{IC DIST4NCES (A), ANGI.ES (O) AND h(1) @t5l6droa TLYSEDSAL EDGE-HGIIIS (A) IN EMEITE 0(1)a-0(3)b a2 3 .279<4' o(1)a-M!(1)-o(3)b x2 93.9(1) o(1)a-o(3)i , 3.063(4) o(1)a-l,n(1)-o(3)t !2 86.1(1) }'i(L)-O(1)a , 2.274(3' Ar(1)-O(2) 1.882(3) o(1)a-0u(13), 3.!01(3) o(1)a.[n(1)-otr(13) x2 97.6(1) !ln(1)-0(3)b , 2.2r3(3) A1(1) -o(3)l 1.971(l) 0(1)a-G(13)p i2 2.891(s) o(1)a-h(1)-ou(13)D a2 82.4(X, l{n(X)-oll(13) , 2 - 110(31 Ar(1)-0(12) 1.920(3) o(3)b-on(13) !2 3.424(4) o(3)b-l{n(X)-oll(13) t2 75.3(L) <{n(1)-D 2.L99 A1(1) -0s(13) 1.895(3) 0(3)b-og(13)p:2 2.64!( L\ o(3)b-!Lr(1) -ol(13)p 12 !04--7-(1) A1(1)-on(14) 1.948(3) o-o 3.100 positions. Refinementof all parameters,including an led to convergenceto an R index of 3.OVoanda l1rRindex isotropic displacementmodel, converged to an R index of 3.2Vo.A model including a refinable structure-factor of 3.77o.Conversion to an anisotropicdisplacement weighting scheme and an isotropic extinction correction model, togetherwith the refinementof all parameters, was tried but did not lead to any improvement in the 324 TI{ECANADTANMTNERALocTsT TABI,E 5. BOND-VAI;ENCE* TABI,E FOR AKATOREITE ln(41 ln(5) ll(1, sl(l, sl(2) 6(r) i(2, ro) r(4) I 't.968 oc, 0.26d i 0.3?i 0.986 o(2, 0.30:t 0.tt' 1.014 1.85? 't.99l o(3) o.rr* f o.2ar o.4a 0.97! o(4) 0.40? t.084 0.t5 2.01? o(5) o.82 0.47t r.059 1.9V o(6) 0.312 o.2&, 1.O8 1.qa o(7) 0.1t9 0.53 r.0m 2.W 0(8, t.o20 t.67 2.67 o(9) 0.98 1.898 ofi0) 0.4* t.qtl 2.1\4 o.ta 0(l1t t.olt 2.W o(12) o.26 o.t& 1.05 0.5 2.005 on(1!) o.4o8t i 0.fl7 1.998 or(14, 0.I'5 o.&9 t.960 0.!26 or(15, 0.9t9 2.W o[c6) o.& 0.t30 0.8i5 2.017 1.976 t.986 t.9d3 1.87t 1.889 2.918 1.984 4.141 4.4r3 t.0 1.0 1.0 r.0 * bond-valence paran€t€rs fron Brorm (1981) refinement. A three-dimensionaldifference-Fourier anions in an octahedralarrangement, and the mean map calculatedduring the final stagesof refinement observedbond-lengths are all --2.20 A. This value is revealedthe locationsof the four hydrogenpositions. closeto the sumof theempirical-radii of Shannon(1976) Subsequentcycles ofrefinement showedthe hydrogen for 16lI\&12+6d trl92- of 2J9 A, indicating that all of positions to be unlikely, as indicated by anomalorrsly the octahedrallycoordinated Mn is in the divalentstate. short O-H bond distances.a conmon featureof refine- Mn(l) lies on a centerof symmetry;hence the two ment of hydrogenpositions using X-ray data.The soft coordinating hydroxyl anionic groups are in a trans constraintthat eachO-H bond distanceshould be -0.98 arrangement.Mn(2) is coordinatedby three oxygen A was imposedby addingthe constraintsas additional atomsand threehydroxyl anionic groups,and the latter weightedobservations in the least-squaresmatrix. Note areal1 in a cis arrangement(Table 4). For Mn(3), thefwo that only the O-H distanceswere constrained,and that coordinating hydroxyl anionic groups are in a cis each hydrogenposition was free to refine around the arrangement,and Mn(4) has only one coordinating associatedoxygen. Refinement of all structuralparame- hydroxyl anionic group. ters, including the hydrogenpositions, converged to an Mn(5) is coordinatedby four oxygen atoms in a R indexof 2.9Voand a wRindex of 3.2Vo.Finalpositional tetrahedralarransement. The b sin?I / l- c sinB-/ FIG.I . Polyhedralrepresentation of the structureof alatoreiteprojected along the a axis.The polyhedralshadings are as follows: Si: crosses,Al: cross-harched,Mn: randomdots. Note the compositionallayering of the structure, with alternating layen of (SiOa) tetrahedraand (Mn$), (MnO) tetmhedraand (AlQ6)octahedra. 326 THE CANADIAN MINERALOGIST I FIc. 2. Polyhedral representationsof the structureof akatoreite:the [Mn9Al2$22]sheet projectedon to (02I ); shadingas in Figure I . Strips of edge-sharing(lvln0o) and (Al0o) octahedraare centeredalong the a axis and are crossJinkedby edge-sharingMnzool dimersoftetrahedra. Flc. 3. Polyhedralrepresentation ofthe strucore ofakatoreiteprojected onto (021);shadings as in Figure l. The [MneAl2Q32]sheet is shownwith the sandwichinglayen of silicate tetrahedra;note the [SiaQ13]sorosilicate ftagments. THE STRUCTUREOF AKATOREITE 32 I of octahedraand the dimers of tetrahedra.A SiOa Hydrogenbonding tetrahedronlinks to both an O(10) of the sharededge of the [Mn2O6]tetrahedral dimerandan anion of an (MnQ6) Thereare four uniquehydrogen positions in akatore- octahedronof the strip of octahedra.This linkage can ite; details of their local stereochemistryare given in only occur where adjacentstrips are in their observed Table4 andFigures 4 and 5. Threeof the four hydrogen configuration;holosymmetrical polyhedra would leadto bondslink acrossadjacent [MneAl2032] sheets (Fig.4); a correspondingdistance too large to be the edgeof a the acceptor anions are always [3]-coordinated and silicatetetrahedron. neverinvolve anionsthat bridgesilicate tetrahedra. The The large intersticesin the [MneAl2Q32]sheet are H(3) atomhydrogen bonds to an adjacentO(12) anion sandwichedbetween [SiaQs3] sorosilicate fragments, and within the same [MneAl2Q32]sheet (Fig. 5); note that theseback-to-back chain fragmentsresemble the rela- H(3) is bondedto an oxygen of a silicate tetrahedron, tionshipbetween adjacent silicate chains in thepyroxene formingan acid(SiO3OH) group. The D(donor)-H(hy- structure. Sorosilicate fragments also link to the drogen)distances are all approximately0.98 A, asforced MneAl2032lsheet yic onebond per tetrahedron,and this by the soft-constraintmethod of the hydrogen-position refinementprocedure. However, this proceduredoes not bond cross-links adjacent [MnsAlzQ:z]sheets into a complexheteropolyhedral framework. explicitly constrainthe H(hydrogen)-A(acceptor)dis- \ bsinr csin0 / Ftc. 4. The hydrogen-bondingarrangements involving the H(1), H(2) and H(4) atomsin akatoreite;cation shadingscorrespond to the polyhedralshadings of Figure l, oxygen atomsare part-shaded spheres, hydrogen atoms are black spheres,donor-iydrogen and hydrogen-acceptorbonds are shownby heavyfull and brokenlines, respectively. 328 THE CANADIAN MINERALOCIST can be rationalizedas occurringto satisfy the bond-va- lencerequirements of its constituentrmions (Table 5). However,the G-Mn-O anglesshow extreme distortions (upto 36') from themean value of I 10o.The reason for this can be seen in Figure 6. The [Mn2O6]dimer of edge-sharingtetrahedra has to link laterally to alternate tetrahedraof the [SiaQ13]sorosilicate group, and this requiresthat the dimensionsof the two gtoups match. The separationof the relevant vertices of the silicate tetrahedrais -5.3 A, whereasthe separationof the relevantvertices of a holosymmetric[Mn2O6] dimer is -4.S A. An unconstrainedsorosilicate fragment could easily shortenby differential rotation of the tetrahedra but this is preventedin akatoreiteby the articulationof the [SiaQ13]group to the strip of octahedra.On the other hand,the [Mn2Ou]dimerhas no suchexternal constraint, andhence all of the mismatch(-0.5 A) is takenup by distortionof the edge-sharingdimer. Hencethe dimer is stretchedalong its long axis to promotethis linkage. The [Mn2O6]dimer also sharesedges with (Mn$6) FIc.5. Thehydrogen-bonding ,urangement involving the H(3) octahedra of adjacent octahedral strips. The strong atomin akatoreite;legend as in Figure4. onces.These values lie betweenI .75and 2. 17 A. in the rangefor the occurrenceof significant hydrogenbond- ing. In addition,the anglesalso are quite typical; three of theD-H-A anglesare close to 160', aboutthe average for such anglesin inorganic structures,and the fourth angleis 137o,within the rangetypical for mineralsand inorganicstructures. The calculatedH-A bond valences(Table 5) are in line with the other bond valencesin the structure.The H(3)-O(12)distance of l.7S A GaUe +) is significantly shorterthan the other H-A distances,and the variation in bond valenceassigned to the O-H and H-A bonds Ieads to bond-valencesums around the donor and acceptoranions close to the ideal value of 2.0 v.u. (valenceunits). Revisionof the akatoreiteformula Read& Reay(1971) defined akatoreite as a hydrous manganesealuminosilicate with the formula (Mns.u, F%.rsNlgo.oaCao.0tA12.oesi7.75oz:.rr(OH)s.er.This for- mula was basedupon results of electron-microprobe analysesand water determinations, with no direct deter- mination of the oxidation stateof manganese(or iron) completed.This structuralsfudy indicates that all ofthe numganeseis presentin the divalent state, and gives eight hydrogenatorns per formula unit. The idealized formula of akatoreiteis revisedto Mny'l2SisOza(OH)s. Distortion of the (MnOl tetrahedron Frc.6. Detailsof the articulationbemeenthe [Si4Q13] fragment andthe [Mn2O6]cluster in akatoreiteprojected on to (021); The (MnO) tetrahedronshows significant distortions legendas in Figure 4. (a) Actual arrangement;(b) similar in bond length from the meanvalue of 2.057 A; these linkagewith holosymmetricalocahedra. THE STRUCTUREOF AKATOREITE 329 extensionofthe dimer significantlydecreases the poten- Canadain the form of a Post-GraduateFellowship to tially destabilizing Mn-Mn interactionsacross all of PCB and Operating,Major Equipmentand Infrastruc- these shared edges, and it seemsprobable that the ture Grantsto FCH. The University of Manitoba sup- stabilizationof this unusualarrangement of edge-shar- portedthis work in the form of a Post-GraduateFellow- ing oc-tahedraand tetrahedra of a largecation 1lal14nz+ - ship and the J.L. Lightcap awardto PCB. Reviewsby 0.66 A, [61111n2+- 0.83 A) resultsfrom the cooperative Drs. Paul B. Moore and Carlo M. Gramaccioli, and interactionof linkage and energeticeffects. editorialwork by Drs. RonaldC. Petersonand Robert F. Martin, improved the clarity and quality of the manu- Relatedsorosilicates script. REFERENCES The detailedstereochemistry of extendedLToQu*i,n > 2 sorosilicateclusters was consideredby Hawthorne BRowN,I.D. (1981):The bond-valencemethod: an empirical (1984);details concerning bondlengths andbond angles approachto chemicalstructure and bonding.1z Structure for this structure are in agreementwith the trends and Bondingin CrystalsII (M. O'Keeffe & A. Natvrotsky, exhibited by the structuresexamined in that study. eds.),Academic Press, New York. Although not noted in that previousstudy, all sorosili- cate clusterswith z > 3 have acid tetrahedralgroups. CRoMER,D.T. & LIBERMAN,D. (1970):Relativistic calculation Akatoreite is no exception; one end of the extended of anomalousscatt€ring factors for X rays.J. Chem.Phys. 53,1891-1898. [7aQ13]cluster is terminatedby an acidsilicate group. Sorosilicate structureswith short clusters (tsrz$rl, - & MANN,J.B. ( 1968):X-ray scatteringfactors computed [Sr:0ro])are basedon sheetsor walls of polyhedra from numericalHartree-Fock wave functions.Acm Crys- (octahedraor octahedraand tetrahedra)that are cross- tallagr. 42,/,,321-324. linked by sorosilicateclusters that extendperpendicular to the sheets.Sorosilicate structures with longerclusters GRAMAccroLr,C.M., Lrnoruo, G. & Hnn, T. (1981):Structure ([ZoQrs]in medaite,Mn6[VSi5O13(OH)], Gramaccioli et of medaite,Mn6[VSi5O I 3(OFI)]: thepresence of a newkind anion. Acm Crystallogr,B.37, 1972- al. l98l) consist of sheetsor ribbons of octahedrain of heteropolysilicate 1978. which the sorosilicatecluster extendsparallel to the (Ca2Mn2[SiaOrr(OH)z] sheetsor ribbons. In ruizite .-* Pner, T. & Lnoruo, G. (1979): Structureof a (OH)2(H2O)2:Hawthorne 1984), discontinuous sheets manganese(tr) arsenatotrisilicate,Mn4[AsSi3O12(OH)]: ofedge-sharingoctahedra are linked by [SiaQrr]clusters the presenceof a new tetrapolyphosphatelikeanion. Acrc orthogonalto the sheets.On the other hand, in tiragal- Cry stallo g r. B.35,2287 -229 1. loite [Mn4AsSi3O,2(OH):Gramaccioli et al. 1979]and akatoreite,the sorosilicateclusters arc orientedparallel HAwmoRNE,F. C. (1984): The crystal structureof ruizite, a to the sheetsof octahedra and show more structural sorosilicatewith an [SiaQ13]cluster. Tschermal Financialsupport for this work was provided by the Received June 2, 1992, revised manuscript accepted NaturalSciences and Engineering Research Council of October i0. 1992.Mineral.56, ACKNOWLEDGEMENTS silicate eastem 416426.