9L
The CanadianMineralo gist Vol. 34, pp. 91,-97(1996)
THE CRYSTALSTRUCTURE OF SHIGAITE, lAlMne*(oH)ele(so+lzNa(H2o)6{H2o}6, A HYDROTALCITE.GROUPMINERAL
MARK A. COOPERand FRANK C. HAWTHORNE Deparnnewof GeolagicalSciences, University of Manitoba"Winnipeg, Manitoba, R3T 2N2
ABSTRACT
The cryslal strucbre of shigaite, tAMnA+(OlD6l3(SOfrNa(H2O)6{HzO}0,rhombohedral, a 9.512(l), c 33.074$) 4,, y 2591.0(8) Ar, Z = 3, R3, hasbeen solved by direct nethods and refined to an R ndex of 4.2Vousing 979 observedreflections measuredwith MoKcl X-radiation.The structuralunit of shigaiteis a planar sheetof edge-sharingoctahedra [AlMna+(OID6]1+. These oxycation sheetsare intercalatedwith oxyanion she€tsof chemical composition Na(H2O)6{H2O}6(SOf2;hydrogen bonding plays a major role in linkage both within the oxyanionsheet and betweenthe structuraluoit and the oxyanionsheet of interstitial species.This work showsshigaite to containessential Na andresults in a significantrevision ofthe chemicalformula- Shigaiteis a hydrotalcite-goup mineral, the Mn2+analogue of motukoreaite.
Keywords:shigaite, crystal structure,chemical formul4 hydrogenbonding, hydrotalcite group.
Somaanr
Nous avons affin6 la stucture de la shigarte, IA1MnS+(OII)613(SO)2Na(H2O)6{HzO}0,rhombo6drique, a 9.512(l), c 33.074(0A, V259L.09)N,z= 3, Rl parm6thoaes directeijusqu'lirnreiiiu n a6l.Z%6nuinsnt979 r6flexionsobserv6es avecun rayonnementMorcr. L'unit6 structuraleest un feuillet d'octabdrese a€tes partag6es,de composition[AlMn3+(OI{)6]11 Ces couchesoxy-cationiques sont intercal6esavec des couchesoxy-anioniques de composition Na(H2O)6{H2O}6(SO)2; les liaisons hydrogdnejouent un role imporanq aussi bien dsns les couchesoxy-anioniques qu'enhe I'unit6 structuraleet la coucheoxy-anionique de I'espbceinterstitielle. Nos rdsulas montrentque la shigaltecontient un atomede sodium,essentiel, ce qui mbne d une revision de la formule chimique. l,a shigaite fait putie du groupe de la hydrotalcite, et est I'analoguee Mn2+de la motukorealte. (Traduit par la Rddaction)
Mots-clAs:shigaite, structure sistalline, formule chimique,liaison hydrogbne,groupe de la hydrotalcite.
Inrnooucnox district), Cape Province, South Africa, and was generouslyprovided by Mr. William M. Pinch. The Shigaite is a sulfate mineral describedfrom Shiga crystal usedin the measurementof the X-ray intensity hefecture, Japan,by Peasoret al. (1985).The orieinal data a niangular {001} plate -0.28 mm on an edge formul4 AI4M&(OII)22(SO/2.8H2O,the tabular habit and 0.04 mm tlick, was mounted on a SiemensP4 atrd perfect {001} cleavageindicate a sheetstructure, automatedfour-circle diffractometer.Cell dimensions and Peacor et aL (L985) suggestedthat shigaite has (Iable l) were derived from the setting anglesof 37 sfructural affinities with lawsonbauerite,(Mn,Mg)e automatically aligned reflections. Intensities were ZnIOI{)22(SO)2.8H2O(Treiman & Peacor1982). As measuredfrom 4 to 60'20 (0 < h < 11, 0 < k < LI, part of our interestin [MQr]-derivativesheet structures 46 < I < 46) with scanspeeds varying between 1.78 (M: octahedrally coordinated divalent or trivalent ard 29.3"201min: a total of 1988 reflections was cation, 0: unspecified anion), we have solved and measuredover one asyrnmetric unit. Psi-scan data refined the structureof shigaiteand presentthe results were measnredon 12 reflectionsbetween 9 and59'20 here. at incremetrts of 5o, and a thin-plate absorption correctionreduced R(azimuthal) from 3.3 to 2.0Vofor a Enpm.nm.rrat glancing angle of 2o. Intensities were correctedfor absorption, Lorentzo polarization and background The material used in this work is from the effects, and then reduced to strucfure factors; N'Chyaning mine, Kuruman (Kalahad manganese 979 reflectionswere classedas observed(lF"l > 5olFl). 92 TTIE CANADIAN MINERALOGIST
TAELE 1. MIgCE.I-ANEOUS INFORMATION FOR SHIGAM identified via the relative scatteringpowers at the atom a (A 9.612(1) Ctystsl dzo (mml O.3Or 0.30 r 0.25 x 0.04 positions,together with the local tetrahedralgeomery. c 33.074(51 Radiation Mor(d/GGphho Tbree atom positions were located with scattering vA1 269r.0(a) Tdd No. of r 1988 powers approximately equal to that of oxygen. One 3p. Gr. R3 'No. ot r 1855 atomis at the origin andis octahedrallycoordinated by P (mmn) 2.4e No. of lR 1699 Do(9.@1 2.21 No. of lF.l>soF 979 one of the other atomswith an interatomicdistance of R(uimdhaf) % 3,?-2,O 2.47 L The central atom was assignedas Na, and the A(obs) % 4.2 coordinating anion was assignedas an oxygen atom larR(obs, % 4.8 group. Call @ntenr: 3lNaAlsMn6(SO.l,(OHlBt2H,Ol of an H2O The third interstitial atom is not a - :(lr.l-lr.llElF.l directly bondedto any cation andwas assignedas HrO. Subsequentleast-squares refinement confirmed a fully occupiedNa site at the origin. Difference-Fouriermaps ' altgr removd ot glgncing reflsctiono following fuIl-matrix least-squaresrefinement showe{ all H atomsin the structurebetween 0.88 and 0.95 A from their associateddonor oxygenatoms. All H atoms were input into the least-squaresrefinement with the soft constraint that all O-H distances are approxi- mately equal to 0.98 A, and all H-H distancesin H2O groupsare approximatelyequal to 1.55 A. Full-matrix Srnuctunr Sonrnou eNn Rmnwvmnr least-squares refinement of all variables with anisotopic displacement parametersfor all non-H All calculationswere done with the SHELXTL PC atomsconverged to an R rndex of 4.2Vo.Final atomic (Plus) systemof programs;R indices are of the form positionsand displacement factors are given in Table 2, given in Table 1 and areexpressed as percentages.The selectedinteratomic distancesand angles for non-H E statisticsindicate a centrosymmetic stucture, and a atomsare given in Table 3, interatomic distancesand solutionwas found in the spacegroup R3 peacor et aL angles involving H atoms are given in Table 4, 1985).Cation and anion positions eqmFatiblewith an and a bond-valencetable is given asTable 5. Observed [MQr] sheetwere locatedand refine4 and the relative and calculatedstructure.factors are availablefrom the X-ray scattering powers were used to establish the Depository of Unpublished Data CISTI, National cation identiti"r. q interstitial (SO/ group was ReseaxchCouncil, Ottaw4 Ontario KlA 0S2.
TABLE2. FINALATOMIC PARAMETERS FOR SHIGA]TE tUn uf u2 us u6 ur Utz trrn -0.0011(11 0.3336(11 0.16671(2) 130(3) 66(4) 62(4t 261(3) 8(41 3(4) 31(3) At(l\ 0 0 0.16476(8) 75(6) 5rt{6) 54(61 118112) 0 0 2743't A^2t 1t3 2t3 1t6 82(9) 60(101 60(10) 1zt6(18) o 0 25(5) NaO0 o 332117t 336(201 336(20) 324(33) 0 0 168(10) s 2t3 1t3 o.o3822(7',t 182(6t 181(6) 181(6) 184(11) o o 9r(3) o(1) 1t3 2t3 0,0063(21 267(19't 3421221 342t22t 118(33) 0 0 171111',t otzt 0.6144(6) O.1690(5) 0.0535(1I 296(181 373(241 189(191 3221211 28(18) 23(20't 138(20) oH(1) 0,5705(4) O.1373(4i 0.1374(1) 100(13) 114(171 72(16t 112(151 -8(141 9(14) 46(14't oH(2) 0.1979(4) 0.O984(4i 0.1346(1) ro9(13) 72(161 101(16) 146(16) 14(14t 21(16) 37(13' oH(3) 0.2336(4) O.4698(4i o.1361 (1) 113(14) 1261171 92117t. 125(16' -2(15) 2(15) 671141 ow(1) 0.1180(5) O.4236t6t o.o556(1) 321(19) 2801221 381(25) 302(211 77120t 34(191 166(21) ow(z) 0.2036(5) 0.1794(5) 0.0601(11 315(181 245l23l 297(221 384(23) -13(21) 18(20) 122118) H(1) 0.684(6) 0.166(7) o.1082(3) r200 Hlz) 0.193(7) 0.109(6) o.1062(2:l .200 H(3) 0.197(6) 0.475(6) 0.1087(61 r200 H{4) 0.172161 O.sO7(6) 0.035(11 t700 H(5) 0.00[(2] 0.394(8] o.064(2' r700 H(6) 0.172(6) O.262(5't 0.055(2) .700 o.242Gl 0.045(2) .700
I Uvalussx 10{; r fixed in rofinemem TIIE CRYSTAL STRUCTTJREOF SHIGAITE 93
(4, ') TABLE3. SELECTEDINTEFATOMIC DISTANCES (A) & ANGLEITI N SHIGAITE TABLE4. DETAILSOF H.BONDING IN SHTGAITE x3 Mn-OH(1)d 2,171141 Al(1)-OH(1)d,l,s 1.915{3) oH(1)-H(1) 0.98(1) H(r)-o(2) 1.83(1) Al(1)-OH(2),e,h x3 Mn-OH{llo 2,17714t l=912G!L oH(2)-H(2) 0.98(t) H(2)€W(2) 1.9SQl Mn-OH(2)o 2,211131
TABLE5. BOND-VALEI{CETABLI FOFSHIGAITE (vz}' Al(1) Al(1 Na H(11 H2) H(3) H{4) H(5) H(5) H(7)
oHtl) 0.357 0.49fr 2,O14 0.351 oHl2) 0.321 o.494dr 2.001 0.326 oH(3) 0.339 o.494dr 0.84 2,@4 0.331 ow(l) 0.16 0.s2 0.85 0.17 2.000 ow(21 o. I 6561 0.14 0.83 0.86 1.995 't.608 o(11 0.1 83. 2.O44 ot2t 1.504610.18 0.14 1.974 '1.o0 2.026 2.952 2.964 0.990 6.020 1.OO 1.@ 1.00 1.00 1.oo 1.oo
I Bofffwl@@ pqrsmoters lrom &om & Altomatt (1985) 94 THE CANADIAN MINERALOGIST occupying the 6c and 3D position, respectively.They The arrangementof interlayer speciesis illustrated are both coordinatedby six OH anions in octahedral in Figure 2. The Na is coordinatedby six (H2O)groups anange-ments,and the
/*a-/
FIc. 1. The structuralunit irl shigaiteprojected down [001]; Mn2+Quoctahedra are shown by random-dotshoding, and Al0o octahedraare shownby dashihading. TIIE CRYSTAL STRUCTLIREOF SHIGAITE 95 +a4
I a
FIc. 2. The interstitial "interlayef' speciesin shigaite projecteddown [001]; Na(H2O)6 octahedraare cross-shade{SO4 tetahedra are random-dot-shaded,oxygen atomsof hydrogen-bondedHrO groupsare shownas black circles,H atomsare shownby small unshadedcircles. G-H bondsare shownbv thick black lines, and H bondsate shown bv broken lines.
I 1/3c a Atfi)
@ At(2) I rn ) & No \- v(11
6€ vl2t
FIa. 3. The crystal structure of shigaite projecteddown a direction -5o from [110]; the legendis as indicatedin the figure, with the addition that Mn atonxi are indicated by the let0ersMn and the symbols> and < that indicatethat the Mn atomslie above and below the plane of the diagram. 96 THE CANADIAN MINERALOGIST valenceclose to the averagevalue (0.15-0.20vu) for mounting medium was analyzed and found to be oxysalts(Hawthorne 1992). The OH(2)goup H-bonds devoid of Na. Both the South African and Japanese to OW(2), the (HrO) group that coordinatesthe samples of shigaite gave 1.5 wt%o Na; tle interstitial Na atom. The OH(3) group H-bonds to corresponding value calculated from the formula OW(l), the (H2O) group that is held in the structure derived from the crystal structure is 2.0 wtTo Na" by H-bonds only. Note that all O-H...O angles are a reasonable agreement considering tlle behavior close to the averagevalue of -165' observedin of the sample under the elecfron beam. We also inorganic solids. analyzedthe material remaining after thennoanalysis The sheet-like character of the sfiuctural unit is by Peacor et al. (1985) and did not detect any Na. in accord with the tabular habit exhibiled by shigaite Presumablythe unidentified volatile identified toward crystals, and the role of H-bonding in linking the the end of their thermoanalysisrun was sometype of structuralunits togetheraccounts for the perfect {001} NaQocomplex. cleavage. Relatedstructures Cumlrcal CoMPosmoN shigaite, NaA13M4+(soJz(oH)rr.12HzO,is the The chemical formula of shigaite was written as Mn2* analogue of motukoreaite,Nao.u(A1:.+Mgs.e) A14M&(O}D22(SO)2.8H2Oby Peacor et al. (1985). (SOt'.3(CO3)0.7(OH)rs.i2H2O,an important phase Ho'flever, the refined structureClable 2) indicatesthe in altered submarine basalts (Rodgers et al. 1977, formula to be significantly different. The fonnula of Zananefro et al. 1989\. The structureof motukoreaite the structural unit is [AlMnt+(OH)6] rather than was describedby Rius & Plana (1986), but the AlrMnr(OH),. Furthermore, Na is an essential details of the interlayer region were not clear owing constituentofthe interstitial species.How are we sure to extensive disorder associatedwith variable SOa that the Na site in the unit cell is actually occupiedby and CO, contents.However, shigaite forms crystals Na? First, the observedscattering power at this site is of much better quality than motukoreaite,and lacks compatiblewith completeoccupancy by Na. Second, (SOr, (COt disorder in the interlayer region; the incident bond-valenceat the Na site is closeto the hence tle hydrogen-bondingdetails of the anionic value of 1.0 ya when calculatedusing the universal sheet are clear in the shigaite structure. All of the bond-valence curve of Brown (1981) for the minerals in this large family of structures consist isoelectronic series Na+, Mg2*, 41:*, Si4+,P5+, 56*. of positively charged[M"(OII)zJ sheets(in which M Similarly, incident bond-valencesums (Table 5) representsdivalent and trivalent cations), between indicatethe type of eachanion presentin the structure. which are intercalated both simple and complex The resulting formula is [AlMn]+(OH)613(SOr2 anions in order to balance the overall charge of Na(H2O)6{H2O}o;in fhis formul4 the structuralunit the structure. The tM,(OtD2ol sheet resemblesthat is enclosedin squarebrackets. in brucite, Mg(OH)2, and the positive charge is Peacor et al. (L985) did not detect Na in their produced by substitution of trivalent cations (e.g., microprobe analysis of shigaite. Howevel they also Al, Fe3*, Cr3*) for some of the divalent analyzed shigaite with a thermoanalyzer attached cations (Mg, Mn2+) while maintaining complete to a quadrupole mass spectrometerto identiff the occupancy of the octahedrally coordinated sites speciesevolved during heating. They noted the loss within the sheet. Shigaite thus belongs to the of an nnidentified volatile toward the end of the large family of layered double-hydroxide thermoanalyzer nm; possibly this was due to strucfures that includes the hydrotalcite and the evolution of an Na-(II2O) complex. However, manasseitegroups @rondel 1941, Ingram & Taylor we cannotexplain the absenceof Na in their electron- 1967), together with numerous "stuffed" supercell microprobe analysis, as the ideal Na2O content of derivatives. our formula, derived from the stucture solution. is 2.7 wt%o.In order to resolve this point, we have ACKNoWI-EDGHVEI.ITS analyzed, by electron microprobe, shigaite from South Africa and both untreatedand heated shigaite We are extremelygrateful to Mr. William M. Pinch from Shiga prefecture, Japan. Samples consisted of Rochester, New York, for generously allowing of cleavage fragments glued to a plexiglass disk. us accessto the material used in this work, and to Operating conditions were 15 kV and 20 nA, the scelerousDr. F.J. Wicks for supplying us with and we useda defocused30-trtm. beam. The untreated shigaite from Japan.We thank Dave Bish and Don shigaite instantaneouslybegan to blister under the Peacor for their commentson the manuscript. This beam and continued to bubble for several seconds work was supported by the Natural Sciences and after the beam was blanked. Despite these adverse EngineeringResearch Council of Canadain the form conditions, a linear plot of accumulatedNa counts of Operating, Infrastructure and Major Equipment against time was attained over 4 seconds.The grantsto FCH. THE CRYSTAL STRUCTUREOF STIIGAITE 97
RmmmqcEs PEAcoR"D.R", Duw P.J.,KAro, A. & WtcKs,F.J. (1985): Shigaite, a new lunganese aluminum sulfate minenl NeuesJahrb, Mineral., Bnot'r, I.D. (1981): The bond-valencemethod: an empirical from the Ioi mine, Shiga Japan. approachio chemicalstructure and bonding. /n Structure Monatsh..453457. and Bonding in CrystalsII (M. O'Keeffe & A. Nawotsky, (1986): ecls.).Academic hess, New York, N.Y. (l-30). Rrus, J. & PmNe, F. Contribution to tle super- structure resolution of the double layer mineral & Alrmuarr, D. (1985):Bond-valence parameters motukoreaite.Neues Jahrb. Mineral., MoMtslL, 2$-n2. obtained from a systematic analysis of the inorganic C.S. crystal structure database. Acta Crystallogr. D.Al, Roocms, k.A., CrflsHor4, J.E., DAvIEs, R.J. & NHsoN, 14+247. (1977):Motukoreaite, a new hydratedcarbonate sulphate, and hydroxide of Mg and Al from AuctrIand, New FRoNDEL,C. (1941): Constitution and polymoryhism of ZeaIand.Mineral. Mag. 41, 389-390. the pyroaurite and sjdgrenite groups. Am- Mineral.26, (1982): 295-31,5. Tnmuan, A.H. & PBecon,D.R. The crystal structure of lawsonbauerite,(Mn,Ir4g)rz.na(SO)2(Off 22'8H2O, and HAwrtoRNE,F.C. (1985): Towardsa structuralclassification its relation to moorcitg.AftL Mineral. 67, 1029-1034. of minerals: the uMryT2oo minemls. Am. Miwral. 70, 455473. ZAMARRtro,I., Pum, F., Vaseusz, A. & CLAGUED.A. (1989): Motukoreaite: a common alteration product in MineraJ.74" 1054-1058. (1992): The role of OH and H:O in oxide and submarinebasalts. Am. oxysalt minerals.Z Kristallogr. n[ 183-206,
INcRAM,L. & Tanon, H.F.W. (1967): The crystal structures Received May 29, 1995, revised.manuscript accepted of sjiigreniteand pyroavite. Mineral. Mag. 36,465479. October31, 1995.