American M ineralogist., Volume 64, pages 376-382 , I979

The crystalstructure of kiittigite

RonBRrcrJ. Hrrr' Department of GeoIogicalSciences Virginia PolytechnicInstitute and Slate Uniuersity B lac ks burg, Vi rgi nia 2406 I

Abstract

Kdttigite,(Zn2ooCoorrNio,n)(AsOn)r.8HrO, from the typelocality (Schneeberg, East Ger- many)crystallizesinspacegroupC2/mwitha:10.241(3),b:13.405(3),c:4.151(2)A,$: 105.21(2)",and Z :2. Isotypywith vivianiteis confirmed:the hydrogenatoms have been locatedand the crystalstructure refined using 812 Zr-filLered MoKa datato an R valueof 0.054(R. : 0.057).The transitionmetals are randomlydistributed over insular single and double(edge-sharing) octahedral groups of O atomsand HzOmolecules connected by AsO' tetrahedrato form complexslabs parallel to (010).These sheets are held together by hydrogen bondingalone, thereby accounting for theperfect {010} of themineral. Shortening of the sharededge relative to the unsharededges within the octahedraldimer (0. l39A) is well within the ratherwide rangeof valuesobserved in otherZn compoundscontaining shared octahedraledges.

Introduction part of a continuing study of Zn stereochemistryand, in particular, of the characteristicsof Zn octahedron The k6ttigite, ideally Znr(AsOn)r.SHrO, shared-edgeshortening. Material from the type local- has long been recognizedas a member of the vivian- ity in Schneeberg,East Germany, was kindly made ite group of with the general formula available for study by B. D. Sturman of the Royal M,(TO4)2-8H,O. The monoclinic members of this Ontario Museum(ROM specimennumber M15537). family include the phosphatesvivianite (M : Fe), bobierrite (Mg), and bari6ite (Mg,Fe), and the arse- Experimental nateskcittigite, parasymplesite (Fe), hoernesite(Mg), The crystal selectedfor data collection was a pale annabergite (Ni), and erythrite (Co). In addition, red transparent platelet displaying well-developed both vivianite and parasymplesitehave been ob- faces,and having the dimensions0.098 X 0.093 servedin a closely-relatedtriclinic modification asthe {010} X 0.037mm. Detailed morphological,optical, chem- minerafs metavivianite and symplesite (Ritz et al., r974). ical and X-ray data for the material have been re- ported by (1976);the The topology of the monoclinic structuretype was Sturman chemicalcomposition determined in that study has been included in shown by Mori and Ito (1950)'zto consistof octahe- the abstract. The crystal was oriented with the axis dral edge-sharingdimers, MrO"(HrO)n, and insular c* slightly displacedfrom the axis of a Picker Fecs I octahedra, MO2(H.O)4, linked by ZOn tetrahedra @ four-circle diffractometer.Unit-cell parameterswere into complex sheetsparallel to (010). However, the refined by least-squaresmethods to give the best fit atomic coordinateswere not refined, and the details betweencalculated and observedangles 20, y, and of this interesting structure remain in doubt. The Q, measured at 25t2"C with MoKal radiation (tr : structure of the triclinic dimorph is still unknown. 0.70926A), for 30 automatically centered reflections The structural analysisof krittigite was initiated as in the range 20 : 30-45". The resultant cell dimen- : : : I Presentaddress: CSIRO Division of Mineral Chemistrv.P.O. sionsc 10.241(3)3,b 13.405(3),c 4.757(2)A,0 Box 124, Port Melbourne, Victoria 3207, Australia. : 105.21(2)' [unit-cell volume : 630.2(2)As]agree 'zNote that monoclinic FedAsO.),.8HrO is (incorrectly) re- well with valuesreported by Sturman (1976). ferred to as symplesite rather than parasymplesiteby Mori and Ito ' fl950). E.s.d.'s, given in parentheses,refer to the last decimal place.

0003-004x / 79 / 0304-0376$02.00 376 HILL: OF KOTTIGITE 377

Table 2. Fractionalatomic coordinates and temperaturefactor coelficientsfor kdttigite

Aton 811or B Bzz Bgs Btz 8rg Bzt

zn(t) 0.0 0.0 0.0 r-8(2) r_4(1) 102(8) 0 -2(3) 0 zn(2) 0.0 0.38521(9) 0.0 2L(L) 13(1) 1ls (6) 0 5(2) 0 As 0.31s41(9) 0,0 o.37 44 (2) 11(1) 11(r) 67(s) 0 -3(r) 0

o(1) o. 1498(6) 0.0 0.37 54 (L4) 15(6) 2L(4) 24(26) 0 -1(10) 0 o(2) 0.4049(7) 0.0 0.7226(L6) 32(7) 13(4) ro2(32) 0 -2(L2) o o(3) 0. 3432( 5) 0.1066(4) 0. 2135(t_2) 25(4) 11(3) r33(2L) 2(3) 10(7) s (6) o(4) 0.0975(5) 0.1144(4) 0.8081(12) 23(4) 22(3) 146(23) 0(3) 11(8) -2(7) o(s) 0,4000(5) 0,2262(s) 0. 717e(13 ) 42(5) 22(3) L4s(26) 3(4) 6(10) 7(8)

H(41) 0. 384(10) 0. 42s(8) 0.364 (22) 2.1 (19) H(43) 0.340 0.37L 0.119 4.0 H(53) 0. 121(10) 0.30s(8) 0.436(24) L.7 (2o) n(s4) 0.044(8) o.236(6) 0. 289(19 ) 0.3(ls) * Thezrnal pazdneteys x LO+ for Zt, As mtd o. Pay,enthesized fifu?es here and in aLL subseqtent tables repreaent the e.s.d. in teyns of the Least significant figure to the Left. The H dtons ote identified bg tuo di'gits indicating the danor and. aceeptor o atons respectiueltJ.

X-ray intensitydata for the structureanalysls were signed to Zn alone. Least-squares minimization of - collectedat25"C usingZr-filtered MoKa radiation (tr the function Zw(lF"l lnl;'zwhere F' and F. are : 0.71069A) and a 20 scan rate of lo per minute. the observedand calculatedstructure factors, and w Backgrounds were determined from l0-second sta- : l/o2, resultedin convergenceat a conventionalR tionary counts at both ends of each dispersion-cor- value of 0.069.At this point it was observedthat the rected(Alexander and Smith, 1964)scan range (mini- isotropic temperaturefactors of the isolated and di- mum width = 3.0o in 20). All reflectionsconsistent meric octahedralatoms were 0.98(4) and 1.05(3)re- with a C-face-centered lattice were collected to a spectively,indicating that there may be little if any maximum 20 value of 60o, using three reflections to difference in the chemistry of the two sites. Refine- monitor instrument and crystal stability at frequent ment with anisotropictemperature factorsa, incorpo- intervals: these showed no significant change. The rating the symmetry restrictionsof Levy (1956),fur- resultant 4045 reflections were corrected for back- ther reducedR to 0.059.The site multiplicitiesof the ground, Lorentz, polarization and absorption effects octahedralatoms were then released(R : 0.057)in a using a p value of 107.7cm-l (the transmissionfac- crude attempt to account for the presenceof Co and tors ranged from 0.402to 0.680).Multiply-measured Ni. The resultant occupancies of 0.951(9) and and symmetry-equivalent reflections (consistent with 0.947(8) are consistentwith the observedchemistry point group 2/m)werc averaged(using weights based and, furthermore, suggestagain that the three transi- on counting statistics)to yield a set of 962 unique tion metalsare distributedrandomly over the octahe- structure factors, each with a standard deviation esti- dral sites. The inclusion of an isotropic extinction mated from the equation o : [o1 + (0.02/')']o'u/2P", parameter as defined by Coppens and Hamilton where 1 is the corrected raw intensity and o 1is derived (1970)produced a value statisticallyidentical to zero, from counting and averagingstatistics. Of thesedata and the parameter was therefore removed from fur- only those 812 observationswith I ) 2o1 were in- ther consideration. cluded in the subsequentleast-squares refinement. A Fourier differencesynthesis utilizing only those data with sind/tr < 0.4A-' was computed,and peaks Location of the hydrogen atoms and refinement ranging from 0.45 to 0.62eA-3 were found for the Refinement of the heavy-atom positions and iso- four hydrogen atoms in the asymmetric unit. There tropic temperature factors was initiated in space were also a small number of other peaks with maxi- group C2/m, using the atomic coordinates deter- mum density l.06eA-s (less than 10 percent of the mined for parasymplesiteby Mori and Ito (1950). magnitude of an O atom peak), but these were all Although significant amounts of Co and Ni sub- located well within the coordination spheresof Zn stitute for Zn in type kdttigite from Schneeberg(Stur- and As and were interpreted as regions of charge man, 1976), the rather small differences in X-ray scattering abilities of the three atoms were ignored in n The form of the anisotropic thermal ellipsoid the initial refinements and octahedral sites were as- expl-(0,,hz + P22k2+ Bssl" + 20,2hk + z7,shl + 2l3,xkl)1. 378 HILL: CRYSTAL STRUCTURE OF KOTTIGITE

Table 3. Magnitudes and orientations of the principal axes of atoms) were obtained from Internationql Tablesfor thermal ellipsoids in kdttigite X-ray Crystallography (1974) and were corrected for R.n.s. dlsDlace- Angle in degrees EO both real and imaginary anomalous dispersioncom- Atm Axis nent (i) +a t'b ft ponents. For H, the spherical scattering factor sug- 0.085(4) 44(5) e0 61(5) gestedby Stewart et al. (1965) was used. Programs 90 zn(l) 2 0.111(4) 90 180 utilized for solution, refinement,and geometrycalcu- J 0. r.19(5) 46(5) 90 151(5) lations were local modifications of Da.re.lrB,DATA- I 0.098(3) 39(6) 90 67(6) zt(2) 2 0.109(3) 90 180 90 soRT,FouRrER, ORXFLS3, ORrrr3 and ORrrp2?. J 0.118(4) 51(6) 90 ls7 (6) 1 0.067(3) 44(4) eo 62(4) Discussionof the structure As2 0. 098(3) 134(4) 90 28<4) 90 J 0.100(3) 900 Kdttigite crystallizes with the topology displayed 1 0.049(26) 81(23) 90 24(23) in Figure I and the bonding dimensionssummarized o(1) Z 0.091(r.9) 171(23) 90 66(23) 0. 139(13) 900 90 in Table 4. The resultsconfirm the proposedisostruc- I 0.098(ls) 69(ls) 90 36(1s) tural relationshipbetween kdttigite and the members o(2) 2 0.108(16) 90 180 90 0.140(L6) 21(ls) 90 126(1s ) of the vivianitegroup of minerals.Indeed, the atomic 1 0,095(12) 106(26) 25(24) 104(18) coordinatesof the nonhydrogenatoms in Table 2 are o(3) 2 0.1r.2(10) 148(36) LL3<27) 98(38) in very close agreement with those determined for 0.L24(r2) 63(35) 100(24) L64(23) parasymplesiteby Mori and Ito (1950)using trial and 1 0.106(10) 20(22) 91(13) 8s(22) o(4) 2 o.L27(t2) 70(22) 99(31) r.70(30) error methods:mean valuesof Ax, Ay, and Az for the (10) 87(17) e(32) 99(31) J 0.142 two structuresare only 0.08, 0.08, and 0.13,A.respec- I 0.116(11) 108(13) 58(16) 139(r.s) tively, reflectingthe closelysimilar radii of Znz+ and o(5) 2 0,145(11) 85(s2) 1s5(28) 115(31)

J 0.154(12) 19(19) 78(50) 120(28) high-spin Fe2+(Shannon, 1976). E(41) 0.16(16) In summary, the structureconsists of insular single H(43) o.23 and double (edge-sharing) groups E(s3) 0.ls (16) octahedral of O E(s4) o. 05(14) atoms and HrO moleculesaround Zn (plus minor Co and Ni), connected by isolated AsOn tetrahedra to form complex sheets,with mirror symmetry,parallel deformation due to chemical bonding. The H atom to (010). Each sheet is corrugated parallel to the c positional and isotropic thermal parameters were axis, with the ridges (formed by the double octa- then included in the refinement,using all 812 reflec- hedra) neatly fitting into the troughs (formed by the tions, but the proton associatedwith the smallestof single octahedra) in the neighboring sheets. The the four residual electron density peaks began to sheets are held together by hydrogen bonding be- wander away from its donor O atom. This H atom tween adjacent single and double octahedra. was therefore reassignedits original coordinates and The transition metals are randomly distributed was held fixed for the remainder of the calculations. over both octahedral sites.Zn(l) is coordinated by The refinement then proceeded smoothly to con- four HrO molecules and two trans O atoms from the vergence(parameter shifts in the final cycle were less AsOn units. Zn(2) is surrounded by two HrO mole- than one-tenthof the correspondinge.s.d.). The final cules in cls configuration, two O atoms from AsOn valuesof R and R,5 were0.054 and 0.057respectively groups, and two O atoms from a mirror-equivalent (0.066 and 0.058 for the entire data set of 962 struc- octahedron on the opposite side of the sheet.Bond ture factors). Values for F, and ^F"(X l0) are listed in length and angle variations within and between the Table 16.Atomic coordinates and thermal parame- octahedra are relatively small (Table 4), with mean ters along with their standard deviations estimated lengths close to the value 2.10A expectedfor Zn2+ from the invertedfull matrix are given in Table 2.The (and high-spin Co'+) in octahedral coordination r.m.s. componentsof thermal displacement,and ther- (Shannon, 1976).However, as usuallyobserved when mal ellipsoid orientations,appear in Table 3. an edge (or face) is shared between coordination Scattering factors for Zn, As, and O (neutral polyhedra, the length of the shared edge, O(2) ...O(2), and the angle it subtendsat the central ' : - R. [>w(lF,l lr"l)'/2w41,t" cation, Zn(2), are both significantlysmaller than any € To obtain a copy of Table l, order Document AM-78-088 from the Mineralogical Society of America, BusinessOffice, 1909 ? K St., NW, Washington,DC 20006.Please remit S1.00in advance All programs are included in the World List of Crystallographic for a copy of the microfiche. Computer Programs (3rd ed. and supplements). HILL: CRYSTAL STRUCTURE OF KOTTIGITE

H54.

lt.'l

Fig. L Unit-cell diagram ofthe kcittigite crystal structure. Thermal ellipsoids for all atoms (B for hydrogen set : 0.5A') represent50 percent probability surfaces.The arsenategroups have been indicated by filled bonds, while the hydrogen bonds are shown as dashed lines. of the other edgesor valence angles within the poly- total of 17 shared edgesobserved in ll compounds hedron. Distortions of this kind are generally dis- containing octahedrally-coordinatedZn atoms. As cussed in terms of the competition between cation- found in a similar analysisof Al octahedraby Burn- cation and anion-anion interactions along and across ham (1963),almost all of the data points lie between the sharedpolyhedral elements(Pauling, 1960;Baur, lines defining the variation of Ou.' . 'Oo, and r972a). Zn...Zn distancein ideal edge-sharingoctahedra For an ideal shared edge model in which the Zn having the extremesof observed Zn-O bond lengths. and Obr atoms are coplanar, all Zn-O6, bond lengths In Figure 2(b) the relativelysmall variation in Zn-O6. are equal,and the vectorsOo.'..Our and Zn...Zn distances has been removed from consideration by are perpendicular bisectors of each other, the rela- replotting the same data with the bridging bonds tionship betweenshared edge length and cation-cat- normalized to a mean length of 2.llA. For all but ion distanceis given by one sharededge (with an e.s.d.of =0. lA: Iitaka et al., 1962) the figure clearly demonstrates an inverse rela- d(Oo".. . Oo.) : 2[d(Zn-Oo")' - . .'Zn)/2|"]qu {d(Zn tionship betweenthe two parameterswhich is consis- In Figure 2(a) this relationship has been plotted for a tent with the other assumptions inherent in the ideal 380 HILL: CRYSTAL STRUCTARE OF KUTTIGITE

Table 4. Interatomicdistances (A) and angles(deg.) in kdttigire*

AsO4 tetrahedron: Zt(L) O octahedron ! zn (2) o octahedron : Aniona : r(H rO)O O(H|O), Ae-0(2) L.669(7> Zn-o(L) x2 2.026(6) zn-O(5)vi x2 2. 088(6) LAs-o(1) -zn(1) LzL.6(4) o(3) x2 1,680(5) O(4)i1 x4 2.L59(6) o(2)vi x2 2.094(4) o(1) L.697(7) average E 2,1I5(s) o(3)vli x2 2,rr7 (6' AB-o(2)-zn(2)vL x2 L32.0(2) average - 1.682(5) average = 2. 100(5) z^(2)x-o(2)-zt(2)vL 94.6(3) x4 o(2)...o(3) x2 2.73g(8) o(1)"'0(4)11 3'027(8) o(5)vt"'o(s)vlil 2'918(Ll) x4 x2 A6-0(3)-zn(2)v11 L20.2(3, o(1) .. o(4)i11 2'893(7) o(2)vr 3'032(7) 2,7oo(9) x2 o(3)...oitil i.eiiii6l o(4)11...o(4)lv 3.084(10) o(3)v11x2 3.031(e) x2 x2 o(1) x2 2.7L2(8) o(4)v 3'068(11) o(3)viii 3'024(8) average- average- 2,743(7) 2.999(7) O(2)ix...o(2)vi 2.840(13) o(3)vll x2 2'895(8) xz 2'944(9) ,o(2)-As-o(3) x2 Log,1(z) 4o(1)-zo-o(4)li x4 92.6(2) 0(3)v111 averase= 2'e68(7) o(1) 106.6a4i oia!flr '+ 8;-.iiri o(3)-As-o(3)1 116.5(4) o(4)il-zn-o(4)iv x2 90.s(3) zo(s)vl-zn-o(s)vill 88,7(3) o(1) x2 106.9(2) o(4)v x2 89.5(3) o(2)vL x2 93.0(2) average= L09,4(2) average- 90.0 O(3)vi1 x2 92.3(2) o(3)viil x2 92'0(2) , o(2)lx-zn-o(2)vl 85.4(3) o(3)v11xz 86'9(3) o(3)vill x2 88'7(3) average- 90.0(2) Water molecules: o, t{ a oo od-E H...Oo Od...Oo LO'-H...Oa Lza-Od-E H...H LH-Od-H

-H(41)...O(l) v\{/^(t\ 1,.03(11) 1.70(11) 2.731(8) L74(g) lOO(6) -H(43)...o(3) L.34 LO2 0.67 2.L4 2.74ii1 r51 118 -u(53)"'0(3) 0.82(11) 2.oo(11) 2.8I7(s) 173(10) (7 o(5) -H(54)...o(4) 100 ) 0.78(9) 2.15(8) 2.896(8) L62(9) 104(6) 1.30(13) 109(r0) * Sytmetrg twfonwtiore for atome outside the aepmet?ic wlit: 1. -y, -x, -y, x, z l,J.L. L-z v. -x, y, I-z vLt. L/2-x, Ll2-y, -z fx. x-L12, Il2+r, z-L 'II. y, -y, x, z-L Lv. x, z-L vI. Ll2-x, Ll2-y, L-z vtIL. x-L/2, Ll2-y, z x. L/2+x, y-112, L+z

shared-edgemodel. However, the wide rangeof cat- For example,while the mean bond lengthfor a very ion-cationand anion-aniondistances suggests that, similarZn octahedraldimer in ZnSeOs.2HrO(Glad- as for the Al octahedra,the detailed geometry is kova and Kondrashev,1964) is only 0.013.Alarger extremely flexible and subject to the influenceof thanin krittigite,the Op,. .'Oo. andZn,..Znsepara- structuralfeatures external to the sharededge itself. tions are0. I 5,A'shorter and 0. I 5A'longer respectively. Indeed,the rangeof Zn.. .Zn distancesacross the 17 sharedoctahedral edges in Figure2 (2.99to 3.45A)is as wide asthe correspondingrange of cation separa- 3 3 tions (3.02 to 3.58,{) observedbetween 42 corner- a2 sharing ZnOn tetrahedra(Hill, in preparation),al-

a t. ,\ a\ though the meanseparation is, as expected,0.17A largerin the lattercase. While the numberand nature .lr of the othercations bonded to th*ebridging O atoms ri \ are undoubtedlyof importancein determiningthe

I l? 33 a4 geometryof the sharededge, recent CNno/2 molecu- Zn Zn (A) Zn Zn lAl lar orbitalcalculations on clustersof edge-sharingAl Fig. 2. Relationshipbetween shared-edge length Or....Or., octahedrahave shown that the minimum energycon- and Zn...Zn distancefor the 17 octahedronshared edges in figurationis alsoa sensitivefunction of the number ktittigite (present study), (Hilt, 1976), tsumcorite (Peterson (Tillmannsand Cebert, 1973),legrandite (Mclean et al., l97l), and arrangementof the sharededges et al., hydrozincite(Ghose, t964), Zn"(OH)s(NOs),.2H,O(Stiihlin and 1978).Since the numberof sharededges per octahe- Oswafd,1970), Zn,(OH)ECI,.tH,O (Allmann, 1968),Zn(OH),. dron for the structuresused in Figure2 variesfrom I ZnSO. (Iitaka et al., 1962), ZnSeOr.2HrO (Gladkova and to 4 it is not surprisingthat a widerange of Oo"'. .On" Kondrashev,1964), ZnSerO, (Meunier and Bertaud,1974'1, and B- and Zn. . .Zn distancesis observed. ZnrPrO, (Calvo, 1965):(a) observedinteratomic distances, (b) distancesnormalized in accordancewith a meanbond length of In Table5 the shared-edgedistortions for the oc- 2.1lA. Curvedlines represent the variationof the two parameters tahedral dimer in kdttigite are comparedin detail for the idealshared edge model and Zn-O distancesas indicated. with the rangesof values,and their means,found for HILL: CRYSTAL STRUCTURE OF KOTTIGITE 381

Table 5. Distortion of shared edge length O0"...Ou., Zn...Zn bond from Zn(l) to O(l), relativeto other bonds distance, mean bridge bond length Zn-O6,, and mean valence within the polyhedron,is also consistentwith this angle Obrznobr resulting from the sharing of edges in ZnO, proposal. octahedral clusters

A(%r..obr)* L(za..zn)ll A(zn-Obr)* A(oo.Znoor)/l Hydrogenbonding

kdttiglre -0.139A +0.107A -0.007A -4.6" The distanceand angleparameters describing the two water moleculesare given in Table4. Although -0.30 +0.r7 -0.07 -8.0 ( r""" * -0,52 the two O-H bondsin the O(4) water moleculeshow other' { nax. +0.45 -0.2L -L6,2 -0.09 [ 'in. -0.o2 +0,07 -o.2 a wide variation in length,the averagevalue for all * (0.82,{) DLetortim lelqtiue to eorcesponding parmeters dnoolting non- four bonds is, as expected,about 0.14.{ bridgi.n4 ani,ons. shorterthan the meanvalue measured in other crys- # Distortdon reLati.ue to the uaLue eopected for regulw oetahedra. talline hydratesby neutrondiffraction (Baur, 1972b; + Data the ten cmpounds (eaeludi.ng kbttigi.te) fw in Figure 2. Ferrarisand Franchini-Angela,1972). Similar shifts of the apparenthydrogen atom position toward the the other compoundsin Figure2.The dataindicate atom to which it is bondedhave been documented in that although the distortionsin kdttigite are some- a large number of structuresand are generallyas- what smallerthan average,they are well within the cribed to the relatively large distortion of electron rangesof observedvalues. The sharededges are, on densitywhich takesplace during formation of the average,7.5 percentshorter, and the Zn.. .Zn sepa- O-H bond (Coppens,1974). Both water molecules rations 5.6 percentlonger, than the corresponding display H-Od-H bond angles(where Oa represents distancesin regular octahedrawith the samemean the donor O atom) closeto the meanvalues of 108 bond length. These distortions may be compared and l09o observedin other hydratesby Ferrarisand with valuesof 5.5and 4.9 percentrespectively for the Franchini-Angela(1972) and Baur (1972b),respec- 14 Al octahedronedges considered by Burnham tively. (1963).The consistentlylarger distortions found for In contrastto the hydrogenbonding schemes pro- the O5"...Oo.distances relative to Zn...Zn arein posedfor the vivianitestructure type by Mori and Ito agreementwith the idealshared-edge model, as is the (1950)and Moore(1971), only oneof thefour hydro- fact (first suggestedby Pauling,1960) that the shared- genbonds in kdttigiteis directedbetween water mole- edgedistortions (for both the Zn and Al polyhedra) cules(Table 4, Fig. l). Moreover,in spiteof the fact generallytake placewith little or no changein the that the Zn-Oa-H anglesare relativelysmall (100- relativelengths of the bondsto the bridgingO atoms I l8o), noneof the hydrogenbond acceptors, Oo, are (Table5). As expected,the valenceangles subtended anionsin thesame coordination polyhedron as Oa. In at the Zn atoms by the sharededges show a wide eachcase the two Oo atomsare not only the anions rangeof values,but the mean distortion(-8') is closestto 06, but theywould be slightly underbonded closeto the correspondingaverage value (-7") ob- were it not for the additional electrostaticbond servedfor the Al octahedrastudied by Burnham. strength(Brown and Wu, 1976)provided by the hy- Bondlengths within theAsOo group are allclose to drogenbonds. In this regardit shouldbe noted that, the meanvalue of 1.6824(Table 4), but theO-As-O althoughnot obviousfrom Table4, the O(l) atomis valenceangles display a large variation. The widest the receptorof two symmetryequivalent hydrogen angle(l 16.5')is associatedwith the tetrahedronedge bonds from H(41). For all bonds the distances connectingthe O(3)vertices of theZn(2) octahedron H. . .O" and anglesOo-H' . .Ooare within the ranges (Fig. I ), and is thereforerelated to the Zn-O(2) bond ofvaluesexpected (Baur, 1972b).From theclassifica- length and Zn. . .Zn distanceacross the sharededge tion schemeof Ferrarisand Franchini-Angela(1972), in the octahedral dimer. The narrowest angle the O(4) water moleculemay be assignedto Class2, (106.6'),on the other hand, is associatedwith the Type H, with onelone pair directedtoward a divalent tetrahedronedge O(1). . .O(2),the lengthof whichis cationand the othertoward a proton. The O(5) mole- controlled by the length of the sharededge in the cule,on the other hand,has a single(divalent) cation dimerand the Zn(l)-O(1) bondlength in theisolated directedtoward one of its lone pairs and is therefore octahedron(Fig. | ). These angular distortions are of Classl', Type"/. thereforean expressionof a compromisebetween the As discussedabove, cohesion between the sheetsof tendencyfor the AsOogroup to assumeregular tet- octahedraand tetrahedraparallel to (010)is provided rahedralgeometry, and the tendencyfor the dimerto solelyby hydrogenbonds, thereby accounting for the distort to accommodatethe sharededge. The shorter perfect{010} cleavage exhibited by the mineral.In 382 HILL: CRYSTAL STRUCTURE OF KUTTIGITE fact,only oneof thefour bonds,O(5)-H(54). . .O(4), Hill, R. J. (1976)The crystalstructure and infraredproperties of servesto connectthe sheetstogether. The remaining adamite.Am. Mineral.,61, 979-986. (1962) threehydrogen bonds are directedtoward O(l) and Iitaka, Y., H. R. Oswaldand S. Locchi Die Kristallstruktur von Zink-hydroxidsulfatl, Zn(OH)2.ZnSO r. A cta Cry stal Iog r., O(3) atomsin the samesheet, an{ provideadditional 1J,559-563. stability to the orientation of the trans-connected International Tablesfor X-ray Crystallography,Vol. IV (1974); Zn(l) octahedron. J. A. Ibersand W. C. Hamilton,Eds. Kynoch, Birmingham, England. Acknowledgments Levy, H. A. (1956)Symmetry relations among coeftcients of the This study was supported by grant DMR75-03879 from the anisotropictemperature f actor. A cta Crys tal I og r., 9, 679. Materials ResearchDivision of the National ScienceFoundation. I Mclean, W. J., J. W. Anthony,J. J. Finneyand R. B. Laughon am grateful to the ResearchDivision at VPI and SU for defraying (1971)The crystal structureof legrandite.Am. Mineral., 56, the computer costs,and to Mrs. Ramonda Haycocks for typing the n47-1t54. (1974) manuscnpt. Meunier,G. and M. Bertaud Cristallochimiedu s6l6nium (+IV). II. Structurecristalline de ZnSerOu.Acta Crystallogr., References 830.2840-2843. Moore, P. B. (1971)The Feir(HrOL(PO.),homologous series: Alexander,L. E. and G. S. Smith (1964)Single-crystal diffrac- crystal-chemicalrelationships and oxidized equivalents.,4z. tometry:'the improvementof accuracyin intensitymeasure- Mineral..56.l-17. ments.Acta Crystallogr.,1 7, 1195-1201. Mori, H. and T. Ito (1950)The structureof vivianiteand sym- Allmann, R. (1968)Verfeinerung der Strukturdes Zinkhydro- plesite.Acta Crystallogr.,3, l-6. xidchloridsII, Zn"(OH)6Clr.lHrO. Z. Kristallogr,,I 26,4l'7-426. Pauling, L. (1960) The Narure o! the ChemicalBond, 3rd ed. Baur, W. H. (1972a)Computer-simulated crystal structuresof Cornell UniversityPress, Ithaca, New York. observedand hypotheticalMgrSiO. polymorphs of low and high Peterson,R. C., R. J. Hill andG. V. Gibbs(1978) The predictions density.Am. Mineral.,57, 709-731. of distortionsin layer structuresusing molecular orbital calcu- - (1972b)Prediction of hydrogenbonds and hydrogenatom fations(abstr.). Geol. Soc. Am. Abstractstlith Progr.,10, 471. positionsin crystallinesolids. Acta Crysrallogr.,828, 1456-1465. Ritz, C., E. J. Esseneand D. R. Peacor(1974) Metavivianite, Brown, I. D. and K. K. Wu (1976)Empirical parameters for Fes(POr)r.8HrO,a new mineral.Am. Mineral.,59,896-89.9. calculatingcation-oxygen bond valencies.Acta Crystallogr., Shannon,R. D. (1976)Revised effective ionic radii andsystematic 832, 1957-1959. studiesof interatomicdistances in halidesand chalcogenides. Burnham, C. W. (1963)Refinement of the crystalstructure of Acta Crystallogr.,432, 75l-767. kyanite.Z. Kristallogr.,I 18,337-360. StAhlin,W. and H. R. Oswald(1970) The crystalstructure of zinc Calvo,C. (1965)The crystalstructure and phasetransitions ofB- hydroxide nitrate, Znu(OH)s(NOa)r'2HrO.Acta Crystallogr., ZnzPzO,.Can J. Chem.,43,ll47-1153. 826,860-863. Coppens,P. (1974) Some implicationsof combinedX-ray and Stewart,R. F., E. R. Davidsonand W. T. Simpson(1965) Coher- neutrondiffraction studies. I cta Cry s tal log r., B3 0, 255-261. ent X-ray scatteringfor the hydrogenatom in the hydrogen - and W. C. Hamilton (1970)Anisotropic extinction correc- molecule.J. Chem.Phys., 42, 3175-3187. tions in the Zachariasenapproximation. Acta Crystallogr.,A26, Sturman,B. D. (1976)New data for krittigiteand parasymplesite. 7I -83. Can.Mineral . 14.437441. Ferraris,C . and M. Franchini-Angela(1972) Survey of thegeome- Tillmanns, E. and W. Gebert (1973) The crystal structureof try and environmentof water moleculesin crystallinehydrates tsumcorite,a new mineralfrom the TsumebMine, S.W.Africa. studied by neutron diffraction.Acta Crystallogr.,828, 3572- A cta Crys tallogr., 829, 2789-2'7 94. 3583. Ghose, S. (196a) The crystal structure of hydrozincite, Zn'(OH)'(CO') A cta Crys tallogr., I 7, 105I - 1057. ". Gladkova,V. F. and Y. D. Kondrashev(1964) Crystal structure of Manuscript receiued,March 6, 1978; ZnSeOr.2HrO.Sou. Phys. Crystallogr., 9, 149-153. acceptedfor publication,June 2 I, 1978