The Crystal Structure and Infrared Properties of Adamite

American Mineralogist, Volume61, pages979-9E6, 1976

Thecrystal structure and infrared properties of adamite

RoosRrcrJ. Hrlr-' Departmentof Geologyand Mineralogy, Uniuersityof Adelaide Adelaide.South Austalia 5001.Australia

Abstract

The crystalstructure of adamite,Znr(AsO.)(OH), has beensolved by direct methodsand refinedby full-matrix least-squaresto R : 0.0402(Rw = 0.0302)using 1472 equi-inclination counter data recordedwith graphite-monochromatizedMoKa radiation.The structureis orthorhombic,Pnnm(Drrfl) with a : 8.306(4),b = 8.524(6),c - 6.043(3)A and Z = 4. Isotypywith andalusiteand the membersof the olivenitegroup of mineralsis confirmed,the Zn atomsoccurring in both six- andfive-fold coordination. ZnO.(OH), octahedrashare edges to producechains parallel to the c axis, and sharecorners with edge-sharingZnO.(OH) trigonal bipyramidsin the a- and b-axisdirections. AsO. groupsconnect the two Zn poly- hedrontypes to producea denseframework structure in whichall O atomsand OH groupsare trigonallycoordinated.

Introduction mensions0.095 and 0.097mm weremounted about The secondarymineral adamite had earlybeen sug- the a and c directions,respectively. Preliminary Weis- gestedto be a memberof the olivenitegroup of senbergphotographs established the crystalsto be mineralson the basisof symmetry,cell dimensionsorthorhombic, Pnnm or Pnn2. Spacegroup Pnnm and composition,but this relationshipwas not con- (Dif;,)was subsequentlyconfirmed by the applica' firmeduntil Kokkoros(1937) reported a two-dimen- tion of statisticaltests (Howells et al., 1950;Rama- sional crystalstructure analysis of the speciesusing chandranand Srinivasan,1959) to the X-ray diffrac- less than 100 X-ray reflections. Other workers tion data.All X-ray datawere collected at 2l"C on a (Strunz,1936; Heritsch, l9it0; Richmond,1940; Mrose Stoe automatic Weissenbergdiffractometer using et al., 1948)have concentrated on physicaland mor- MoKa radiation monochromatizedwith a graphite phologicalproperties. crystal(I : 0.7107A). fne latticeparameters were Olivenite,Cuz(AsOTXOH), libethenite, Cur(POr)- determinedfrom <^rscans of ft00,0k0, and 00/ reflec- (OH), eveite,Mnr(AsO.)(OH), and adamiteare iso- tions and the resultsrefined by the method of least structuralwith the high-temperaturemineral andalu- squares.These values, together with other physical = = site, AlrSiOu(Strunz, 1936;Heritsch, 1940; Moore constants for adamite are: a 8.306(4),2b :6.043(I and Smyth, 1968),and arecharacterized by the pres- 8.524(6),c A, v: 427.85A', formula = = : enceof M2+ cationsin both five-and six-foldcoordi- weight 286.68,Z 4, F(000) 536e,D.(water : : nation.In addition,the componentCor(AsO.XOH) immersion) 4.434(8)B.cffi-s, D, 4.45g.cm-'. hasbeen found in solid solutionwith adamite.and a The intensitieswere measured by the c,r-scantech- continuousseries exists between the syntheticcom- nique using a proceduredetailed by Snow (1974)' pounds(Keller, l97l). Data were gatheredfor a quadrant of reflections about both crystals (a axis, Okl-llkl; c axis, Experimental hk}-hkl}). Standardreflections monitored for each Two fragmentsof adamitefrom the Ojuela mine, reciprocallattice layer showedno sensiblechange' Mapimi, Durango,Mexico, with (roughlycubic) di- Lorentzand polarizationcorrections appropriate for usewith a highly mosaicmonochromator (Whitta- ker, 1953)were applied, and the datafrom both axes I Presentaddress: Department of GeologicalSciences, Virginia PolytechnicInstitute and State University,Blacksburg, Virginia 24061. 2 e.s.d.'sgiven in parentheses,refer to the last decimal place,

979 980 RODERICK J. HILL

(representing5258 intensities) were then scaledto- A of O(2), indicatingthat the peakprobably repre- getherby a non-iterativeleast-squares method (Rae, sentsan accumulationof residual(probably absorp- 1965)to yield 1472unique reflections. Of thesedata, tion) errors in the data, rather than a physically 202had intensitiesless than threetimes the standard meaningful atom site. Nevertheless,electrostatic deviationof the countingstatistics and wereconsid- bond strengthconsiderations, presented below, in- eredto be "unobserved."No absorptioncorrections dicatethe involvementof the O(2) atomin a bondto wereapplied. H, and for this reasonthe difference-map-suggested (but not refined)H atom positionhas been included Structuredetermination and refinement in the final list of atomiccoordinates. The As and (two) Zn atomswere located by appli- Scattering factors for As, Zn, and O (neutral cation of the SymbolicAddition Procedure(Karle atoms) were obtained from InternationalTables for and Karle, 1966)and refinedby least-squaresmini- X-ray Crystallography(1962) and were correctedfor - mizationof thefunction >w(lF"l lF")'whereFo and the real part of the anomalousdispersion (Cromer, F" are the observedand calculatedstructure factors 1965);for H, the valueswere those of Stewartel a/. and the weights(w) are derivedfrom countingstatis- (1965).Programs used for solution,refinement, and tics.A subsequentdifference map yieldedthe four O geometry calculationswere local modificationsof atoms.Isotropic refinement converged with a con- FAME,UMULTAN,6 FoRDAp,t Onnls (Businget al., ventionalR indexof 0.075.Anisotropic temperature 1962),Onrrn (Businget al., 1964)and.ORrnp (John- factor refinementsincorporating the symmetry re- son,1965). strictionsof Levy(1956) for the six atomsin special The final least-squaresparameters of the atoms positionsreduced R to 0.043. and their standarddeviations (estimated from the The high intensityreflections were observed to be- invertedfull matrix) are givenin Table l. Table 2 havein a mannerconsistent with the occurrenceof presentsthe anisotropicthermal ellipsoid data from extinction,and the observedstructure factor was TableI transformedto theparameters of dimensions therefore replaced by the expressionF,K-'[l + (r.m.s.vibrations in A) and orientationsof principal f(0)l?F.'zGlua,where K is the scalefactor normally ellipsoidaxes, while the geometryof the structureis applied to l4l , f(0) is (l t cos220)/Isin?j(t+ givenin Table3. The observedand calculatedstruc- cos'21))1,and G is a secondaryextinction parameter. ture factorsare compared in Table4.8 Full-matrix refinementusing only the observedre- flectionsthen convergedto a final R value of 0.0402 Discussionof the structure andRul : 0.0302(R : 0.050and Rry : 0.031for all Asidefrom minor differencesin coordinates(when reflections).The value was of G 5.9(4)X l0-ae-2. transformedaccording to the relationshipl/2-x, From a considerationof the adamiteasymmetric | /2-y, | /2-z), the adamitecrystal structure (Fig. I is unit contents,the hydrogen ) atomin the formulaunit identicalto thatof andalusite(Burnham and Buerger, must occur either on the two-fold axis, or on the 196l), and the membersof the olivenitegroup of mirror planeat z : 0 or z : Vz.The position two-fold minerals(Bragg and Claringbull,1965). The Zn(l) may be eliminatedsince the protonwould thenhave atoms occur in elongate octahedra which share to exist inside a ZnO" octahedronor on one of its "equatorial"edges to produceinfinite chains in thec- sharededges: Baur (1972, 1973) has pointed out that axis direction.These chains share corners with in- this is an unlikelysituation. Difference maps were sular AsOo tetrahedra and with pentacoordinated thereforecomputed using both the full data set and Zn(2) atoms.These trigonal bipyramidsshare an thosedata for which sindltr< 0.4,in an attemptto locatethe proton on one or other of the mirror planes.One peakwas observed on the zerolevel of 6 Part of the Symbolic Addition Package by E. B. Fleischer, A. the unit cell,in bothsyntheses, at a distanceof 017 A L. Stone and R. B. K. Dewar of the University of Chicago, used in from theO(2) atom and with this study to calculate a set of normalized E values. an intensityappropriate 6 A program for the automatic solution of crystal structures,the to that of a hydrogenatom. However, least-squares modus operandi of which is described by Germain et al. (1971). refinementof this H positionconverged to within0.5 7 A program to execute Fourier summations written by A. Zal- kin of the University of California, Berkeley, California. 8 To -76-027 3 obtain a copy of Table 4, oider Document AM ftom The form of the anisotropicthermal ellipsoid is the Mineralogical Society of America BusinessOffice, 1909 K St. expl-($,,h2 + Pz2k2+ Bssf,+ 2Bphk + 2pnhl + 2prskl)l N.W., Washington,D.C. 20006.Please remit $1.00in advancefor t : - Pp l2w(lF.l l4l )'/ZwFo,1,r, a coov of the microfiche. CRYSTAL STRUCTURE OF ADAMITE 981

Tnsls l. Atomic coordinates and temoerature factor coefficients for adamite

Atom 0 ?.t z Btr Bzz 8ss Btz Brs Bzz -14 As 2s048(6) 24394(s) rl2 181(s) 79(4) 286(4) (s) 0 0 -t 0 zn(l) 0 o 24737(10) s29(8) 260(6) 3l-1(10) s9(7) 0 zn(2) L3482(7) 36423(6) 0 267(7) 128(6) 444(L2) -L\t ) 0 0 -e 0 o(1) 0760(4) 1447(4) rl2 11r5\ 2L(4) 24(6) (6) 0

0(2) = oH 1079(5) 1268(4) 0 3s(s) 11(3) s9(7) 1(4) 00 -2 (4) o (3) 3960(5) 1063(4) Ll2 33(s) 8(4) r26(e) 00 -10 -6 (3) (3) o (4) 268s(3) 361s(3) 2778(4) 4L(4) 28(s) 3e(s) (3) 11 H 0.20 0.l3 0

Positional para4eters and anisotroplc temperature factors x 105 for As and Zn; x 104 for 0'

The error in the final flgure is lndicated in parenthesis.

edge to produce Zn O6(OH)2 dimers within the smallZn(l)-O-Zn(l) angle(93.6o) is againa func- framework.The threeO atomsare eachcoordinated tion of the sharededge between Zn atoms,while the by (two)Zn andAs, whilethe OH groupis trigonally 16.6' difference between the sum of the three coordinatedby Zn alone:all four anionsare less than Zn-O-Zn anglesand 360' may be rationalizedin 0.5A from the cationplane. terms of the presenceof a proton bondedto the In detail(Table 3) the four As-O bondsare statis- centralanion to completean approximatetetrahedral tically identical,but a significantlevel of bond angle array.In this casethe anionorbitals are probably sp' distortionhas resulted in a loweringof the symmetry hybridized. of the AsOagroup from 43m (ideal tetrahedron)to The Zn(2) atom is situated0.075 A out of the m.The sumof the anglessubtended by theAs andZn atomsat O(3) and O(4) is 360' and 358.7"respec- Te,srl 2. Magnitudesand orientationof principalaxes of tively,suggesting that the orbitalson both anionsare thermalelliosoids in adamite

sp'zhybridized. However, for O(1) the sum of the Rms disPlacenent Angle, in degrees' to anglesis 344.5o,halfway between those appropriate Aton Axls A +a +b +c to trigonal(360') and truncatedtetrahedron (328') 1 0.053(r) 8r(3) 8(3) 90 coordination.For O(l) and O(3) the Zn atomsare As 2 0.073(1) 90 90 180 situatedacross shared edges (between octahedra and 3 0.080(r) 8(3) e8(3) 90

trigonal bipyramids,respectively) which hasresulted 1 0.076(1) 90900 in considerablynarrower Zn-O-Zn angles(95.5' and zn(r) 2 0.082(2) 116(1) rs4(1) 90 0 .146( 1) 26(r) 116(1) 90 104.3o,respectively) relative to Zn-O-As (average: 3 126.2'). For O(4) there is no edgesharing and the 1 0.069(2) 9o(3) o(3) 90 zn(2) 0.091(1) 90 90 180 anglesare more regular. 0.097(r) o(3) 90(3) 90 The octahedroncontains aZn(l) atom at the cen- 0,066(9) 90 90 o ter of an approximatelysquare planar array of two o(r) 0.076(16) 123(11) L47(rr) 90 O(l) atomsand two OH groups[previously referred 0.rr4(11) 33(11) 123(11) 90 to as O(2)l at an averagedistance of 2.057A. Each 0.065(10) 92(Lo) 2(10) 90 o(2) 0.104(7) 90 90 r80 octahedronshares an OH-OH and an O(l)-O(1) 0.1r1(7) 2(10) 88(10) e0 edgewith adjacentoctahedra in the c-axisdirection, (Pauling,1960), being o.oss(12) 86(9) 4(e) 90 the sharededges, as expected 0 (3) 0.107 (8) L76

Tlst-n 3. Adamiteinteratomic distances and angles

As04 tetrahedron zn(1)00 (oH), octahedron Zn(2)00 (0H) trlgonal blpyranld

- - - As o(1) r.678(4) zn(r) o(1) 2.062(3) x2 za(Z) oH {rr 2,036(4) (3) (4) (3)-' (4) 0 r.684 oH i{ 2,05r(3) xz 0 r. e99 0(4) r.682(3) x2 0(4)-- 2.26L(3) x2 o(3)v 2.079(4) o(4) 2.013(3) x2 Average = I.682(2) Average= 2.L25(L) Average = 2,028(2) o(1) - o(3) 2.678(6) o(r) - o(r)iii 2.772(7) oH - o(3)it 2,876(5) o(4) 2.788(4) x2 oH 3.037(2) x2 ,.. o(4) 2.933(4) x2 0(3)-0(4) 2.767(4) x2 o(4)rv 3'Os7(4) x2 0(3)^"- 0(3)v 2.s04(7) - o(4) o(4)l 2.68s(s) o (4) il 3'2r2(4) xz 0(4) 3.s31(5) x2 oH - oHlli. 2.809(7) o(3)v - o(4) 3.007(4) x2 Average- 2,745(2) 0(4)11 2.823(4) x2 o(4) - o(4)va 3.358(5) 0 (4)av 3.124(5) x2 Average = 3.076(2) Average= 3.007(2) o(r) - As - 0(3) 10s.6(2) o(1) - zn(l) o(t)lii s4.5(2) oH - zn(2) - o(3)av 90.9(2) o(4) Lr?.2(r) x2 0H 9s.2(L) x2 0(4) 92.8(1) x2 o(3) 0(4) r10.6(r) x2 o(4)iY 8e.e(1) x2 o(3)av o(3)v 75.7(2) 0(4) 0(4)a 10s.9(2) o(4) ra 95.9 (1) x2 o(4) 123.3(1) x2 OH 0H111 86.4 (2) 0(3)v o(4) 94.5(1) x2 Average = I09.5(1) o(4):1 8r.5(1) x2 o(4) o(4;vr 113.0(2) o(4)av 92.7(r) x2 oH 0(3)v 155.6(2)* o (4)ii o(4)av r72.3(L)* Average equatorlal " 119.9(1) Average = 90.1(1) Averageaxlal = 90.2(1)

Symetry transfomations for atoms outslde the asymetric unit:

iii. x, !, z v. 4-x, \+y, \-z iv. x-\, \-y,1-z vi. x, !, 2

Diatances, in A, and angles, ln degrees, wlth e.s.d.ts glven in parentheses ln terns of last declnal place, *Excluded fron the average,

planeformed by O(3) and two O(4) atoms.Another atomsexcept O(2), and O(4) to a lesserextent, show O(3)atom andan OH groupoccur at slightlygreater close agreementbetween the sum of the bond distancesabove and below this plane to producea strengthsand valence.The markedcharge deficiency distortedtrigonal bipyramid about the cation.Each on the O(2) atom in adamitesuggests that the proton of thesepolyhedra share a "vertex"edge, O(3)-O(3), is in a positionwhereby it is directlybonded to O(2). with another bipyramid to produce discrete Assumingthat all of the deficiencyon O(2) is dueto ZnrO.(OH),groups. The sharededge, of length2.5O4 this bond,the Brownand Shannoncurves predict an A (subtendingan angleof 75.7oattheZn(2) atom), is O(2)-H distanceof 0.94 A. ttris is in reasonable considerablyshorter than the unsharededges (of av- agreementwith the "observed" bond distancefrom eragelength 3.147 A). It appearsto be the shortest the structurefactor refinement(0.77 A). O-O contactdistance documented for any kind of Zn In the light of theseconclusions it is interestingto oxyanion. The edge shorteninghas resultedin the comparethe correspondingbond strengthsums for maintenanceof a distanceof 3.221A betweenZn the mineralandalusite (Burnham and Buerger, l96l). cationsin adjacentbipyramids without the needto As shown in Table 5, all the atomsin andalusite, invokebond stretching.For comparison,the Zn-Zn includingthe atom (O") equivalentto O(2) in ada- distancesacross the O(l)-O(l) and OH-OH shared mite, are essentiallyelectrically neutral: there is no edgesin the octahedronare 3.053and 2.990A, re- need(in the absenceof other information) to advo- spectively. cate the presenceof H atomsin the structure.Aside The balanceof the electrostaticcharges, computed from small differencesin averagebond lengthsfor with the empiricalbond-strength-bond-length curves analogouspolyhedra, the two structuresare remark- of Brownand Shannon (1973) is givenin Table5. All ably alike.The very shortO(3)-O(3) distancein ada- CRYSTAL STRUCTURE OF ADAMITE 983

Frc. l. Unit cell diagramfor adamite(with 507oprobability thermal ellipsoids). mite is mirrored by an extremelyshort O"-O" dis- the structuretype is capableof accommodatinga tance(2.247 A) in andalusite,and by and large the widevariety of cations,including Znz+, Cu2+ , Mn'*, other polyhedrondistortions are similar in both Co2+,Al3+ (in the octahedraland trigonal bipyra- structures.Indeed, the charge neutrality ofandalusite midal sites)and As6+,P6+, and Si{+(in the tetrahe- is a functionof the valenceof the cations,rather than dral sites).In regardto the discussionpresented of any major rearrangementof the anionswithin the same framework due to cation size differences.In Trnr-B 5. Brown and Shannon(1973) bond-strength sums (p) both structuresthe strongestbonds are those in- fbr adamiteand andalusite volvingthe tetrahedralcations, namely Asv andSiIv. In adamitethe O(2) atom is the only oxygenatom not Adamlte Aodaluslte bonded to As, and it is unable to counteractthe AEom p Valence ao AtoE p Valence A resultant bond strength deficiencyby its bonds to As 4.994 5 0.1 S1 3,959 4 0.8 divalentZn. On the other hand. in andalusite.the zn(l) L.994 2 0,3 A1, 2.928 3 2,4 I equivalentatom is ableto neutralizethe deficiencyby zn(2) 2.044 2 a1 ALz 3.045 3 1.5 formingbonds to the trivalentAl atoms.The andalu- o(1) 2.009 2 0.5 o, 2.0L9 2 1.0 site frameworkcan thereforeexist without the pro- o 0(2) r.170 2 4L.5 o 1.880 2 6.0 ton, whereasthe additionalbond strengthinherent in o(3) 2.039 2 2.0 o 2.L55 2 7.8 the O(2)-H associationis necessaryto maintain the o(4) L.907 2 4.7 o. 1.944 2 2.8 stabilityof adamite. From the diversityof compositionof the members * The percentage difference between the valence and p. of the olivenitegroup of minerals,it is apparentthat 984 RODERICK J. HILL

WAVELENGTH(MICRONS} 56749 zF U80 G lrlo- 60 t! C)

Fe40 F = 220 G F o 2000 1000 1600 1400 t2o0 400 2o0 FREQUENCY(CM.I) Frc 2. Infraredabsorption spectrum for adamite. above,all the speciescontaining divalent and pen- Resultsand discussion tavalent cations are observedto contain an OH The free arsenateion belongsto point group43m, group,despite significant differences in the sizeof the but in adamitethe site symmetryof the ion hasbeen cations. loweredto ra. Under thesecircumstances all degener- acyof the (nine)vibrational modes is lost(Hertzberg, Infrared spectrum 1945;Adler and Kerr, 1965),and the absorption nine Experimental spectrumis expectedto display the full set of vibrational modes,represented by three Vr, one Vr, The spectrumdisplayed in Figure2 wasobtained threeVo, and two % bands.The absorptionbands in on a Perkin-Elmermodel 521 Grating Infrared Spec- the infrared spectrumof adamite(Fig, 2) havebeen trophotometer. Sampleswere prepared by gently assignedto variousmodes of vibration of the AsOl- grindingapproximately 1.5 mg of the mineralwith group as displayedin Table6. They arein agreement 250 mg of AR grade,dessicator-dried, potassium with assignmentsin relatedcompounds (Nakamoto, bromidein an agatemortar under alcohol. This blend 1970). was then vacuum-pressedfor two minutesin a tool- The XY, moleculesin trigonal bipyramidalconfig- steeldie at about 6000p.s.i. to form a translucent uration (idealsymmetry6m2) have a possible12 non- pelletapproximately 12 mm in diameterand I mm degeneratevibrational modes when their symmetryis thick. The spectrumwas recordedin double-beam lowered,as in adamite.However, frequency assign- mode(KBr standard)for the region200-4000 cm-1. ments for this group could not be located in the literature,despite an exhaustivesearch. vibrational frequency TesI-r 6. Infraredspectral assignments for adamite The relationship between (z) and internucleardistance (R) for diatomicmole- culesas given by the expressionsfor force constant Group Mode rrequency (cm ) (Badger,1934) and frequency(Barnes et al.,1944)is

o-H stretchlng i520 well known.Using the observed values of z andR for a numberof phosphateminerals containing Zn in Zn-O-H stretchlng 3400 tetrahedralcoordination (Hill, 1975),this relationship comb inatlons 1620 has beencalibrated for the Zn-O systemsuch that a 880, 820, 795 predictionof the vibrationalfrequency of Zn in five- 730 The indicate AsOO fold coordinationmay be made. results 530,510,470 that for an averageZn-O distanceof 2.028A in the trigonal bipyramid in adamite,the vibrational fre- ZnO- 380,320 quencieslie in the regionbelow 380 cm-1. An analo- 5 gouscalculation using the frequency(250 cm-r) as- CRYSTAL STRUCTURE OF ADAMITE 985 signedto Ni-Cl stretchingmodes involving terminal BeocEn, R. M. (1934) A relation between internuclear distances ligands of a similar pentacoordinatedimer in and bond force constants.J Chem. Phys 2,128-13'. eNo V. Z. WIllIlus (1944) (Goedkenet al., l97O)indicates BlnNEs, R. 8., R. C. Gonr, V. Ltoorl [C?H,6N,],[NiCl,] Infrared Spectroscopy Reinhold, New York. that the equivalentmode in adamiteshould occur at Bnun, W. H. (19'12)Prediction of hydrogen bonds and hydrogen about 330cm-'. Moreover,the AsFugroup, with a atom positions in crystalline solids.lcta Crystallogr. B28, 1456- closerelationship to ZnOuin termsof bond length 1465 and reducedmass, displays absorption frequencies in - (1973) Criteria for hydrogen bonding II. A hydrogen bond a coordination polyhedron around a cation. Acta the range809 to 128cm-l (Nakamoto,1970). The in the edge of Crystallogr. B.29, 139-141. bandsat 380 and 320cm-1 in the adamitespectrum Bnecc, W. L. eNn G. F. CLlnINcsuLl (1965) The Crystalline may thereforerepresent vibrations of the ZnOufunc- State-Volume IV. Crystal Structures of Minerals. Bell, London. tional group rather than Z, modesof AsO.. It is BnowN, I. D euo R. D. SneNNoN (1973) Empirical bond- interestingto notethat a similat arrayof IR absorp- strength-bond-length curves for oxides. Acta Cryslallogr A29, tion bandsin the range500 to 270 cm-t is obtained 266-282. BunNnllr, C. W nNo M. J. Busncsn (1961) Refinementof the for tarbuttite,Znr(POn)(OH), the structureof which crystal structure of andalusite. Z Kristallogr. 115,269-290. also contains pentacoordinatedZn atoms (Hill, BusrNc, W. R., K. O. Me.nuN rNo H. A. Levv (1962)ORFLS, a t97s). Fortran crystallographic least-squares program. U S. Natl. The largeabsorption bands at 3400and 3520 cm-', Tech Inform Seru. ORNL-TM-305, Oak Ridge, Tennessee. _ (1964) with peak AND ORFFE, a Fortran crystallo_ only a minor at about 1620cm-1, indicate graphic function and error program. U.S. Natl. Tech. Inform the presenceof OH groupsin the crystalstructure, Seru. ORNL-TM-306, Oak Ridge, Tennessee. and the absenceof crystal-hydratewater (Yukhnev- Cnor,rrn, D. T. (1965) Anomalous dispersion correctlonscom- ich, 1963;Gillespie and Robinson, 1964;Farmer, puted from self consistent field relativistic Dirac-Slater wave 1964).In fact,the small 1620cm-1 peak may repre- functions. Acta Crystallogr. 18, 17-23 Fnnuen, V. C. (1964) Infra-red spectroscopyof silicatesand re- senta combinationof lowerfrequency fundamentals lated compounds. ln, H. F. W. Taylor, Ed., The Chemistry of (Farmer,1964) and does not necessarilycorrespond Cements.Academic Press,London. to bendingmodes of watermolecules present in trace GenvrrN, G., P. MrrN nuo M. M. WoorrsoN (1971)The appli- amounts.Thermal evidence supports this conclusion, cation of phase relationships to complex structures III. The dehydrationoccurring in a singlestep at 570'C(Hill, optimum use of phase relationships Acta Crystallogr. A27, 368-3'76. t97s). Grllrspte, R. J. eNo E. A. RosnlsoN (1964)Characteristic vibra- Since thermal and X-ray evidence indicates the tional frequencies of compounds containing Si-O-Si, P-O-P, presenceof only one structurally distinct OH group S-O-S, and Cl-O-Cl bridging groups. Can. J. Chem. 42, in adamite, the two absorption peaks at high fre- 2496-2503. (1970) quency are interpreted as having different origins: the Gororrr, V L., L. M. V,rr-r-enrNoAND J. V. Que,clreNo Stabilization of high-spin five-coordinate nickel (II) complex sharp peak at 3520 1 cm- is assignedto OH stretching anions by appropriate counterions. J. Am Chem Soc 92, vibrations displacedto lower frequenciesfrom their 303-307. values in the free state by hydrogen bonding effects Heozt, D. (1965) Infrared spectraof strongly hydrogen-bonded (Farmer, 1964),while the diffuseband at 3400cm-1 is systems.Pure Appl. Chem. 11,435-453. assignedto stretching vibrations of Zn-O-H groups. Hrnrtscu, H. (1940) Die Struktur des Libethenites Cur(OH)[PO.]. Z Kristallogr. 102, l-12. The absenceof bands in the 1300 and 2700 cm-l Hrnrznrnc, G. (1945) Molecular spectra and molecular stucture regions confirms that no acidic As-OH groups are IL lnlrared and Raman spectra of polyatomic molecules. Van presentin the structure(Hadzi, 1965). Nostrand, New York. HrLL, R J. (1975) The crystal structure of the mineral scholzile and Acknowledgments a study of the cryslal chemistry of zinc in some related phosphate and arsenate minerals. Ph.D. Thesis, University of Adelaide, The author is grateful to Dr. J. B. Jones for his help in the South Australia. realization of this work, to Mr. M. Raupach, Division of Soils, HowELLs, E. R., D. C. Pnrlltps eNo D. Rocens (1950)The proba- C.S.l.R.O., Adelaide, for permitting use of the IR spectro- bility distribution of X-ray intensities. II. Experimental in- photometer, and to Mr. J. E. Johnson for supplying the specimens vestigation the X-ray detection of centresof symmeluy.Acta of adamite from his private collection. The researchwas supported and . by a C.S.l.R.O. PostgraduateStudentship. Crystallogr. 3, 210-217 International Tables for X-ray Crystallography (1968) Vol. lII. 2nd Kynock, Birmingham, England. References ed. JoHNsoN,C. K. (1965)ORTEP, a thermal ellipsoidplot program Aolrn, H. H. nxo P F. KERR (1965) Variations in infrared for crystal structure illustrations. U.S. Natl. Tech. Inform. Seru. spectra,molecular symmetry and site symmetry of sulfate miner- ORNL3794, Oak Ridge, Tennessee. als. Am. Mineral. 50, 132-147. KAnr-e, J. rNo I. L. Krnre (1966) The symbolic addition pro- 986 RODERICK J. HILL

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