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The Crystal Structure and Infrared Properties of Adamite

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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.
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