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Y: 55.Sa(3) 7:787.183 A3 Po:6.42 Gmfcc Spacegroup: Pl Or PI Pz:6.39 Gmfcc

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Y: 55.Sa(3) 7:787.183 A3 Po:6.42 Gmfcc Spacegroup: Pl Or PI Pz:6.39 Gmfcc THE AMERICAN MINERALOGIST,\,OL. 55, JULY_AUGUST, I97O THE CRYSTAL STRUCTURE OF HEMIHEDRITE W. JouN McLo.rN, CrystallographyLaboratory, (Jniaersityof pittsburgh, Pittsburgh, p ennsyhtania1 5 Z 1 3r AND JouN W. ANruoNv, Department oJ GeoLogy,IJniaersit^v of Arizona, Tucson,Arizona,9 57 2 1. ABSTRACT Hemihedrite is a triclinic minerar having the composition ZnF:[pb;(cror)asioa], 1vi1h cell dimensionsa:9.497 it,b:tt.+43 A, c:10.841 A,a:120"30,,A:92"06,,2:55o50,. Three-dimensional counter data were collected and the structure was solved from the Patterson function and refined to an R factor of 0.04. The structure is similar to those of the tsumebite series and contains a zinc coordinated by four oxygens and two fluoride ions; the lead environments are quite varied. Chromium and silicon are regularly four-coordinated by oxygen. Evidence from the crystal structure determination tends to cast some doubt on the morphological interpretation that the speciesis acentric. INrnonucrrox The new mineral hemihedrite was found in Arizona and is describedin the precedingpaper. The present investigation deals with the crystar structure, its determination and the compositionalproblems which it brought to light. The chemical formula for hemihedrite was thought to be ZnPbs(CrOn)rO,at the outset of this structure determination. This composition was significantly modified when it was realized.that the proposed composition of hemihedrite could not provide atoms necessary to occupy siteswhich emergedduring the courseof the structure determi- nation. New chemical work showed that the predicted elementsfluorine and silicon were indeed present. The composition based on the refined structure was establishedas ZnFr[pb6(CrO+)eSiO<]r. Cnysrar Dera Hemihedrite, triclinic, ZnFr[pbs(CrOn)rSiOn]z a:9.a97Q) d a:120.50(4). D:11.443(s) p:e2.r0(4) c: 10.8a1(4) "y: 55.sa(3) 7:787.183 A3 po:6.42 gmfcc Spacegroup: Pl or PI pz:6.39 gmfcc Numbers in parentheses are standard deviations and refer to the last decimal places. rPresent address: Department of Geology, university of Arizona, Tucson, Arizona, 85721. 1103 1104 W. JOHN MCLEAN AND JOHN W. ANTITONY ExprruunNral Anumberofcrystalsofhemihedriteweregroundtospheresandtheoneselectedfor previously selected (pre- data collection had a radius of 0.052 mm. The unit ceII had been photographs ceding paper) and its parameters refined using data obtained from weissenberg calibrated with NaCl taken on ali three axes with CuKa radiaiion (a:1.5418 A) and with difiractometer powder (o:5.6402 ir at22'C). These data were used in conjunction data were collected L"u.,rr"-"nt to refine the cell parameters by least squares. Intensity reflections obtainable with on a Picker four-circle automatic difiractometer. Of the 3607 wete considered to be observed on the cuKa radiation ,2790 were collected, of which 2428 corrected for absorp- basis that their intensities were greater than 1.5o. The intensities were thelnternational' Tables tion by linear interpolation of the sphericalcorrections takenfrom pp' 304 and 305 (1959), with a pR ol 6 4' Jor X:ray crystatlography' Vol. II, SrnucrunB DnrBnurNerroN The intensities were reduced to structure factors and a three-dimen- Pb sional Patterson function was calculated which indicated that the by structure was essentiallycentric. The Patterson was solved routinely choosing a vector, assuming it to be a double Pb-Pb interaction, and searching the Patterson manually for two single Pb-Pb interactions separat; by twice the double interaction vector' This established the p*itior$ of iwo pb atoms and a third was located manually by selecting a another vector and assuming it to be a double interaction between third Pb and one of the known Pb's. After several trials a vector was found for which this assumption was valid and the position of the third three- Pb established. These three atoms were used to calculate a dimensional multiple minimum function from which the remaining two pb atoms were obtained. A second multiple minimum function was addi- calculated, based on the five Pb atoms, which clearly showed four posi- tional positions, but one of these was at a center of symmetry' The tion afthe center of symmetry was ignored and one Zn and two Cr atoms of inserted into the other three positions, giving a conventional R factor (1) 0.31. A difference Fourier synthesis was calculated which indicated (2) that the atom which had been put in as zt wasactually cr, indicated that there waszn in specialposition at0,!,0 and that there were no other specialposition atoms, (3) indicated shifts for the other atoms, and (4) showed 16 oxygenJike atoms which formed four independent tetra- hedra, three around the three Cr atoms and the fourth coordinating an atom whose scattering power appeared to be about twice that of an oxygen.Theapplicationofthesefindingswiththefourthtetrahedral utoir a.ru*ed to be Cr reduced the R factor to O'21. Three cycles of full matrix least-squaresrefinement with isotropic temperature factors reduced R to 0.06. The temperature factor of the fourth cr assumedthe anomalously high value of 3.0. A difference Fourier then revealed a STRUCTURD OF HEM IH EDRITE 1105 seventeenthoxygen-like atom in the region of the special position Zn. A fourth least squarescycle with this atom added, the fourth Cr changedto lZn, the addition of correction for anomalous dispersion resulting from the Pb, Zn, and Cr atoms, and all 27 atoms varied isotropically, reduced R to 0.05. The structure model at this point was based on the apparent centric nature of the structure and the originally assumed composition, 2lZnPbs(CrO+)sOal,modified by the appearanceof the additional oxygen- Iike atoms to give 2lZnPbs(CrOa)aFrO].All atoms occurred in centric pairs except the Zn atoms where the presenceof one at a center of sym- metry forced the other to be disordered in the centric refinement. The possibility that the structure is actually noncentric is very strongly sug- gestedby the crystal morphology and an attempt was made to test this possibilityby refining the structure in the spacegroup Pl. At this point it was necessaryto go to a block diagonal least-squaresprocedure. The Ieast-squaresrefinement was preceded by three cycles of difference Fourier synthesis which began with one Zn atom as the only noncentric part of the structure and ended with all atoms shifted into slightly acentric positions, the mean differencein positional parameters from the centric casebeing about 0.009.Two least-squarescycles produced poorer agreement(R:0.06) and the number of parameterswas nearly doubled. While this result is not conclusive becauseof the difficulties inherent in making the shift from a centric to a noncentric refinement and the shift from full matrix to block diagonal least-squares,the very satisfactory centric refinement tends to indicate that the structure is not significantly acentric. The assumedcomposition 2[ZnPb5(CrOa)sFrOlrequires disorder of the general position Zn. Moreover, the distance from this position to the oxygen-like atoms surrounding it is about 1.64 A. A normal Zn-F or Zn-O distance is greater than 1.9 A. Ther" difficulties are avoided if the fourth tetrahedral position is assumed to be occupied by silicon rather than zinc. The Si-O distanceis about 1.63A for independenttetrahedra, and the scattering power of Si is very close to that observed for this position. This assumption obviates the necessity to postulate disorder of Zn in the general position. The composition of the crystal of hemi- hedrite from which the data were collected is therefore indicated to be ZnFztPbr(CrOa)aSiOnlz.Substitution of a full Si atom for the hz"lI Zn and centric full matrix least-squaresrefinement of isotropic F and O atoms and the five Pb atoms, three Cr atoms, and Si atom with aniso- tropic temperaturefactors for one cycle reducedthe R factor to 0.041. At that point the refinementwas terminated. The resulting atomic param- 1106 W. JOHN MCLEAN AND JOHN W. ANTHONY ii*iH +dl$\c -i44NN44 \oo,N$o+\ocao\<i odroNQaNo iiN$$NNN:O N4r.+OON€ 5556555-55 €+r6D\ONO:.JJJ'J J.i 33333 3330 d\oocoeiro 6ro€o4rN@: stslsv3slsy3c, sNrOc)rmdr QP=TF===PP ++r€6NN\o4 5555555-65 3+ts:d€nd O\rOrQNN\O ?r'rrrrrll o €aG'aeeac didirD4aN sra,Ssrsrsvsvs) ;-€DNiNONC' O',44O€O\rcO 6NN+4S+NNO \o@i\on4:@ o,o,€oo@N\ ==xxx=xxxx NO<i900$ I o ?rrrrrrrrr l', <riN:n4c)N F NANNNNNN & 33333 s339 c)cl,4<rO\diD oo\icoco\oLa)c?oo o4€6€NON B *0$4sasv$e O\4sNC)<1c|'+ =x=xxxxxxx D+rN+NOO\€ E d H o :iiii €4D€ ^o: N s d1 b \o iC\4OHNqNiO o\HiHiHi- bO+rN+14dl+r: g )<xxxxx==xx ts N€AOrOo+to\ ;. l- _i n.-_- .o .o. n € oo<1 s€o9!N O osNcONHrNis rN€N\O<tr\O\O\c) E 6+hSN6€$+S €dOc!\oDiOr o :x-xx=x=xx ts ONNiNINNi bo i-aNoioNo NNNNNNNNN €4\od+rNs+t\o 3333e 3330 ios\o6irN€ ooo!troNN4N\O F oi+1+TNNNOoN dHg;:Emmgd ! N€OTDOOi@€ o d o +r<'r4ND.o\oo NNSNNNNNN o 33333 0039 €9@+orNN4 €NN4OONcO.O\O €O\N€NOOcOcO erO<i€OOD4<'i +:rH*r+OOn Ft+HnorNN4 eaNOiii:OS eoN!+.+DoFov o i 6ND\O€€dlN<r 333e3 3339 NNONNNNNN *\O\ONHOe.]OOI 44c}1 OrdcaN4 NONdINOiTO gESg:8e3\N dl 4oa€icQN<r NNo\r9oo\4.$a Hr:oa4$rD @ o ^^^^^ ooo30900 a rNO$14 iNO fifffififS666a STRUCTURE OF HEMI H EDRITE rr07 Tarr,e 3. coupenrsoN ol rrrn srnucrurrs or TsuunrrrE ANDHrunrr,onrrn. Tnr cnr,r, eno Penaurrnts or HnurnrnRrrE ARECoNvrnrnn ro ruosE oF rcs P2r/yn Psnulocrr-r. ron CoupenrsoN.. Name Tsumebite Hemihedrite Formula CuOHPbzPOTSOT Zno aPbFPbr(CrO.r)rSiOr Space group P21/m PI abc o,' b' cl Unit cell 7.8s A.
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