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The Crystal Structures of Cavansite and Pentagonite'

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The Crystal Structures of Cavansite and Pentagonite' American Mineralogist, Volume 58, pages 412-424, 1973 TheCrystal Structures of Cavansiteand Pentagonite' Howeno T, EvnNs, Jn. U.S. GeologicalSuruey, Vllashington,D.C. 20244 Abstract Cavansite and pentagonite, dimorphs of Ca(VO)(Si.O'").4tIO found in Malheur County, Oregon, are both orthorhombic and represent novel, layer-silicate structure types. Zigzag pyroxeneJike (SiO'), chains, joined laterally into sheets parallel to the a+ plane, are present in both with tetrahedral apices pointed alternately plus and minus along the b axes. The lateral linkage in cavansite results in a network of 4-fold and 8-fold rings, but in pentagonite the network is entirely made up of 6.fold rings. The vanadyl groups VOa and Ca'* ions lie in mirror planes between thE silicate layers and are coordinated alternately to pairs of tetrahedral apices along the chains on opposite sides of the mirror planes. Vanadium is in square-pyramid coordination, and Ca is in 7-fold coordination in both structures. The HaO molecules are poorly resolved, have high apparent thermal parameters, and are probably zeolitic. The special features of the structures favor twinning in pentagonite (commonly found in fiveling groups) but prohibit twinning in cavansite (in which twinning has not been observed). Refinement of the crystal structures (to R : 0.109 for cavansite, and R ; 0.081 for pentagonite) gives bond lengths with estimated eriors of + 0.02 A. Discoveryof Cavansiteand Pentagonite crystal diffractometer measurementson the crystals In 1960, an unusual blue mineral was found in used for the final structurerefinements. These differ a roadcut near Owyhee Dam, Malheur County, slightly from thosereported from X-ray powder data Oregon.Staples, Evans, and Lindsay (1968) found by Staples, Evans, and Lindsay (1973) who at- that this mineral consistsof two polymorphs of the tribute such observedvariations in cell parametersto new compoundCa(VO) (Si4O10).4HzO. These sep changesin the amount of zeolitic water in the crystal arate specieswere named cavansiteand pentagonite, structure, as affectedby temperatureand humidity. and their description and natural history are de- For the structure analysis,X-ray intensities for scribed in the precedingpaper (Staples,Evans, and 998 independent reflections were measured on a Lindsay, 1973). As often happensin suchstudies, a General Electric quarter-circlecrystal orienter using full understandingof the nature and chemistry of Zr-fi7tered MoKa radiation. The intensities were these two species was achieved mainly through measuredmanually, using a stationary-crystal,sta- (peak-height) crystal structure analysis.The results of a prelimi- tionary-counter technique with two nary crystal chemicalstudy were reported previously backgroundmeasurements per reflection.These data by Evans and Staples (1970). The structure analy- included all those with Bragg angle less than 47", ses were carried out at the U.S. GeologicalSurvey, and of these361 were lessthan an arbitrarily chosen and are describedin detail in this paper. threshold value. Without any other preliminary considerations StructureDetermination of Cavansite (some earlier attempts to solve the structure from Buerger precessionphotographs of a prismatic two-dimensional Patterson maps had been aban- crystal of cavansite showed it to be orthorhombic doned), the three-dimensionaldata were normalized with spacegroup symmetryPcmn or Pc2p. Powder (to obtain E values) and treated directly by the diffraction and other crystallographicdata have been symbolic addition procedure of Karle and Karle given by Staples,Evans, and Lindsay (1973). The (1966), on the assumptionthat the spacegroup is unit cell parametersgiven in Table I for cavansite centrosymmetric(Pcmn). The calculation was car- and Table 4 for pentagoniteare derived from single- ried out on an rBM 360 Model 65 computer using l Publication authorized by the Director, U.S. Geological xRAY67,the unified systemfor crystal structurecom- Survey. putationsof J. M. Stewart(1967). Usingthe termi- 412 CRYSTAL STRUCTURES OF CAVANSITE AND PENTAGONITE 413 TenrB 1. Structure and Thermal Parameters for Cavansite* Theroal parameters (xlO+)3 Atm Type x y B,A (equlv. ) tsrr Bzz Bsa Btz Brs Bzz ca 4(c) 0,0829(8) 0,25 0.3810(7) r.s3(10) 37(6) 2s(3) 3s(s) 0 5(6) 0 v 4 (c) 0.4033(7 ) 0.25 0.s27L(6) 1.s4(9) 42(s) 22(3) 39(s) 0 -2(s) 0 sir 8(d) 0.09s4(7) 0.0336(s) 0.1839(6) 1.13(8) 24(5) 22<3) 23(s) -2(3) -10(s) 2(3) siz 8(d) 0.3165(5) 0.0431(s) 0.392s(6) 1.09(8) 24(5) 18(3) 26(s) 3(3) -s(4) -3(3) o1 8(d) 0.0845(17) 0,1509(11) 0.176s(15) 1.3(2) s6(14) L2(6) 21(11) 2(e) 3(r3) -e(8) o2 8(d) 0.2945(18) 0.1591(12) 0.4124(16) r..9(3) 51(r7) 2s(8) 3s(13) -2(e) -17(13) -e(e) 03 8(d) 0,4417(t4) 0.0203(12) 0,2952(L4) t-.3(2) 32(r3) 28(8) rr(10) -8(8) 6(10) 0(e) 04 8(d) 0.167r(15)-0'0118(u) o,0423(r4) L.2(2) 41(14) L7(7) 18(1r) -e(8) s(12) -e(8) 05 8(d) 0,L847<L7) -0.0062(13) 0.3175(16) r.8(3) s3(1s) 33(8) 20(L2) -14(10) 20(r4) 5(9) o5, 4(c) 0.5515(25) 0.25 0.4s70(32) 2.8(s) 37(22) 4s(rs) 101(33) 0 1(23) 0 o7(H2o) 8(d) 0.947L(22) 0.1186(16) 0 .470o (24) 4.0(4) 118(26) 4s(L2) 113(2s)-40(14) 28(22) 1(ls) o6(n2o)4(c) 0.3709(41) 0.25 0.1387(2s) 4.6(8) 2r8(s6) 68(21) 9Qr) o LL2(27) O og(Hao)4(c) 0.8092(44) o,25 0.2806(58) 8.4(1.2) 137(48)1r-6(33) 303(87) 0 117(60) 0 * Space group: Pt * (DLZ|). Unlr ceLt: a-g.792e)8, b-L3,644G)&, c=9.62ge)L; Z=4. standard errors Lndl.cated ln parentheses ln terms of last slgnlflcant flgures, nology of this system,the following programs were actually single,but a parallel group of two or more linked together for the computation: DATRDN(raw individuals deviating from perfect alignment with data reduction); DArFrx (data normalization); each other by 0.1 to 0.5 degree.This fact leadsto SIGMA2(search for hkl vectortriplets); pHAsE(sym- severe fluctuations in the ratio of peak height to bolic addition procedure); FoURR (Fourier syn- integratedintensity from reflectionto reflection.The thesis). The one-shot calculation was completely use of a 0-20 scanning technique with the Picker successful,requiring a total of 6 minutes of machine instrumentwas expectedto overcomethis difficulty. time (in actualfact, the last link was separatedand In this experiment,a crystal of dimensions0.06 x carriedout on a secondnight). The sharpenedelec- 0.08 x 0.28 mm was usedto measureintensities of tron density map (E map) showedthe structure in 1703 independentreflections (all with Bragg angles almost all its details. The silicate framework was < 60'), usingNb.filtered MoKc radiation.Of these, clearly revealed, and the V and Ca atoms were 903 had intensity values less than 3o based on sharply imaged on the mirror planes (with perhaps counting statisticsfor each reflection and were given somequestion as to which was which), althoughthe zero weights in all subsequentcalculations; the re- HzO moleculeswere not so well resolved. maining 800 reflections were treated with unit The initial model was refined by least squares weights.No correctionswere madefor absorptionor analysis (using oRFLs; Stewart, 1967) and con- extinction. verged in four cycles to a structure that gave a The new data setwas analyzedby the leastsquares conventionalreliability index R ,: 0.098.While the method using onrrs, and in the final anisotropic postulatedstructure was thus proved, this result was modesa program (nnINe) written by L. W. Finger not considered satisfactory,mainly becausein the of the GeophysicalLaboratory, Washington, D.C. refinement of isotropic thermal parameters,B took S'tableconvergence of 34 structure (positional) and on negativevalues for 10 of the 13 different atoms 68 thermal parameterswas reachedat R = 0.109. (all but the H2O molecules).The excellentquality This time the thermal parameters (equivalent iso- of the structureitself in terms of its electrondensity tropic B values)varied frorn 1.1 to 2.8 A'zfor the mapping and interatomicdistances led to the conclu- cations and silicateframework, and 4.0 to 8.4 A'z sion that the anomalousthermal parametersresulted for the H2O molecules,and were consideredto be from systematicerrors in the data measurements. acceptable.The HzO moleculeshave large and highly Therefore a new crystal was selectedand the data anisotropic thermal rnotions, and probably are not set remeasured on the Picker automatic diffrac- well representedby the structure factor functions tometer. assumedfor them. A full least-squarescycle which Careful diffractometer scans showed that any included a variable populationparameter for each crystal of cavansitethat is large enoughfor collecting H2O molecule did not lead to any value for these reasonably good three-dimensional data is not parametersthat wassigniflcantly different from unity. 4t4 HOWARD T. EVANS, IR, Trsrv 2, Calculated and Observed Structure Factors for Cavansite* rc rc Fo Fc I B ]n K ! 5 9 A l' 16 624455t 61331-34 6642t25 2t -24 6 9 3 r9 2r 6ro a tr -24 612 a 65 -43 72444-52 Itb4r-45 20 _2S ?6319-26 1932r-25 22 2i 953ZO20 ro5;26-2t 10e34-29 22 -2t r292423 1392411 l79r!19 r 3 9 re-22 22e{345 24921-24 2692921 212 9 4t -t5 -22 ro9zl-27 -rl 16 rt tl 5l \z I5 4 1 a 2L 2l 14912-tA 45e42.1 4 6 e 29 li 4194?tr 4991142 tt 21 412 9 20 -r0 509t779 5292926 51956-51 5492r.6 ll 51956-61 5r0 9 1) -42 609t2-24 62912-20 6r918-21 lzstl-26 zl a2943-52 22 s5923-27 r6e19-32 52 5l 5t -a5 0 2r0 a9 5( ,' ,; o rL0 46 {7 i o 4 r0 19 25 0 ?lo 51 43 o eto t? ,? 0 lo to 23 _t3 I O r0 l3 5 I rLO 1l -il l6lo29-5 I ?ro 5l -41 I 3t0 L9 -26 1 9 l0 30 -12 t rr r0 3r -ro l 12 ro 25 -30 2 t to )4 -t1 t 2ro 22 aa 3 5r0 25 -rl r 6 to lI l, 4 210 27 -3 1 tto 24 -2r 2\ 2r r 4lo 24 17 r I ro rs -a 3 0 t0 rr -13 9 AtO 2a -24 | 0rI ]r (o I rrr 4{ {r r trr 47 4l l lL 11 2? 25 2 r LL ?l -29 2 2Lr 56 5' 2 3 tL 19 -22 2 4ll 24 -26 2 t tl J2 _3O 26112531 2 7Ir 5r -40 2 etl (0 _4t r l0 lr 2? 28 eolla0-1? 3t -]r 44u3t-34 5 0 Ll r4 -l( 5rlI40l{ 5 2Ll 22 -29 a5LL5E55 5 6 ll 20 -17 3 O lr l5 It 0 612 rO t0 2 0 lz 10 -rt 2t 2r 2.12tt-ll toL262-43 ) 512 t1 2? 20 5 0t2 30 -21 5 ar2 2s -5 6 2r2 23 -?A r 4 l) 25 -ls r 5 rr 19 -r] r o lr 23 re * In this table only refleclions that were measured aboue the 3o threshold are listed.
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