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CRYSTAL STRUCTURE of JOAQUINITE 8'T'l

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CRYSTAL STRUCTURE of JOAQUINITE 8'T'l The American Mineralogist, Volume60, pages872-878, 1975 Crystal Structureof Joaquinite Entc Dowrv Departmentof Geologicaland GeophysicalSciences, Princeton Uniuersity,Princeton, New Jersey08540 Abstract The crystal structure of monoclinic joaquinite, ideal formula NaFe2+Ba2RETirSis OrsOH.HrO, has been determinedby the Patterson method from 2630 diffractometer data, and refinedby least-squaresto a weighted,R of 0.075.The spacegroup is C2, and cell parametersarca = 10.516+ 0.003A, b:9.686 + 0.003A,c: 11.833+ 0.0044,and,B = 109.67+ 0.003o,with Z : 2.The newindependent determination largely confirms a previous one by Cannillo, Mazzi, and Rossi(1972), but thereare important differences,including the spacegroup and severaladditional atoms found in the presentstudy (Na, OH, HrO). The principal structuralunit is a four-memberedSLO1, silicate ring, equivalentsof which are linkedby TiO" octahedrato form sheetsparallel to (001).Sheets are stacked facing alternately up and down in the c direction;Ba atomsand watermolecules occur midway between sheets on one level, and Na. Fe, and RE atoms on the alternatelevels. The Ti octahedraare somewhatout of the planeof the sheetson the Ba-HzOside, and theyshare an edgewith each other to link sheetsin the c direction.The Fe and Na polyhedra,which alsolink sheets,are very irregular;Fe is 5-coordinated,and Na is 6-coordinated.The orthorhombicform ofjoa- quiniteis evidentlyrelated to the monoclinicform by a typeof unit-celltwinning on an (001) pseudo-mirrorplane through Ba and HzO. Macroscopictwinning of the monoclinicform on this plane and disorderedstacking of monoclinicand orthorhombicforms also occur. Introduction intimately intergrown in the material from San Benito County. The I mm sized crystals show Joaquiniteoccurs as small(-l mm) honey-brown orthorhombicexternal morphology (Palacheand crystals associatedwith benitoite, neptunite, and Foshag,1932), but X-ray studiesby Laird and Albee natrolitein SanBenito County, California (Louder- (1972)and Cannilloet al (1972)have disclosed that backand Blasdale,1909; Palache and Foshag,1932), theseusually consist of finely twinnedor disordered and alsoin Quebec(Bell, 1963) and South Greenland monoclinicjoaquinite intermixedsometimes with (Semenovet al, 1967).Satisfactory analyses have orthorhombicjoaquinite, the whole simulating beenobtained only recently,as the presenceof rare- orthorhombicsymmetry. earth elements(RE) and strontium had not been A recentstructural study oI monoclinicjoaquinite recognizedearlier. The compositionof materialfrom by Cannillo et al (1972) was unfortunately California and Greenland is somewhat different; overlookeduntil the presentstudy was entirelycom- Semenov et al (1967) suggestedthe formula pleted.Most of the importantaspects of the structure NaBarFe2+CerTirSi.OruOHfor joaquinite from proposedby them havebeen confirmed in thepresent Greenland,and Laird and Albee(1972) suggested the study; however,it seemsthat they were unable to formula Bas.r(Srr.zRE..uTho.r)r.,(Cao.zNas.aFes.lLio.s resolveall the atoms,largely due to the fact that they Mgo.r)z.uTis.rSis2.ooes.z(OH)rB.Bfor material from were forced to use intensity data from precession California.However, the formulasare very similar in photographsof a twinnedcrystal. They also assumed the ratiosof the cations. that the spacegroup was C2/m, insteadof C2, as The existenceof monoclinicas well asorthorhom- found in this study. A lower residual,more bic crystalsof joaquinitehas also beennoted only reasonablebond lengths,and better agreemehtwith recently(Laird and Albee, 1972;Cannillo, Mazzi, formulasproposed on the basisof microprobedata and Rossi, 1972). No chemical differenceswere demonstratethat the present determination is a detectedbetween the two modifications.which occur significantimprovement over that of Cannillo,Mazzi, 872 CRYSTAL STRUCTURE OF JOAOUINITE 873 and Rossi.The improvementis a consequenceof associatederrors. For least-squaresrefinement, the the availabilityof superiormaterial. Since the present structure factors were weighted according to the determinationwas completelyindependent of the reciprocalof the variance.Atomic scatteringfactors earlierone, it will be largelydescribed as such,with were taken from Cromer and Waber (1965),with commentswhen there are important differencesin anomalous dispersioncorrections from the Inter- procedureor results. national Tables for X-ray Crystallography. The program systemCnYu, most recentlymodified by Experimental G. N. Reeke,Rockfeller University, was usedfor The materialused in thisstudy, a typicalspecimen Fouriersyntheses. from SanBenito County, consisted of equantcrystals with intermixedorthorhombic and twinnedmono- Determinationof the Structure clinic forms. From severalcrushed pseudomorphs, The threestrongest peaks in thethree-dimensional only one crystal could be found which was suit- Pattersonsynthesis, at x : 0.42,y : 0.25,z : 0.451'x able for structure analysis;this crystal had dimen- : 0.48,y: 0.0,z : 0.005;andx : 0.34,y: 0.0and sions 0.1 X 0.1 X 0.03 mm and proved to be z = 0.90,were assigned to theBa-RE cross vector, the monoclinic,flattened parallel to (001).Many other Ba-Ba vector,and the RE-RE vector,respectively. crystalsexamined under the microscopeshowed ob- Theseinteratomic vectors are almost consistent with vious twinning and severalpromising crystals the spacegroup C2/m; however,the Ba-Ba vector mountedon theprecession camera showed admixture falls slightly,but very distinctly,off the line x : 0.5,z of orthorhombicand twinnedmonoclinic forms as : 0. In principle, this is sufficientto determinethe describedby Laird and Albee(1972), as well as oc- spacegroup as C2, rather than Cm or C2/m, which casionaldiffuse reflections as described by Cannilloel wasassumed by Cannilloet al (1972).Inaddition, a al (1972). Precessionphotographs of the crystal relativelyhigh concentrationof Pattersonmaxima selectedshowed extinctions consistentwith space was observed'onthe Harker planey : 0, with no groups C2, Cm, or C2/m. Cell parametersdeter- significantpeaks exactly on the Harker line x : 0, z mined by least-squaresrefinement of 20 reflections : 0. The spacegroup C2 wastherefore adopted for measuredon the Pickerdiffractometer are a: 10.516 structuredetermination; the resultsproved this as- + 0.003A, b :9.686 + 0.003A, c : 11.833+ 0.004 sumptionto havebeen justified. A, and0 : 109.67+ 0.03o.The chemical composition After locationof theBa and RE atoms,completion wasassumed to bethe same as the average analysis of of the structure proceeded(slowly) by means of Laird and Albee(1972), and to conform to the for- repeateddifference Fourier syntheses,supplemented mula they gave(above). in the later stagesby least-squaresreflnement. The crystal was mounted on a Picker Fncs-l Atomic positionalparameters and isotropictem- diffractometerwith thea* axisapproximately parallel peraturefactors are listed in Table l. to the phi axisof the instrument.Intensities for 2630 Cationswere assigned to siteson the basisof bond independentreflections were collected in a quadrant lengths (Table 2) and charge balance as well as of reciprocalspace, to a maximumsin d/tr valueof electrondensity. The Fe andNa sitesmust both have 0.80.The data werecorrected for backgroundand considerablesubstitution of othercations, and one or Lorentz and polarizationfactors using the program both areapparently not fully occupied,judging from AcACA,written by C. T. Prewitt.Of the 2630reflec- theanalyses (Laird and Albee, 1972). Occupancies, as tions,2244had intensitiesgreater than four timesthe Fe us Na, wererefined for thesetwo sites;other oc- background and were acceptedfor least-squarescupancieswere not refined,the populationof the RE refinement.Absorption correctionswere also made sitebeing assumed to be the sameas in the analysesof with Ace,c,l,using a Gaussianquadrature subroutine Laird and Albee(1972). written by G. E. Harlow, PrincetonUniversity; the All oxygenatoms exceptthose designated as OH method for this subroutinewas suggestedby and HrO receivePauling electrostatic bond strengths Burnham(1966). The linearabsorption coefficient is which total reasonablyclose to 2.0. The atom 98.6cm-1 and transmissionfactors ranged from 0.43 designatedOH is bondedonly to oneFe andtwo RE to 0.75. The programsRptNn II and Blorre, by atoms. The atom designatedHrO could only be L. W. Finger,Geophysical Laboratory, were used to bondedto two Ba atoms,and theseare at relatively calculatestructure factors, for least-squaresrefine- longdistances (3.13 A). ment,and to calculatebond distances and ansles and ln the final least-squaresrefinement, all cations 874 ERIC DOWTY Tlst-e l. Atomic Coordinates and Isotropic Thermal Parameters tional y coordinate of the RE atom to shift by about for Monoclinic Joaquinite* 0.0031(about 10sigma for that atom), and that of the o Ba atom to shift by about 0.0014(about 3 sigma)in Atom x vz xxB. " the same direction with respect to the rest of the Ba 0.2366(t) o.o o.oo58(1) o.94 (21 atoms, which remained essentiallyunchanged relative RE 0. 1667(1) o.7467(r) o.4sr8(r) I . L1(2') Ti 0.0490(3) o.736e(51 o.r276(3) 0.36(4) to each other. +,++Fe O.O o.3694(6) o.s 0.6(1) The final unweightedR was 8.6 percent and the +,++Na O.0 0.o54(3) 0.5 5.9(8) weighted R was 7.5 percent. These values are un- sil 0.3?73(5) o.7343('71 o.2426(41 o.66(8) satisfactorilyhigh, and a number of areasin the final si2 0.3085(5) o.2674('7\ O.2432(5\ o.e1(e) densities. The si3 0.1206(6) o. o375( 7) O.29L7(5) o.80(9) difference map show high electron si4 0.0929(6) 0.4365(6) O.2709(6',t 0.68(9) principal problems seemto be in three areas,the RE Na atom. When an ++01 0.o o.a61(2) 0.0 0.6(3) atom, the OH atom, and the ++O2 0.0 o.606t 2\ 0.0 0.8(3) isotropic temperature factor was used for the RE 03 0.116(2) 0.881(2) 0.251(1) L.2 (2) positive peakswere obsErved 04 0. r88 ( 1) 0.385(2) 0.19r(r) o.e(2) atom, three subsidiary 05 o.22612) 0. I15( 2) O.237(1\ r.3(3) near it in the differencemap, onejust below it in the c centered just above it, but 06 0.233(1) 0.715( 1) 0.133 ( 1) o.7(2) direction, and two 07 o -472(2) 0.60s(2) o.22e(2) 1.6(3) separated from each other along the b direction.
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