American Mineralogist,Volume 67, pages8U-808, 1982

Crystal structure of synthetic hafnon, HlSiO4, comparisonwith and the actinide orthosilicates

J. ArexeuoBn SpBBneNo BnreN J. Coopen

Department of Geological Sciences Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061

Abstract

A crystal structurerefineme-nt of a synthetichafnon, HfSiO4, space group l4llamd with a: 6.5725(7)4,c = 59632(qA,Z: 4,D: 6.97gmcm-3, produced positional parameters for oxygenofy = 9.965t(13)and z: 0.1948(14)at R : 0.054.Hafnon hasa zircon structure and the similar ionic radii of Hf (0.83A) and Zr (0.844) accountsfor the interatomic distancesand anglesofhafnon and zircon beingidentical within the statederrors, although hafnon has systematicallysmaller distancesand correspondsto a zircon at 29.5 kbar pressure.Combining the hafnon,zircon and structurerefinements allows prediction of the oxygenpositional parameters and bond lengthsfor the actinide(Pa, U, Np, Pu and Am) orthosilicatesas well as possibleextent of the solid solution among these zircon- structuresilicates.

Introduction greater compositionalsubstitutions for Zr and Si and yield discordantages (see Speer, 1980a,chemi- The most commoncompositional substitution for cal zoning). A refinement of haf- Zr in zircon is Hf (see Speer, 1980afor discussion non was performed to determine if any geometrical and references).Most natural have a Zr:Hf or dimensionaldifferences accompany the physical ratio of 40:1, which correspondsto their crustal property differences between hafnon and zircon. abundanceratio. Synthetic HfSiOa is isostructural The comparisonof the hafnon structure with that of with zircon and was called hafnon by Curtis et al. zircon and thorite permits an understandingof the (1954),a name eventually approved for the rare, crystal chemistry of the solid solutions among haf- natural end member by the International - non, zircon and the actinide orthosilicates. ogical Association. Hafnon is found in pegmatites with Ta and Nb , in particular where there Experimental is enrichmentin the pegmatiteof the even-oddpair Hafnon crystals were grown in a platinum cruci- t2111-tt7u over the 4ozr-41Nbpair. ble from a Li2MoOamelt which was about5 percent Hf and Zr areextremely similar in their chemical by weight HfO2'SiO2.The starting material was properties; Hf was distinguishedfrom Zr only in spectroscopicgrade, 99.99% HfOz from Apache 1922.This similarity is a result of the similarity in Chemicals,Inc. and Puratronic SiO2from Johnson their electronic structures and ionic radii which Matthey Chemicals,Ltd. The crucible was held at permits complete miscibility in most Zr-Hf com- 1000"Cfor four days in an air-atmosphere muffie pounds.While there is little changein the geometry furnace. The crystals were separated from the and dimensionsof these structureswith chemistry, quenchedflux in hot water. there are greater changesin some physical proper- The crystals were tetragonal dipyramids, and ties (e.9., density and thermal neutron capture wavelengthdispersive electron microprobe scans of crosssection). hasa crosssection of0.18 the crystals detected only Hf and Si. The space barn whereasHf has a larger value of 115barn. The groupof hafnon,determined by precessionmethod, presenceof Hf in zircon increasesits susceptibility is l41lamd. Unit-cell parameterswere obtained by to metamictization, and Hf compositional zoning least-squaresrefinement of 20 automatically-cen- causesa nonuniformsusceptibility. Hf-rich zonesin tered reflections in the 20 range of 74 to 93" (Table naturalzircons are usuallv more metamict. contain 1). Data were collectedon a crystal0.12 x 0.14 x 0003-004x/82/0708-0804$02.00 E04 SPEERAND COOPER:SYNTHETIC HAFNON 805

Table l. Crystallographicdata, positional and thermal tering of Cromer and Libermann (1970). Hf is parametersfor synthetichafnon, HfSiO4 located at (0, 314,1/8), Si at (9, 314,5/8) and O at (0, tem- Space Broup: I 41/ and y, z). Initial atomic coordinatesand isotropic perature were those of a zircon refinement UniL-cell paraneters: factors this study D SH RGS by Hazen and Finger (1979).Because of the high a, A t.stzs(t)t 6.581 6,573 6,569 correlation among most of the parameters, the c, A 5.9632(4) 5.967 5.964 5 .967 refinementwas done in parts. The scalefactor and v, A3 25'1.6 258.4 258.I 257.5 isotropic temperature factor for Hf were each re- I p, gm cm 6.97 6.95 6.97 9'n' fined separately.The R-factor, 4 : (lF"l - lF.l)/ I (l4oko) 2 u 403. 8 cn- lFol, decreasedfrom 0.239 to 0.091.The silicon Positional and thernal paraneters: isotropic temperaturefactor and oxygen positional Aton x y z 6tr B:: B parameters and isotropic temperature factor were zt3 o.o o.75 o.r25 0.0035(1) 0.0034(2) next variedsimultaneously, keeping the scalefactor (8) si 0.0 0.75 0.625 0.05 and Hf temperature factor fixed, reducing R to o 0.0 0,0655(13)0.1948(14) o.73(7) 0.089.Finally, the isotropic temperaturefactor for Hf was converted to anisotropic temperaturefac-

iParenthcsized figures represent esd's of last units cited. tors, all parameterswere permitted to vary, and an 2Mass attenuation coefficients are from International Tables for isotropicextinction correction was made. The final X-ray crystallography, vo1, 4. R : 0.054and G : 0.96.Table 22lists observed and 3zr is in site a wlth slte slmetry 42 m. calculatedstructure amplitudes. Btt = Bzz and Bl2 = 813 = 823 = 0 References: D, Durif (I961); sH, salt and Hornung (1967); Results RCs, Rangkrishnan gL aI. (1959. Refined structural parametersfor hafnon are pre- sentedin Table I and selectedinteratomic distances and angles are listed in Table 3. Orientation and 0.22mm in size, mountedwith [110]parallel to the magnitude of the thermal ellipsoid are phi axis of an automated Picker four-circle diffrac- included in Table 3. Descriptions of the zircon tometer. Intensity measurementswere made with structuraltype are given by Speer(1980a) or Robin- Nb-filteredMoKa radiation using the constantpre- son e/ al. (1971).The isotropic temperaturefactor cision intensity techniqueof the Krisel X-ray dif- for silicon is small. Determination of light-atom fractometercontrol system(Finger and Hadidiacos, parametersin a structure with heavy atoms is 1981).All 1386reflections in one octant of recipro- difficult by X-ray methods and at best leads to large cal spacefrom 3o to 95' 20 were collected. Three uncertaintiesin the refined parameters.In hafnon, reference reflections were monitored every two the silicon makes the smallestcontribution to the hours. The intensity sums of these reflectionsvar- structure factors and the only variable parameter is ied less than 4 percent from the average.The data its temperature factor. Temperature factors are were corrected for Lorentz, polarization and ab- sensitiveto absorptioncorrections, but the difficul- sorptioneffects and then were symmetry averaged ty with the silicon temperaturefactor is believedto to produce 203 reflectionswith intensitiesgreater resultfrom its high correlationwith the scalefactor, than 2o.t Becausethe crystal is a highly modified Hfprr and HfFn.The correlation coefficientsare tetragonalprism, absorptioncorrections were made 0.34,0.78 and 0.60respectively. These three param- assumingan ellipsoidalshape. Maximum and mini- etersare alsohighly correlated(>0.60 amongthem- mum transmissioncoefficients were 0.066and 0.028 selves. Perhaps the greatest error in the silicon respectively.The refinementwas accomplishedus- temperaturefactor lies in its optimistically small ing the neutral atom scatteringfactors of Doyle and esd.A completelist of correlationfactors is includ- Turner (1968)and coefficientsfor anomalousscat- ed in Table 2.

rProgramsutilized for data handling, refinementand geometry calculations were local modifications of oer,cI-Ie, DATASoRT, 2To obtain a copy of this table, order Document Am-82-205 onxprs3 and onrrr,3. These programsare included in the World from the Mineralogical Society of America, Business Office, List of Crystallographic Computer Programs (3rd ed. and sup- 2000Florida Avenue, NW, Washington,DC 20009.Please remit plements). $1.00in advancefor the microfiche. E06 SPEER AND COOPER: SYNTHETIC HAFNON

Table 3. Selectedinteratomic distancesand anglesfor synthetic for volume compressibility,synthetic hafnon corre- hafnon spondsto a zircon at29.5 kbar pressure.This would

sl tetrahedron dLstances (1,) and angles (') suggestthat hafnon would transform to the schee- s1-0 t41 1.620(8) lite-structure at 30 kbar less than the approximately o-ot L2l 2 ,425(r5) o-o t4l 2,749(15) 120kbar transition of zircon determinedby Reid o-s1-o 96.e (6) (1969) o-st-o 116.1(3) and Ringwood and the 160kbar transition of zircon to the KAlF4-structure postulated by Hazen zr trlangular dodecahedron dlstancee (l) and angles (") (1979). Hf-0 t41 2.115 (8) and Finger Perhapsof more importance, the Hf-o' t4l 2.260(9) ubiquitouspresence of Hf in solid solution would Average Hf-o 2. 188 lower the pressureof any transitions experienced o.ot t2l 2.425(16) 0-0. . t8l 2.827(L3) by natural zircons. o-ott t4l 3.049(r2) 0-0 t4l 2.478(L7) o-Hf-o 64.e (4) Hffion, zircon and the actinide orthosilicates o-Hf-0 80.4 (2) 0-Hf-o 68.9 (4) The oxygen y and positional parametersreport- o-Hf-o 92.2 (L) z ed for zircon (Hazen and Finger, 1979)are slightly orlentatlon aod nagnltudes of hafnLuo themal ellipeolds larger than in hafnon, but both are identical within iIf r il 0.078(2) A rl 0.088(2)A the stated effors. The refinement of a synthetic Catl.on-catlon dlstances thorite (Taylor and Ewing, 1978) clearly shows Hf-Itf 3,6086(3) Hf-st 2.98L6(2) larger values for the oxygen positional parameters Hf-st 3.6086(3) (Fig. 1a). This ditrerenceof the oxygen position and, more important, the expandedcell dimensions *Blacketed nunbera are bond oultlDltcltles. accommodatesthe larger actinide elements in the **ParenthesLzed flgures represent eds's of the lasL unlts c1ted. zircon-typestructure. The oxygen positional pa- tEdge shared beteeen tetrahedron and dodecahedron. z +iEdge ehared between two dodecahedra. rameter has a larger difference than y with increas- ing ionic radius for these three isostructuralcom- pounds.The resultingpositions of the oxygenare at increasing distances perpendicular to the direction Discussion joining the two adjacentmetal atoms. This allows the greatest increase in bond length with least Comparison with zircon repositioning of the oxygen. Using the oxygen The unit cell volume of hafnon is 1.24 percent positional parametersof hafnon, zircon and thorite, smallerthan that of zircon, reflectingthe 1.2 per- valuescan be predicted for the other actinide ortho- cent differencein the Shannon(1976D ionic radius silicates(Fig. la) as well as the Me-O and Me-O9 vIrIHf between (0.834) and vrrrZr (0.84A). But the bond distances(Fig. lb) on the basisof their ionic ionic radius of Hf is close enough to Zr that the radii. positional parameters of the hafnon structure are Hafnon is the only zircon-structure silicate with a identical to previously reported zircon refinements metal atom smaller than Zr. Those that remain are within the errors reported (Robinson et al., l97l; natural (Th : thorite, U : coffinite) and synthetic Finger, 1974;Hazen and Finger, 1979).However, (Pa, Np, Pu, Am) actinide orthosilicateswith ionic the Hf-O distancesin hafnon are systematically radii between 0.95 and 1.05A. There is complete smaller,a result primarily of the smallercell dimen- solid solution between hafnon (Hf : 0.83A) and sionscaused by the smallerHf atom. The average zircon (Zr : 0.84Ar, but some uncertainty regard- Hf-O distanceis 2.188A,0.014smaller thantheZr- ing the wide miscibility gaps found on the ZrSiOa- O distanceof 2.198Ain zircon (Hazen and Finger, USiO4, ZrSiOa-ThSiOa,and USiO4-ThSiOajoins 1979).Of the several zircon refinements at various (seeSpeer, 1980a,bfor discussionor referencesto pressuresreported by Hazen and Finger (1979),the studiesof synthetic and natural orthosilicates).As hafnon structure correspondsmost closely to the mentioned above, hafnon is a structural equivalent 23.2and 28.9 kbar refinements.They found that the to a zircon at 29.5 kbar pressure.Using Hazen's O positional parameters in zircon did not vary (1977) concept that temperature, pressure, and between0 and 50 kbar, and therefore the changesin composition are structurally analogousvariables, interatomic distances and angles are related to the actinide orthosilicates are structural equivalents changesin sizeof the unit cell. Using their equation of hafnon and zircon at elevated temperatures. SPEER AND COOPER: SYNTHETIC HAFNON E07

of zircon (Hazen and Finger, 1979)is characteristic E o.2l of all zircon-structure silicates, there will be limited c o solid solutionof Zr and Hf in actinideorthosilicates. ,6 3 0.20 The differences in ionic radii between the smaller o g radii zircon-structuresilicates (Hf and Zr; 0.83- o- po.re 0.844) and the larger radii ones (Am-Th; 0.95- g 1.054), makesprediction of limited solid solutions c relatively easy. It remains to be seen if the size o ooz c'l I differencesamong the actinides are small enough to x trcon allow completesolid solutions. o 0.06 Acknowledgments

J.A.S. was supportedby U.S. Departmentof Energycontract DE-AC05-7EET27001awarded to J. K. Costain and Lynn Glov- er, III. B.C. was supportedby the NationalScience Foundation, Earth ScienceSection, grant EAR77-23114to P. H. Ribbe and G. V. Gibbs.The authorswant to thankthe ResearchDivision at 2.\ VPI & SU for defraying the computer costs. We appreciate a review of the paperby P. H. Ribbe.

References = Z.l Cromer,D. T. and Liberman,D. (1970)Relativistic calculations orl c aa' of anomalousscattering factors for X-rays. Journal of Chemi- o cal Physics,53, 189l-1898. Curtis, C. E., Doney, L. M. and Johnson,J. R. (1954)Some 7r propertiesofhafnium oxide, hafnium silicate, calcium hafnate, c the American Ceramic Socie- o and hafnium carbide. Journal of -o Hf ty,37, 458-465. Doyle, P. A. and Turner, P. S. (1968)Relativistic Hartree-Fock Zircon x-ray and electron scatteringfactors. Acta Crystallographica, 424,390-397. 2.1 Hofnon Durif, A. (1961)Structure du germanated'hafnium. Acta Crys- tallographica, 14, 312. Finger, L. W. (1974) Refinement of the crystal structure of 08 0.9 1.0 l.l zircon. Carnegie Institution of Washington Year Book, 73, 544-547. ionic rodius, A Finger,L. W. and Hadidiacos,C. G. (lgEl) X-ray diffractometer Fig. l. Variation of the oxygen positional parameters(a) and control system,Krisel Control, Inc. 16 Farsta Court, Rock- Me-O and Me-O' bond lengths(b) with ionic radii for the zircon- ville, Md. 20850. structure silicates:hafnon (this work), zircon (Hazen and Finger, Hazen, R. M. (1977)Temperature, pressure and composition: 1979)and thorite (Tayor and Ewing, 1978).Arrows point to the structurally analogous variables. Physics and Chemistry of predicted values for the actinide orthosilicates of Am, Pu, Np, Minerals.l. 83-94. U. and Pa. Ionic radii are from Shannon(1976). Hazen, R. M. and Finger, L. W. (1979)Crystal structure and compressibilityof zircon at high pressure.American Mineral- ogist, 64, l96J0l. Conversely, hafnon and zircon are structural equiv- Ramakrishnan,S. S., Gokhale, K. V. G. K. and Subbarao, zircon-hafnon. alents of the actinide orthosilicates at high pres- E. C. (1969) Solid solubility in the system Material ResearchBulletin. 4. 323-328. sures. If the small thermal expansion of zircon Reid, A. F. and Ringwood,A. E. (1969)Newly observedhigh- (Subbarao and Gokhale, 1968) is characteristic of pressure transformations in Mn3Oa, CaAl2O4 and ZrSiOa. hafnon, the high-temperatureactinide composition- Earth and Planetary ScienceLetters, 6, 205-208. al analoguesofhafnon and zircon would correspond Robinson,K., Gibbs, G. V., and Ribbe, P' H. (1971)The garnet. to valuesabove the meltingof zircon and decompo- structure of zircon: a comparison with American Mineralogist, 56, 782-790. sition of hafnon. This would confirm the often Salt,D. I. and Hornung,G. (l%7) Synthesisand X-ray study of observedlimited solid solutionof actinidesin zircon hafnium silicate. Journal of the American CeramicSociety, 50, and hafnon. Similarily, if the small compressibility 549-550. E08 SPEER AND COOPER: SYNTHETIC HAFNAN

Shannon,R. D. (1976).Revised effective ionic radii and system- Subbarao, E. C. and Gokhale, K. V. G. K. (1968) Thermat atic studiesof interatomic distancesin halides and chalcogen- expansionof zircon. JapaneseJournal of Applied Physics, 7, ides.Acta Crystallographica,A32,751-167. 1126. Speer,J. A. (l9E0a) Zircon.In P. H. Ribbe, Ed., Orthosilicates, Taylor, M. and Ewing, R. C. (1978)The crystal structuresof the p- 67-112' Mineralogical Society of America Reviews in ThSiO4polymorphs: huttonite and thorite. Acta Crystallogra- Mineralogy Volunie 5. phica, B34, 1074-10j9. Speer,J. A. (1980b)The actinideorthosilicates. In p. H. Ribbe, Ed., Orthosilicates, p. ll3-135: Mineralogical Society of Manuscript received, October 12, 19El; America Reviews in Mineralogy Volume 5. acceptedfor publication, March 16, 1982.