The Structure of Zircon: a Comparison with Garnet

THE AMERICAN MTNERALOGIST, VOL,56, MAY_JUNE, 1971

THE STRUCTURE OF ZIRCON: A COMPARISON WITH GARNET

Knrrn RouNSow, G. V. Grsns, eNn P. H. Rrnnr, Departmentof Geological,Sci,ences. Virginia PolytechnicInstitute and State U niaersity, Blacksburg, V irgi,nia 2406I

Aesrn,{cr

An anisotropic refinement of a non-metamict zircon la:6.607(I), c:5.952() A; ILt/arndi Z:4, p:4.714 gm/cc; R:0.0191 from Kragero, Norway, produced these positional parameters for oxygen: y:.0661(1); z:.1953(1) The principal structural unit is a chain of alternating edge-sharing SiO+ tetrahedra andZrOs triangular dodecahedra extending parallel to c. The chains are joined laterally by edge-sharing ZrOs dodecahedra and are responsible for zircon's prismatic habit and cleavage, its extreme birefringence and optically positive character. Similar chains occur in garnet extending in three mutually perpendicular directions, but they are cross-linked by 106 octahedra as well as by XOs dodecahedra. Oxygen in zircon is coordinated by one Si at 1.622(1) A and two Zr at 2.131and 2.268 ^. The mean bond angle at oxygen is 120o, suggesting that its orbitals are s22 hybridized, leaving a lone pair available to form a double bond with Si. In garnet oxygen is four-coordinated and the mean bond angle is = 108", suggesting sp3 hybridization with no lone pair-s available for double bonding. This may explain why the Si-O bond in garnet is =0.02 A longer than in zircon. As in garnet, the SiOagroup is a tetragonal bisphenoid elongated parallel to c, and the Si vibration ellipsoid is prolate in the same direction. Zr vibrates isotropically, but oxygen has its maximum vibration axis normal to the plane of the coordinating Si and Zr atoms. The distortions of the ZrOs triangular dodecahedron from an ideal hardsphere model can be rationalized in terms of cation-cation reoulsion across its shared edEes.

INrnonucrtoN Zircon, ZrSiO+,occurs as an accessorymineral in granitic and syenitic igneousrocks and is a common detrital componentof sedimentaryrocks. Its relativelv high density and resistanceto weatheringhave contributed to its extensiveuse in heavy mineral studies. The crystal structure of zircon was determined independently by Vegard (1926), Binks (1926), Hassel (1926), and Wyckoff and Hendricks (1927). Krstanovi6 (1958) has since refined the structure using two- dimensional methods and obtained an Si-O bond length of 1.612 A. White and Gibbs (1967) observed that this bond length was inconsistent with the SiKB wave-length shift measuredfor twelve minerals with only Si in tetrahedral coordination. This descrepancyled to the present three- dimensionalrefinement, which yielded an Si-O bond length of.1.622 A in better agreementwith the value predicted by White and Gibbs. During the course of our study, Krstanovi6, Djuri6, and Ili6 (1968) reported refinedoxygen coordinates [o:0.06a(1) ; z:0.t94(l) ] for a metamict and 782 STRUCTURE OF ZIRCON 783 a non-metamict zircon using three-dimensionaldata but they did not give cell dimensionsor bond lenqthsfor either srrucrure.

ExprulmnrAl DBt.trr-s

The zircon crystal used in this study was obtained from a syenite in Kragero, Norway. Sharp diffraction maxima on weissenberg photographs indicate that this specimen is non- metamict. This is substantiated by the fact that the cell dimensions agree with those given by Holland and Gottfried (1955) for gem-quatity, unirradiated zircon. Electron microprobe analysis indicated that the only impurity present in detectable amounts is hafnium (-1 'fhe rvt. percent). unit cell parameters are a:6.607(1); c:5.982(l) A, Z:4, p:4714 gm/cc. The space group is l\t/amd. More than 550 non-zero intensities were measured with an equi-inclination weissenberg single-crystal diffractometer using Nb-filtered Mo radiation and a scintillation counter. The intensities were recorded on a stripchart, integrated with a planimeter, corected for Lp and, absorption effects and converted to structural amplitudes. These were submitted to an isotropic least-squares calculation (Busing, Martin, and Levy, 1962) using the positional parameters for oxygen (0 0, 0.067, 0 198) determined by Krstanovi6 (1958), where the origin was chosenat 2f m. Form f actors were taken from the rnternational Tables, vol. rII for Si and o, that for zt'+ was taken from cromer and (1965) ,s Waber and corrected for the reai part of anomalous dispersion.The lFobs were weighted according to a schemeproposed by Hanson (1965), and the weights were adjusted to give approximately equal (uAF2) for ten equally populated groups of increasing 's ir(obs) | thereby making {zerAF2) essentially independent of magnitude of the individ- ual lF(obs) I's A relatively large number of low angle, high intensity reflections were rejected in the final refinement 's because of extinction effects, and the unweighted residual for 498 i 1t(obs) | was 0.020 (iveighted R:0.024). The anisotropic refinement reduced R to 0.019 (weighted R:O.022), and an 1l-factor test (Hamilton, 1965) showed that this relinenent was more significant than the isotropic one. 'Ihe finalpositional and thermal parameters are given in Tables 1 and 3. The interatomic distances and bond angles are listed in Table 2.

DtscussroN The principal structural unit in zircon is a chain of alternating edge- sharing SiOr tetrahedra and ZrOs triangular dodecahedra extending parallel to c (Fig. 1). The chains are joined laterally by edge-sharing

T.q.sI,n 1 Postrtoxer, PAnauntnns, ANrsomoprc'Irlrpnr-a.tunn Iracrors, lNo EqurvalnNr Isornoprc Tpupnrerurc Facrons lon ZrncoN

Atom r 0"2 9sa \tz 9n Fzt (equ1v../

Zr 0.O O.75 0.125 0.00096(8)30.00096(8) 0.0012(1) 0.0 0.0 0.0 0.23(1)

si 0.0 0.7s 0.62s 0.0014(1) 0 0014(1) 0.0027(3) 0.0 0.0 0.0 0.45(2)

o 0.0 0.0661(1) 0.19s3(1) 0.0037(2) 0 0031(2) o.oo29(2) 0.0 0.0 _0 0000(2) 0.s3(2)

3 Estinated standard deviations are in parenthesesand refer to the last deciml place. 784 ROBINSON,GIBBS, AND RIBBE

'Insrn 2.Ixtrn,lrourc Drsrancns (A) ,l-ltl Axcr,rs (Drcntns) rN Zrncor

S'iOa Tetrohe.d.ron si-o t4l' r.622(r)b

04 Distances Angles at Si o-o 1212.43O(2)r e7.0(1) o-o t412.7s2(2) 116.06(8)

Mear' 2.645 to9.7r

ZrO s T rian gutra+D oilecahedr on Zr-o(L) t4l 2.268(t) zr-o(B) t4l 2.r3r(r)

Mean 2.200

04 Distances Anglesat Zr o(A)-o(A')l2l 2.430(2)t 64.8(1) o(A)-(B) t8l 2.8+2(r) 80.41(2) o(A)-o(B)t4l 2.494(2)d 6e.00(s) o(B)-o(B')t413 .07| (2) 92.23(r)

M:an 2.770 78.77

Catio n-C ation Dis tances Angles ot o*ygen Zr -Zr' 3.626 (2) 111.02(s) Zr -Si 2.eer(2) ee.17(6) Zr'-Si 3.626(2) 14e.81(s)

[]" Multiplicity. 0b Estimated standard deviations refer to the last decimal place. t Edge shared between tetrahedron and dodecahedron. d Edge shared between two dodecahedra.

Tesr-n 3. Mecrrruup AND ORTENTATToNSol PnrNcrp,c'r Axns ol rrm TrrenuAl ErLrPSorDs

Angle (') with respect to: Atom rms displacement Axis A

Si rr o.Oss(s), 0 90 90 f9 0.Oss (6) 0 90 90 0.070(4) 90 90 0 (3.4)

Orr 0.072(3) 90 9r.4 (r3 .7) 1.4(13.6) r2 0.083(3) 90 r.4(r3.7) 88.6(13. 7) rl 0.0e2(3) 0 90 90

u Estimated standard deviations are in parentheses. STRUCTUKE OF ZIRCON 78s

Frc. 1. The chains of alternating edge-sharing SiOa tetrahedra andZrOs triangular dodecahedra extending parallel to c and joined laterally by edge-sharing dodecahedra. dodecahedra(y' Figs. 1 and 2) and are responsiblefor zircon's prismatic habit and (110) cleavage,its extreme birefringence and optically positive character. Similar chains occur in garnet (Fig.3), extending in three mutually perpendicular directions, but they are cross-linkedby ZOo octahedra as well as by XOs dodecahedra.In zircon octahedral voids are present but contain no cations. The structural similarities of zircon and garnet ac- count for their similar hardness,density and high refractive indices. Fro. 2. The chains of alternating edge-sharing SiOn tetrahedra arrdZrOe dodecahedra projected on (001) and showing the edgesharing between dodecahedra.

Frc. 3. A comparison of the alternating edge-sharing tetrahedra and dodecahedra chains in zircon with similar chains in sarnet. STRUCTURE OF ZIRCON 787

5t o o) (\I Zr'

G6\

Frc. 4. The coordination, interatomic distances and angles for oxygen.

Orygen'coord,inationand the Si-O bond. The_oxygen in zircon is coordinated in a olanar arrav bv one Si at 1.622A and two Zr at 2.131 and 2.268h (nig. +), and it h*. it, maximun vibration axis normal to this plane (Table 3). The length of the Si-O bond is similar to the predicted value of 1.624A for a three-coordinatedoxygen (Brown and Gibbs, 1969).The mean bond angleat oxygenis 120o,suggesting that its orbitals are sp2hybridized, leaving a lone pair perpendicular to the planar array available to form a double bond with Si. In garnet oxygen is four-coordinated by one Si, two { X2+} cations and one I Z3+] cation, suggestingsy'3 hydridization of the oxygen orbitals, with no lone pairs available for double bonding (Griffith, 1969). This may explain why the Si-O bond in garnet (Novak and Gibbs, 1971)is 0.01-0.034longer than in zircon,but fails to explain the long Si-O bond of 1.641A in topaz (Ribbe and Gibbs, 1971)where oxygen is three-coordinatedby two Al and one Si.

Cation coordination pol,yhedra.The SiOn group in garnet has site symmetry 4=Sq and in zircon42m= Dza,but both are tetragonaldisphenoids 788 ROBINSON,GIBBS, AND KIBBE

Frc. 5. The Dzatriansular dodecahedron(after Hoard and Silverton, 1963)for ZrOsin ziicon.

elongated along the S+ axis. In zircon the Si vibration spheroid is prolate along this axis in conformity with the distortions of the disphenoid (Table 3). The two O-O edges shared with the ZrOs dodecahedron are short, 2.4304, oppositeO-Si-O anglesof 97.0o.The unsharededges are 0.32A longer, opposite O-Si-O angles of 116.1" (see Table 2). The Si-Zr re- pulsion acrossshared edgesprobably accounts for the elongation of the SiOagroup along a. The ZrOs polyhedron can best be describedas a triangular dodecahedron (Hoard and Silverton, 1963). It has symmetry D2ain contrast to the dodecahedronin garnet with 222: D2 symmetry. In zircon two edgesare shared with SiO+ groups and four with other dodecahedra (2.4944). There are two sets of unshared edges, eight at 2.8424 and four at 3.0714 (seeFigure 5.) Similarly in garnet two edgesare sharedwith SiO+ tetrahedra and four with other dodecahedra,but there are onlv eight unshared edges;the remaining four are shared with FO6 octahedra. There are two non-equivalent Zr-O distances. The Zr-O bonds to the edges shared between tetrahedra and dodecahedra are both 2.268 A; STRUCTUREOF ZIRCON 789 thoseto edgesshared by dodecahedraare 2.131and 2.268A, as are those to the eight short unsharededges. The bonds to the four long unshared edgesare 2.Bl h, as expected.There are also two non-equivalentX-O distances in the dodecahedraof garnet, but two short onesare opposite the edgesshared by tetrahedra and dodecahedraand two long ones are opposite the longest unshared edges.The steric details of the dodecahedron in zircon areconsistent with Pauling's rules, but in garnet such is not the case(Novak and Gibbs, 1971).An explanationfor this discrepancy was offered by Gibbs and smith (i965) who concluded that unreasonably long Z-O bond lengths and short unshared O-O distanceswould result if Pauling'srules were satisfied.

ACKNOW LEDGE\,ENTS

The authors are grateful to the National science Foundation for support from grants GA-1133 and GA-12702. K. R. was supported in part by a National DefenseEducation Act fellowship.

RBrrnnNcns

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