Mineralogical Magazine, June 2010, Vol. 74(3), pp. 567–575 The crystal structure of zigrasite, MgZr(PO4)2(H2O)4, a heteropolyhedral framework structure 1, 2 F. C. HAWTHORNE * AND W. B. SIMMONS 1 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada 2 Department of Earth and Environmental Sciences, University of New Orleans, New Orleans, Louisiana 70148, USA [Received 28 April 2010; Accepted 1 July 2010] ABSTRACT ˚ The crystal structure of zigrasite, ideally MgZr(PO4)2(H2O)4, a 5.3049(2), b 9.3372(4), c 9.6282(5) A, ˚ 3 À3 a 97.348(1)º, b 91.534(1)º, g 90.512(4)º, V 472.79(5) A , Z = 2, triclinic, P1, Dcalc. 2.66 g.cm , from the giant 1972 pocket at Newry, Oxford County, Maine, USA, has been solved and refined to R1 3.75% on the basis of 2623 unique reflections (Fo >4sF). There are two P sites, each of which is solely occupied by P with <PÀO> distances of 1.532 and 1.533 A˚ , respectively. There are two Mg sites, both of which are occupied by Mgand are octahedrally coordinated two O anions and four (H 2O) groups with <MgÀO> distances of 2.064 and 2.075 A˚ , respectively. There is one Zr site, occupied by Zr and ˚ octahedrally coordinated by six O anions with a <ZrÀO> distance of 2.065 A. The (ZrO6) octahedron shares corners with six (PO4) tetrahedra, forminga [Zr(PO4) 2] sheet parallel to (001). These sheets are stacked in the c direction and linked by (MgO2(H2O)4) octahedra that share O atoms with the (PO4) groups. The structure is formally a heteropolyhedral framework structure, but the linkage is weaker in the c direction, accountingfor the marked (001) cleavage. KEYWORDS: zigrasite, phosphate, crystal structure, Newry pegmatite, Maine, malhmoodite Introduction Experimental details ZIGRASITE, MgZr(PO4)2(H2O)4, was described as a The single crystal of zigrasite used in this work, a new mineral from the giant 1972 gem tourmaline- pale tan fragment with dimensions bearingpocket at the Dunton Quarry, Newry, 25 mm660 mm6100 mm, was removed from a Oxford County, Maine, USA, by Hawthorne et al. polished thin section. Electron microprobe analysis (2009). It occurs as complex aggregates of three subsequent to collection of the X-ray intensity data distinct phases, zigrasite and two unnamed showed it to be uniform and close to end-member phases: the Ca analogue of zigrasite, CaZr(PO4)2 composition. The crystal used for structure (H2O)4, and Zr(PO3OH)2(H2O)4. Zigrasite is determination was analysed, usinga Cameca SX- associated with tourmaline, microcline, quartz, 100 electron-microprobe, by Hawthorne et al. albite, beryl, amblygonite-montebrasite, chil- (2009). Determination of the crystal structure (see drenite-eosphorite and apatite, and crystallized below) shows that zigrasite contains four (H2O) as one of the last minerals duringpocket groups per formula unit, and the empirical formula formation. for zigrasite was calculated on the basis of 12 anions including4 (H 2O) groups per formula unit: 2+ (Mg0.97Fe0.01Zn0.01)S=0.99(Zr0.99Hf0.01)S=1.00 P2.00O8(H2O)4,ideallyMgZr(PO4)2(H2O)4. * E-mail: [email protected] DOI: 10.1180/minmag.2010.074.3.567 # 2010 The Mineralogical Society F. C. HAWTHORNE AND W. B. SIMMONS Data collection and crystal-structure ref|nement four O anions with <PÀO> distances of 1.532 and 1.533 A˚ , respectively, close to the grand <PÀO> The single crystal of zigrasite was mounted on a distance in minerals of 1.537 A˚ given by Bruker AXS SMART APEX diffractometer Huminicki and Hawthorne (2002). There are equipped with a CCD detector and usingMo- Ka two Mg sites, both of which are occupied solely radiation. The intensities of 5497 reflections were by Mgand are octahedrally coordinated by two O collected to 60º2y using30 s per 0.2º frame, and an anions and four (H2O) groups with <MgÀO> empirical absorption correction (SADABS, distances of 2.064 and 2.075 A˚ , respectively. Sheldrick, 1998) was applied. The refined unit- There is one Zr site, solely occupied by Zr and cell parameters for the triclinic cell (Table 1) were octahedrally coordinated by six O anions with a obtained from 6768 reflections with I >10s.The <ZrÀO> distance of 2.065 A˚ . crystal structure of zigrasite was solved by direct There are 12 anion sites in the structure of methods with the Bruker SHELXTL Version 5.1 zigrasite (Table 2), labelled O(1) to O(12). The system of programs (Sheldrick, 1997) and refined incident bond-valence sums from the P, Mgand in space group P1¯ to R1 = 3.65% and a GooF of Zr cations in the structure to these anions fall into 1.018. Refinement of the structure was based on two groups: O(1) to O(8) lie between 1.64 and 2623 intensities of unique observed reflections (Fo 2.00 v.u. (valence units) and O(9) to O(12) lie >4sF). Scatteringcurves for neutral atoms were between 0.30 and 0.42 v.u. (Table 4). taken from the International Tables for X-ray Accordingly, O(1)ÀO(8) are assigned as O Crystallography (1992). R indices are given in anions and O(9)ÀO(12) are assigned as (H2O) Table 1, and are expressed as percentages. Details groups. In accord with this assignment, eight H of the data collection and structure refinement are atoms were located at the final stages of structure given in Table 1, final atom parameters are given in refinement and their parameters were refined with Table 2, selected interatomic distances and angles the soft constraint that the O(donor)ÀH distance in Table 3, and bond-valence values in Table 4. A be close to 0.98 A˚ . The stereochemistry of the list of observed and calculated structure factors has resulting hydrogen-bond arrangement is given in been deposited with the Principal Editor of Table 3. Mineralogical Magazine and is available from www.minersoc.org/pages/e_journals/dep_mat.html. Structure topology The Zr octahedron shares corners with six (PO4) Description of the structure tetrahedra to form a pinwheel cluster (Moore Atom coordination 1973) that we may express in its most general [6] [4] There are two P sites, each of which is solely form as [ M( TO4)6]. In zigrasite, this cluster occupied by P and tetrahedrally coordinated by forms a fragment of a [Zr(PO4)2] sheet (Fig. 1) TABLE 1. Miscellaneous information for zigrasite. a (A˚ ) 5.3049(2) Crystal size (mm) 25606100 b 9.3372(4) Radiation/monochromator Mo-Ka / graphite C 9.6282(4) No. of reflections 14396 a (º) 97.348(1) No. in Ewald sphere 5497 b 91.534(1) No. unique reflections 2766 g 90.512(1) No. Fo >5s(F) 2623 ˚ 3 V (A ) 472.79(5) Rmerge % 1.6 Space group P1¯ R1 % 3.75 Z 2 wR2 % 8.72 3 Dcalc (g/cm ) 2.66 Cell content 2 [Zr(PO4)2Mg(H2O)4] R1 = S(Fo À Fc)/SFo 2 2 Ý wR2 =[SwFo À Fc) / Sw Fo ] , w =1 568 2 TABLE 2. Atom positions and displacement paramters (A˚ ) for zigrasite. xyzUeq U11 U22 U33 U23 U13 U12 Zr 0.25362(4) 0.74773(3) 0.49040(2) 0.00794(9) 0.00537(13) 0.00899(13) 0.00943(13) 0.00079(8) À0.00038(8) À0.00084(8) P(1) 0.26500(14) 0.45008(8) 0.68108(8) 0.0129(2) 0.0104(3) 0.0144(3) 0.0137(3) 0.0016(3) À0.0005(3) À0.0010(3) P(2) 0.23232(14) 0.06111(8) 0.32252(8) 0.0123(2) 0.0102(3) 0.0132(3) 0.0133(3) 0.0014(3) À0.0003(2) À0.0013(3) THE CRYSTAL STRUCTURE OF ZIGRASITE Mg(1) 0 0 0 0.0185(3) 0.0126(7) 0.0290(8) 0.0139(7) 0.0030(6) À0.0010(5) À0.0030(6) Mg(2) Ý Ý 0 0.0199(3) 0.0179(7) 0.0242(8) 0.0168(7) 0.0004(6) À0.0017(6) À0.0002(6) O(1) 0.3139(5) 0.5903(3) 0.6181(3) 0.0234(5) 0.0225(12) 0.0209(11) 0.0280(12) 0.0073(9) 0.0006(9) À0.0004(9) O(2) 0.2402(4) 0.4846(3) 0.8389(2) 0.0197(5) 0.0173(11) 0.0272(12) 0.0140(10) 0.0010(8) À0.0005(8) 0.0020(9) O(3) 0.4845(4) 0.3458(3) 0.6475(2) 0.0195(5) 0.0165(11) 0.0217(11) 0.0205(11) 0.0026(9) 0.0025(8) 0.0055(9) O(4) 0.0220(4) 0.3765(3) 0.6176(3) 0.0228(5) 0.0159(11) 0.0254(12) 0.0254(11) À0.0014(9) À0.0035(9) À0.0062(9) O(5) 0.0048(4) 0.1546(3) 0.3686(3) 0.0219(5) 0.0169(11) 0.0233(12) 0.0248(11) À0.0003(9) 0.0040(9) 0.0035(9) O(6) 0.2539(4) 0.0557(3) 0.1641(2) 0.0188(4) 0.0136(10) 0.0286(12) 0.0145(10) 0.0043(8) 0.0009(8) 0.0032(9) 569 À À O(7) 0.4720(4) 0.1273(3) 0.3959(2) 0.0201(5) 0.0162(11) 0.0222(11) 0.0213(11) 0.0021(9) À0.0032(8) À0.0056(9) O(8) 0.1909(5) À0.0929(2) 0.3600(3) 0.0200(5) 0.0214(11) 0.0144(10) 0.0247(11) 0.0054(8) À0.0042(9) À0.0035(8) O(9) 0.2710(5) 0.8701(3) 0.8935(3) 0.0247(5) 0.0160(11) 0.0356(14) 0.0218(11) 0.0014(10) 0.0006(9) À0.0026(10) O(10) 0.1377(6) 0.1631(3) 0.8947(3) 0.0310(6) 0.0298(14) 0.0305(14) 0.0333(14) 0.0064(11) À0.0013(11) À0.0092(11) O(11) 0.2305(5) 0.3942(3) 0.1149(3) 0.0275(5) 0.0221(12) 0.0312(14) 0.0300(13) 0.0056(11) 0.0039(10) 0.0012(10) O(12) 0.3467(6) 0.6878(3) 0.0819(3) 0.0313(6) 0.0354(15) 0.0309(14) 0.0270(13) 0.0018(11) À0.0031(11) 0.0095(12) H(1) 0.436(6) 0.912(7) 0.877(7) 0.084(8)* H(2) 0.203(12) 0.819(7) 0.805(4) 0.084(8)* H(3) 0.135(13) 0.260(3) 0.947(6) 0.084(8)* H(4) 0.112(13) 0.156(7) 0.793(1) 0.084(8)* H(5) 0.075(7) 0.449(6) 0.133(7) 0.084(8)* H(6) 0.273(13) 0.360(7) 0.205(4) 0.084(8)* H(7) 0.320(13) 0.764(5) 0.022(6) 0.084(8)* H(8) 0.420(12) 0.717(7) 0.176(3) 0.084(8)* * constrained to be equal duringrefinement F.