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The Canadian Mineralogist; June 2002; v. 40; no. 3; p. 947-960; DOI: 10.2113/gscanmin.40.3.947
© 2002 Mineralogical Association of Canada
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Articles

RECONSIDERATION OF THE CRYSTAL STRUCTURE OF PARANATISITE AND THE CRYSTAL CHEMISTRY OF [[6]M2 [4]T2 {phi}12] SHEETS

Elena Sokolova§ and Frank C. Hawthorne

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

§ E-mail address: sokolova{at}ms.umanitoba.ca

The crystal structure of paranatisite, ideally Na4 [Ti O Ti O (SiO4)2], from the Khibina–Lovozero alkaline complex, Kola Peninsula, Russia, a 9.181(2), b 4.800(1), c 9.811(2) Å, V 432.37(1) Å3 has been refined in the space group Pmc21 (Z = 2) to a residual R value of 5.1% using 651 observed (| Fo | > 4{sigma}F) reflections collected with a single-crystal diffractometer, a serial detector and MoK{alpha} X-radiation. The empirical formula, space group and crystal chemistry of paranatisite are revised. There is one unique Si tetrahedron, three unique Na octahedra and two unique Ti square pyramids with <Si–O> = 1.629, <Na–O> = 2.430 and <Ti–O> = 1.971 Å. The crystal structure of paranatisite is described in terms of seven polyhedral layers, with emphasis on the role of three types of sheets. Two sheets consist of (TiO5) square pyramids, (SiO4) tetrahedra and (NaO6) octahedra; the third sheet consists of (NaO6) octahedra and (SiO4) tetrahedra. These sheets are based on the 44 plane net in which the vertices are colored to represent occupancy by different cations and vacancies ({square}). A sheet can be characterized by the unit [[6]M2 [4]T2 {phi}12], where M generally represents [6]- or [5]-coordinated cations, T represents [4]-coordinated cations, {phi} stands for an anion (O, F, Cl, OH, H2O), and (M{phi}n) and (TO4) polyhedra share common vertices. This sheet occurs as a component in the structures of several minerals: sulphohalite, Na6 [(SO4)2] F Cl; natisite, Na2 [Ti O (SiO4)]; garnet, [[8]M3 [6]M2 T3 O12]; griphite, (Mn2+,Ca,Na,Li)24 Ca4 Fe42+ Al8 (PO4)24 F8; girvasite, Na Ca2 [Mg3 (OH)2 (PO4)2 {PO2 (OH)2} (CO3)] (H2O)4; olmsteadite, K2 [Fe42+ Nb2 O4 (H2O)4 (PO4)4]; gainesite, Na2 [Zr2 Be (PO4)4]; rhomboclase, (H5O2) [Fe (SO4)2 (H2O)2]; and several uranium minerals related to meta-autunite, Ca [(UO2) (PO4)]2 (H2O)6.

Keywords: paranatisite, crystal structure, Khibina–Lovozero complex, crystal chemistry, 44 net, mixed M—T sheet.






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