THE CRYSTAL CHEMISTRY OF Li-BEARING MINERALS WITH THE MILARITE-TYPE STRUCTURE: THE CRYSTAL STRUCTURE OF END-MEMBER SOGDIANITE

The crystal structure of end-member sogdianite from the Dara-i-Pioz alkaline massif, northern Tajikistan, a 10.1240(3), c 14.3198(5) Å, V 1271.1(1) Å3, space group P6/mcc, Z = 2, has been refined to an R index of 2.0 % using 576 observed (|Fo |> 4sF) reflections collected with single-crystal diffractometer with MoKa X-radiation. Electron-microprobe analysis gives a composi-tion (Zr1.98 Hf002)S2.00 (K0.99 Na0.01)S1.00 Li2.97 Si12.01 O30. The sogdianite end-member has the typical structure of the double-ring silicates of the milarite structure-type: the A octahedron and T(2) tetrahedron are completely occupied by Zr and Li, and tetrava-lent Zr at the A site induces the lack of Na at the B site. The simplified chemical formula Zr2 K Li3 Si12 O30 corresponds to the ideal end-member formula of sogdianite (Zr,Ti4+,Fe3+,Al)2 (M,Na)2 K [Li3Si12O30] with Zr dominant at the A site and M (vacancy) dominant at the B site: Zr2 M2 K [Li3 Si12 O30].