The crystal structure and cation ordering of Phase-X-(K1−x−n)2(Mg1−n[Al,Cr]n)2Si2O7H2x: A potential K- and H-bearing phase in the mantle

Abstract

Phase-X, a potassium di-magnesium acid disilicate, is a high-pressure synthetic compound—a potential K-bearing silicate in the mantle—with space group P 6 3 cm (no. 185), a = b = 5.028(2) A, c = 13.216(3) A, V = 289.34 A 3 , Z = 2. The structure has been determined with 1521 CCD measured intensities and refined by the least-square method to R = 0.0187. The structure is built up of octahedral MgO sheets and layers containing disilicate groups, Si 2 O 7 , (with distinct Si1 and Si2 tetrahedra linked by the apical O2 atom) alternating along the c axis. The octahedral sheet is based on a hexagonal closest-packed array of two layers of non-equivalent O atoms, O1 and O3; two-thirds of all edge-sharing M octahedra are filled. Within the framework of the Si 2 O 7 groups are channel structures parallel to [100], [010], and [110] that contain K atoms disordered in the middle of a large trigonal cavity (the A site). The FTIR spectrum in the OH stretching region shows a sharp peak at 3602 cm −1 due to OH − ordered in one anion site; the position of hydrogen, which operates in a charge-balancing substitution for the partial occupancy of the A site (K 1− x □ x ) A ↔ H x □ 1−x ) H , is undetermined. Densification in phase-X is affected by the greater compression of the empty octahedra in the octahedral layer and by constraining the trigonal A cavity containing the K atom to the size of the Si 2 O 7 disilicate group. This dense packing contributes to the relatively high zero-pressure calculated density of 3.38 g/cm 3 .