Crystal structure of anhydrous phase X, K_1.93(Mg_2.02Cr_0.02)Si_2.00O₇

Abstract

The crystal structure of anhydrous phase X with composition K_1.93(Mg_2.02Cr_0.02)Si_2.00O₇ was refined by means of X-ray diffraction analysis of a single-crystal synthesized at 16.5 GPa and 1700 °C. The structure is trigonal with space group P31m and the cell parameters are a = 5.0756 (1), c = 6.5969 (2) Å, and V = 147.179 (6) ų. The structure is closely related to those of the space group P6₃cm, which have previously been identified to have hydrous and anhydrous phase X's. However, these differ in the arrangements of Si₂O₇ groups and MgO₆ octahedra.