CRYSTAL-STRUCTURES OF MG12SI4O19(OH)2 (PHASE B) AND MG14SI5O24 (PHASE ANHB)

The crystal structures of two high-pressure magnesium silicates with mixed four- and six-coordinated Si have been solved and refined. The crystals were synthesized in the splitsphere, cubic anvil apparatus (USSA-2000) located at the Stony Brook High-Pressure Laboratory. Phase B, Mg12Si4O19(OH)2, crystallizes in space group P2(1)/c with 40 atoms in the asymmetric unit. Phase AnhB, Mg14Si5O24, occurs in space group Pmcb with 18 atoms per asymmetric unit. Final values of the standard R-factors were 0.056 and 0.029 for the observed reflections of phases B and AnhB, respectively. Both structures contain two types of layers, one with edge-shared Mg and Si octahedra, and the other with Mg octahedra and Si tetrahedra and the stoichiometry of humite for phase B, and forsterite for phase AnhB. Each octahedral layer is flanked by two of the tetrahedral layers, with a total of six layers per unit cell. Each Si octahedron shares all 12 edges with Mg octahedra, forming a cluster with 13 octahedral cations. This dense packing contributes to the relatively high zero-pressure densities of 3.368 and 3.435 g cm-3 for phases B and AnhB, respectively. This study also demonstrates that high-pressure materials do not always have simple structures.