Crystal structure of 2Mg ₂ SiO ₄ .3Mg(OH) ₂ , a new high-pressure structure type

Phase A[2Mg₂SiO₄·3Mg(OH)₂], a high-pressure hydrous silicate which is a possible H₂O-bearing mineral phase in the mantle, is hexagonal with a = 7.8603(2), с = 9.5730(2)A; P6₃, and Z = 2. The structure has been determined with 1653 counter-measured intensities and refined by the least-squares method to R = 0.059. The structure is based on a slightly distorted ABCB packing of anions (O²⁻, OH⁻) in which one-half of the octahedral sites are filled by Mg and one-fourteenth of the tetrahedral sites by Si. Two types of layers alternate in the structure parallel to (001), one consisting of networks of Mg octahedra and the other of blocks of three Mg octahedra. The first layer is identical to that found in welinite, Mn²⁺$Mg32+$SiO₇, and the second one is similar to a layer observed in hematolite, (Mn²⁺,Mg,Al)₁₅(OH)₂₃(AsO₃)(AsO₄)2. In phase A the chains of Mg octahedra are linked to other similar chains by Mg octahedra to form sheets. Therefore, no isolated chains of Mg octahedra are observed in the structure, though they are important constituents in the humite group minerals of composition mMg₂SiO₄·nMg(F,OH)₂ with 1 ≤ m/n ≤ 4.