Recent studies of high-pressure hydrous magnesium silicates have revealed a wide variation of structures and chemistries. These materials are composed of layers of octahedra that have ''rock-salt'' topology modified by vacancies and silicon replacing magnesium, and by layers with octahedral magnesium, tetrahedral silicon and olivine-humite series stoichiometry. There are two ways to form a close-packed oxygen substructure from these types of layers. One scheme has a 1:1 ratio of the two layers and a four-unit or 9.4 ¿ repeat. The second stacking sequence has two olivine-series layers for each octahedral unit and a six-layer or 14 ¿ repeat. A general formula for the compositions of these phases is m{Mg4n+2IVSi2nO8n(OH)4}. Mg6n+4-2mod(n,2)VISin+mod(n,2)O8n+4, where mod(n,2) is the remainder when n is divided by 2; n=1, 2, 3, 4, ∞; and m=1, 2, ∞. This formula has been used to predict compositions and room pressure-temperature densities. These phases have mixed tetrahedral and octahedral coordination for silicon and could be important constituents of the mantle transition zone. ¿ American Geophysical Union 1989