Structural parallels between the high - pressure B phases and the leucophoenicite series

The high-pressure phases denoted ANH-B, hydrous B, and SHY-B (Finger et aI., 1991; Pacalo and Parise, 1992), containing both [4]Siand [6]Si, are members of a homologous (polysomatic) series in which the [4]Si-containing portions are cOlnposed of olivine-like (0) slabs alternating with thinner goethite-like (G) slabs in the ordered arrangements 000, OOOG, and OG. Leucophoenicite, OOOG, containing the same kind of slabs, represents the only macroscopic member of the leucophoenicite series (White and Hyde, 1983). It has the same slab sequence as the hydrous B phase; lamellar micromembers were found with 04-6G. The principal difference between the two series is that the leucophoenicite series lacks the purely octahedral layers with [6]Sithat are intercalated with the [4]Si-containing ones in the B phases. These relationships allow one to predict further members for both series. They also suggest possible homologues that are intermediate between the two series, e.g., between leucophoenicite and hydrous B and between the hypothetical OG leucophoenicite and SHY-B, and contain a smaller proportion of [6]Si-based layers than these B phases. Mn might playa stabilizing role in the B series. Hypothetical B phases based on 0 slabs alternating with norbergite-like (N) slabs are much richer in Mg and have lower H20-storing capacity.