Mixed magnesium-aluminium hydroxides. II. Structure and structural chemistry of synthetic hydroxycarbonates and related minerals and compounds

A structural scheme is proposed for the synthetic Mg-Al hydroxycarbonates described in Part I (Gastuche, Brown & Mortland, 1967) based on chemical and X-ray powder data. The structure consists of positively-charged brucite-like layers in which Al replaces Mg up to a maximum of about one in three sites. The positive charge is balanced by an interlayer sheet containing carbonate groups and water molecules. The idealized structural formula for the Al-rich material is: [Mg 4 Al 2 (OH) 12 ] 2+ [CO 3 .3H 2 O] 2− . A similar structural scheme seems to apply to many other materials including the minerals of the pyroaurite group, the compound 4CaO.Al 2 O 3 .13H 2 O and the related mineral hydrocalumite, a nickeliferous magnesium hydroxide mineral described recently and to many synthetic products that have been referred to as double sheet structures.