Crystal structure of coalingite

SUMMARY. The crystal structure of coalingite (Mg10Fe2(OH)24(COa).2H~O) has been determined using single-crystal X-ray methods. The mineral is trigonal, with space group R~m, a~t = 3"I2, ca = 37"4/~, Z = 89 and (000I) cleavage. The structure is of a layer type, and is based on a structural element about ~2'5 A thick in the c-direction and consisting of two brucite-like layers and one dis-ordered layer containing carbonate ions and water molecules and resembling those in sj6grenite and pyroaurite. The unit cell comprises three of these structural elements tacked together in the c-direction. The Mg ~+ and Fe z+ ions are randomly distributed among all the cctahedral sites of the brucite-like layers. The structure closely resembles those of sj6grenite and pyroaurite, but has two brucite-like layers between each CO~--H20 layer where these have one. There is a tendency to random interstratification, and the crystals appear to contain intergrown regions of brucite and of sj6grenite or pyroaurite. Coalingite-K probably has a similar structure ~ but with three brucite-like layers between each CO~--H~O layer; its idealized formula is probably Mg16Fe~(OH)36(COz) 9 2H20. COALINGITE forms platey crystals and has the approximate composition Mgx0Fe2(OH)24(COz).2H20. It resembles brucite, pyroaurite, and similar minerals