Mineralogical Magazine

Tile crystal structures of sjogrenite and pyroaurite, two stacking modifications of approximate composition Mg.Fe.(OH),.(CO.).4H.O, have been determined by X-ray diffraction using three-dimensional methods. Sjogrenite is hexagonal, with a 3,13, C 15.66 A, space group P6.!mmc, Z = !; pyroaurite is rhombohedral, with aH 3,13, CH 23.49 A, space group R3m or R3m, Z = i. Both structures are based on brucite-like layers, with magnesium and iron distributed among the octahedral positions. The cations appear to be largely disordered, although ordered regions may occur in some crystals. Between the brucite-like layers are the water molecules and carbonate groups. These are statistically arranged, with their oxygen atoms distributed among a larger number of possible sites.