Summary: The crystal structures of sjögrenite and pyroaurite, two stacking modifications of approximate composition Mg₆Fe₂(OH)₁₆(CO₃).4H₂O, have been determined by X-ray diffraction using three-dimensional methods. Sjögrenite is hexagonal, with a 3·13, c 15·66 Å, space group P6₃/mmc , Z = ¼; pyroaurite is rhombohedral, with a_H 3·13, c_H 23·49 Å, space group R3¯m or R3m, Z = ⅜. 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.