The crystal structure of shigaite, [AlMn22+(OH)(6)](3),(SO4)(2)Na(H2O)(6){H2O}(6) a hydrotalcite-group mineral

The crystal structure of shigaite, [AlMn22+(OH)(6)](3)(SO4)(2)Na(H2O)(6){H2O}(6), rhombohedral, a 9.512(1), c 33.074(5) Angstrom, V 2591.0(8) Angstrom(3), Z = 3, R (3) over bar, has been solved by direct methods and refined to an R index of 4.2% using 979 observed reflections measured with MoK alpha X-radiation. The structural unit of shigaite is a planar sheet of edge-sharing octahedra, [AlMn22+(OH)(6)](1+). These oxycation sheets are intercalated with oxyanion sheets of chemical composition Na(H2O)(6){H2O}(6)(SO4)(2); hydrogen bonding plays a major role in linkage both within the oxyanion sheet and between the structural unit and the oxyanion sheet of interstitial species. This work shows shigaite to contain essential Na and results in a significant revision of the chemical formula. Shigaite is a hydrotalcite-group mineral, the Mn2+ analogue of motukoreaite.