THE CRYSTAL-STRUCTURE OF SHIGAITE, [ALMN22-GROUP MINERAL((OH)(6)](3),(SO4)(2)NA(H2O)(6)(H2O)(6) A HYDROTALCITE)

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 method s and refined to an R index of 4.2% using 979 observed reflections mea sured with MoK alpha X-radiation. The structural unit of shigaite is a planar sheet of edge-sharing octahedra, [AlMn22+(OH)(6)](1+). These o xycation sheets are intercalated with oxyanion sheets of chemical comp osition Na(H2O)(6){H2O}(6)(SO4)(2); hydrogen bonding plays a major rol e 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, th e Mn2+ analogue of motukoreaite.