THE CRYSTAL STRUCTURE OF SHIGAITE, [AlMn2+2(OH)6]3(SO4)2Na(H2O)6{H2O}6, A HYDROTALCITE-GROUP MINERAL

MARK A. COOPER and FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2

Abstract

The crystal structure of shigaite, [AlMn2+2(OH)6]3(SO4)2Na(H2O)6{H2O}6, rhombohedral, a 9.512(1), c 33.074(5) Å, V 2591.0(8) Å3, Z = 3, R, has been solved by direct methods and refined to an R index of 4.2% using 979 observed reflections measured with MoK X-radiation. The structural unit of shigaite is a planar sheet of edge-sharing octahedra, [AlMn2+2(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.