The crystal structure of shigaite, [AlMn (super 2+) 2 (OH) 6 ] 3 (SO 4 ) 2 Na(H 2 O) 6 {H 2 O} 6 , rhombohedral, a 9.512(1), c 33.074(5) Aa, V 2591.0(8) Aa 3 , Z = 3, R3, has been solved by direct methods and refined to an R index of 4.2% using 979 observed reflections measured with MoKalpha X-radiation. The structural unit of shigaite is a planar sheet of edge-sharing octahedra, [AlMn (super 2+) 2 (OH) 6 ] (super 1+) . These oxycation sheets are intercalated with oxyanion sheets of chemical composition Na(H 2 O) 6 {H 2 O} 6 (SO 4 ) 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 Mn (super 2+) analogue of motukoreaite.

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