Crystal structure of Cr-bearing Mg(3)BeAl(8)O(16), a new polytype of magnesiotaaffeite-2N′2S

The crystal structure of a new polytype of magnesiotaaffeite-2N′2S, ideally Mg(3)BeAl(8)O(16) (trimagnesium beryllium octa­aluminium hexa­deca­oxide), is described in space-group symmetry P-3m1. It has been identified in a fragment of a mineral sample from Burma (Myanmar). The new polytype is composed of two Mg(2)Al(4)O(8) (S)- and two BeMgAl(4)O(8) (N′)-modules in a stacking sequence N′SSN′′ which differs from the N′SN′S-stacking sequence of the known magnesiotaaffeite-2N′2S polytype. The crystal structure can be derived from a close-packed arrangement of O atoms and is discussed with regard to its polytypism and its Cr(3+) chromophore content.