Rhodizite: Structure and Composition

The mineral rhodizite could not be assigned a chemical formula on the basis of chemical analyses. The solution of the crystal structure reported here reveals that CsB(12)Be(4)Al(4)O(28) is the ideal formula. The oxygen atoms occupy all but four sites per cell of cubic close packing whose period is (1/2)a. The boron, beryllium, and aluminum atoms occupy interstices of this array, the first two in tetrahedral coordination, the last in octahedral coordination. Together these atoms form a continuous network whose composition is B(12)Be(4)Al(4)O(28), which should be neutral. The alkali atom is enclosed in a cage in this network and presumably is not behaving as an ion.