The structures of the minerals alunite KAl3(SO4)2(OH)6 and crandallite CaAl3(OH)6[O½(OH)½)]2 have been investigated from a geometric standpoint. The similarity of this structural type with the pyrochlores A1+xM2O6 or A2M2O7 is shown. The framework KAl3(OH)6O6 is strongly related to the host lattice M2O6, i.e. M4O12: it is built up from distorted hexagonal tungsten bronze layers Al3(OH)6O3 similar to the M3O9 layers observed for pyrochlores, and from KO3 layers which correspond to the MO3 layers of pyrochlores. The stacking sequence of the A13(OH)6O3 and M3O9 layers is however different in both structures; it yields, due to the tilting of the octahedra, a coordination number 12 for K+ in the KO3 layers, while M of the MO3 layers is characterized by an octahedral coordination.

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