The structure of wenkite1

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Abstract
The mineral wenkite Ba4(Ca0.9□0.1)6(Al0.4,Si0.6)20O39(OH)2(SO4)3 · nH2O 2 was found by Papageorgakis (1962) in metamorphic marbles of the Ivrea zone (Alps). The structure determination indicated that the basic symmetry is P31m, however, by perfect submicroscopical twinning on (0001) or positional disorder, physical properties and diffraction effects show P[unk]2m symmetry (a = 13.515 Å, c = 7.465 Å) This suggests that wenkite might possess a domain structure. Wenkite has an interrupted Al/Si framework with the same hexagonal building units which are found in cancrinite and gmelinite. They are linked through an additional tetrahedron which is only bonded with three corners to the framework. In between these hexagonal cavities, which contain Ba(1), larger highly distorted cavities are formed which contain Ca and Ba(2)SO4. A chain of BaSO4 molecules extends parallel to the z axis in the center of these cavities which are defined by eight-membered TO4 rings. The framework contains systems of channels parallel to the z axis with 6- and 8-membered rings and a system of kinked channels parallel to the x axis with 10-membered rings.

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