Optical microscopy, transmission electron microscopy, and X-ray diffraction were used to study armenite samples from the Armen mine, Norway; from Rémigny, Quebec, Canada; and from Wasenalp, Vallis, Switzerland. All armenite samples studied can be described as cyclic twins of orthorhombic domains leading to pseudohexagonal morphology. Electron and X-ray diffraction patterns indicate various types of orthorhombic superstructures. The crystal structures of samples from Rémigny and the Armen mine were refined from single-crystal X-ray data in the space group Pnna with a = 13.874(2), b = 18.660(2), c = 10.697(1) Å, Z = 4. The structure belongs to the double-ring silicate type but deviates from hexagonal symmetry. This deviation occurs for two reasons: (1) H2O is ordered on B′ sites, which causes the lattice to be primitive orthorhombic, and Ca is sevenfold coordinated by six O atoms of the tetrahedral framework and one H2O; (2) partial (Si,Al) ordering reduces the symmetry to orthorhombic. The Pnna structural model exhibits some completely (Si,Al)-ordered tetrahedra and additional tetrahedral sites for which T-O distances indicate an Si/Al ratio of 1.

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