Vanadian derbylite (vd) [102785-52-4] at the Buca della Vena mine occurs as elongate prismatic crystals, in assocn. with black anatase; in contrast, the V-free derbylite occurs as squat prismatic crystals. The vanadian derbylite contained V2O3 2.00-7.01% or 0.19-0.68 formula units V3+ per formula unit of Sb.

The refinement of the crystal structure of Theigarhorn epistilbite showed the presence of 2 tetrahedral configurations, as in the Elba dachiardite. These 2 configurations, obtained by the rotation of 2 tetrahedrons around one edge, have both C2 symmetry, with different frequency (82 and 18% resp.); thus, the resulting space group is C2. The presence of these 2 acentric domains is interpreted as a tendency to avoid the T-O-T angle of 180° present in the topol. symmetry C2/m of epistilbite. The amt. of Al, obtained from the T-O mean distance, has been cor. by a method which takes into account the influence of T-O-T angles on the measured T-O distance, and resulted in good accordance with Al content from the chem. anal. A remarkable degree of ordering in the Si/Al distribution has been found. Two extra-framework cation and 6 water mol. sites have been located. Both cation sites have a square antiprism coordination; these polyhedrons form clusters of 4 antiprisms, which are linked together as in a chessboard.