Abstract

Pitiglianoite, ideally Na6K2Si6Al6O24(SO4)·2H2O, is a new aluminosilicate that occurs in ejected blocks near Pitigliano, southern Tuscany, Italy. It is hexagonal, space group P63, with a = 22.121(3), c = 5.221(1) Å, and Z = 3. Pitiglianoite is colorless with a virreous luster; Dmeas = 2.37(4), Dcalc = 2.394 g/cm3. Optically it is uniaxial negative with ω = 1.508(1) and = 1.506(1).Its crystal structure has been refined to R = 0.065 using 2799 reflections obtained with Mo radiation on a four-circle automatic diffractometer. Pitiglianoite belongs to the cancrinite and vishnevite group of minerals, with a three-dimensional framework of alternating [SiO4] and [AlO4] tetrahedra with alkali cations, H2O molecules, and sulphate groups in the channels and in the cages of the structure. The cell parameters of pitiglianoite are nearly identical to those of microsommite. However, the chemical composition of the two minerals is different, according to the coupled substitution Na+ + H2O = Ca2+ + Cl between pitiglianoite and microsommite. The cell volume of pitiglianoite is three times that of vishnevite, which has a very similar chemical composition, because of the ordering of Na+ and K+ cations and (SO4)2− groups within the channels parallel to the threefold axes.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.