A tetragonal natrolite

Abstract

Tetragonal natrolite has been found as white or, less frequently as, dull pink micro-crystalline aggregates in hydrothermal veins in naujaite, at two separate localities in Ilímaussaq. The aggregates are built up of sheaf-like groups of prismatic crystals, the greatest dimension of which is about 0.01 mm. The single crystals are bounded by a single prism and either a basal pinacoid or a bipyramid. The density is 2.210 ± 0.002. The mineral is uniaxial, optically positive. n_ε = 1.4093 ± 0.001, n_α = 1.4080 ± 0.001. DTA and TOA curves of both ordinary natrolite and tetragonal natrolite are presented; tetragonal natrolite dehydrates at a slightly lower temperature than ordinary natrolite. The mineral is tetragonal, space group probably 142d. The unit cell dimensions are: a₀ = 13.0403 ± 0.001 Å, C₀ = 6.619 ± 0.001 Å, and Vol. = 2252 ų. The chemical composition of the mineral falls within the range of composition of natrolites as reported in the literature.