On the mineralogy and paragenesis of tugtupite Na8Al2Be2Si8O24(Cl,S)2 from the Ilímaussaq alkaline intrusion, South Greenland

Abstract

Tugtupite, Na₈Al₂Be₂Si₈O₂₄(Cl,S) ₂, is colourless to red, tetragonal, pseudocubic and forms triplet twins on {101}. c: a = 1.0269 ± 0.0003, d = 2.33 g/cm³, n_ε = 1.499 ± 0.001, n_α = 1.495 ± 0.001. a₀ = 8.637-8.643, c₀ = 8.867 - 8.870, V₀ = 662 ų. The description of the crystal structure is taken from Danø (1966). The mineral occurs in hydrothermal veins in the Ilímaussaq alkaline intrusion, South Greenland, and in the Lovozero alkaline intrusion, Kola Peninsula (beryllosodalite). A number of occurrences of tugtupite in Ilimaussaq are described. Tugtupite is most commonly associated with albite, analcime and Li-mica and is generally secondary after chkalovite. The paragenesis and origin of the mineral and a number of associated beryllium silicates are discussed.