Stronalsite, SrNa2Al4Si4O16, a new mineral from Rendai, Kochi city, Japan.

Abstract

A new mineral stronalsite, SrNa2Al4Si4O16, is the strontium analogue of banalsite and named after the chemical composition. It is orthorhombic, Ibam or Iba2, a=8.415(4), b=9.901(4), c=16.729(9)A, Z=4. Average of five microprobe analyses is SiO2 39.13, Al2O3 32.70, CaO 0.17, SrO 15.72, BaO 2.29, Na2O 9.99, total 99.91%, corresponding to (Sr0.94Ba0.09Ca0.02)Σ1.05Na1.99Al3.95Si4.01O16 on the basis of O=16. It is white in colour with a vitreous luster and white streak. H.(Mohs)=61⁄2. No cleavage. Density (g/cm3): 2.95(meas.), 2.95(calc.). It is optically biaxial and positive, 2V=32°(meas.), dispersion indiscernible. Refractive indices: α=1.563(2), β=1.564(calc.), γ=1.574(2). Optic orientation: a=Y, b=Z, c=X. Colourless in thin section. Non-fluorescent under short and long wave ultraviolet lights.It occurs as veinlets cutting meta basic tuff xenolith enclosed in serpentinite quarried at Rendai, Kochi City, Japan, in association with slawsonite and pectolite. The formation was favoured by a silica-poor condition and concentration of strontium coming from a disintegration of basic plagioclase into such a strontium-excluding calc-silicate as grossular-hydrogrossular. A strontium-sodium-aluminum silicate from Mt. Ohsa, Okayama Prefecture (Kobayashi et al., 1984) has been proved to be the second stronalsite, in which strontium is partially replaced by barium and minor calcium.