Abstract

A sodium-rich stilbite occurs in the drusy parts of a granite porphyry, Kii Peninsula, Japan. Chemical analysis gave SiO2 58.16, A12O3 17.17, MgO 0.74, CaO 4.08, Na2O 3.60, K2O 0.85, H2O- 4.65 and H2O+ 13.34. Sr, Ba, Rb and Cs were undetected. This corresponds to K0.52 Na3.30 Ca2.07 Mg0.57 Al9.57 Si26.55 O72 · 28.87 H2O.

Unit-cell constants calculated from X-ray powder data are very close to those of normal stilbite; a= 13.67, b= 18.16, c = 11.31 and β= 129°10′. The sodian stilbite is nearly colorless with brownish yellow tints, luster is vitreous to slightly pearly, and cleavage is {010} perfect. Hardness = 3.5 and Sp. Gr. = 2.18, 2V(–) =43.2, cΛX = 9°, α=1.482, β=1.489, γ=1.496, γ-α = 0.014, all ±0.002.

Hydrothermal treatment of normal calcium stilbite at temperatures between 400°C and 200°C and at water vapor pressuers between 15 and 100 atm produced wairakite at higher pressures.

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