STILBITE FROM BOMEKI, MIYAGI PREFECTURE, JAPAN

Abstract

A calcium-rich stilbite occurs in the amygdaloidal cavity of the Upper Miocene dacite dyke at Bomeki, Miyagi Town, Miyagi Prefecture. Chemical analysis gave SiO₂ 56.38, Al₂O₃ 15.89, CaO 7.94, Na₂O 1.08, K₂O.10, H₂O⁺16.23 and H₂O^- 1.93wt%. Fe, Mn, Mg and P were not detected. This corresponds to Na_1.00K_.06Ca_4.07Al_8.96Si_26.98O₇₂•28.98H₂O.
Cell-parameters calculated from x-ray powder data are a₀=13.43, b₀=18.21, c₀=11.21Å and β=128.9° closing to those of stilbite. The Bomeki stilbite is nearly colorless to faintly white in color and the cleavage is perfect on (010). Mohs hardness=3-4, sp. gr.=2.22 and refractive indices α=1.492, β=1.500, γ=1.502 (γ-α=0.010), all±0.002.
The amount of exchangeable cations and cations-exchangeable capacity are only 37% of the theoretical value calculated from the chemical composition.