Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXI, No.4


Borosilicates (Datolite, Schorl) and Alumosilicates (Andalusite, Sillimanite) in the Oketo Rhyolite, Hokkaido

Watanabe, Jun;Hasegawa, Kiyoshi

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The second occurrence of datolite in Hokkaido, after the first report from the Furano mine (Sako et al., 1957; Sato et al., 1967), is herein described from the Oketo Rhyolite (Watanabe et al., 1981a, 1981b), Kitami Province, Hokkaido. Datolite from the Oketo Rhyolite occurs in the small veinlets cutting the metasomatic facies of the rhyolite of the Miocene intrusion. Optical data of the datolite are: α=1.625, β=1.652, γ=1.669. 2Vx=71.0-72.0°. Lattice constants are: a=9.62Å, b=7.60Å, c=4.83Å, c/a=0.50, β=90°11'. X-ray data is shown in Table 1. Chemical data are: Ca4.02B4.11Si3.90O15.96(OH)4.04 to Ca4.23B3.68Si4.11O15.93(OH)4.07 on the basis of 20(O, OH), namely as the empirical formula is given Ca1.00-1.06B1.03-0.92Si0.97-1.03O3.99-3.98(OH)1.01-1.02 for the formula of datolite; CaBSiO4(OH). The mineral is associated with dravite-schorl tourmaline including minor content of uvite molecule, andalusite and sillimanite with a very small quantity of fluorite and topaz, showing the mode of occurrence different quitely from any of the known cases in Japan and world. Also, some data of the associated minerals are presented.