Baghdadite from Fuka,Okayama Prefecture,Japan

概要

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Baghdadite was found in a spurrite zone in skarns at Fuka, Okayama Prefecture, Japan. It occurs as anhedral grains up to 0.5 mm in length and prismatic crystals up to 0.6×0.4×0.2 mm, in association with gehlenite, spurrite, tilleyite, perovskite, grandite garnet and vesuvianite. The empirical formula of the mineral is (Ca3.03Na0.01)Σ3.04(Zr0.83Ti0.15)Σ0.98(Si1.99Al0.01Fe0.01)Σ2.01O9 on the basis of O=9, which is consistent with the ideal formula Ca3ZrSi2O9. The unit cell parameters are a=10.429(2), b=10.170(2), c=7.365(1)Å and β=91.01(1)°. The mineral is optically biaxial negative with refractive indices α=1.735, β=1.747, and γ=1.755, and calculated 2V=78°. The Vickers microhardness is 803 kg mm−2 (25 g load) and the density is 3.36 g cm−3. It is likely that baghdadite at Fuka was formed by a reaction of pre-existing rankinite and/or kilchoanite, and ZrO2 evolved from an intrusive igneous liquid at a high temperature.