Abstract
The biotite crystals described in this paper are found as an ingredient mineral of hornblende biotite granite exposed on Myogo-ho, Kawasaki-machi, Shibata-gun, Miyagi Prefecture. They are also contained abundantly in the sand derived from the weathered rock. These crystals occur as six-sided short prisms or plates. Hardness: 2.5-3. Colour: brownish black with brownish streak. Indices of refraction: α=1.620-1.621, β=1.658, γ=1.658-1.659. Further, they are characterized by the small optic angle ?? (-)2V ?? 0° ?? and weak dispersion (υ>ρ). A chemical analysis gave the following result for this mineral: SiO2 33.92, TiO2 2.91, Al2O3 13.88, Fe2O3 3.39, FeO 26.66, MnO 1.35, MgO 4.86, CaO 0.75, K2O 7.46, Na2O 1.70, H2O (+) 3.53, H2O (-) 0.16, total 100.57%. Moreover, the X-ray powder diagram of this specimen agrees with that of biotite rich in ferrous iron on its octahedral layer.
