R1: Characterization and description of minerals (Joint Session with The Gemmological Society of Japan)

[R1P-09] Proto-ferro-anthophyllite in a granitic pegmatite from Nomi Island, Hiroshima Prefecture, Japan

*Masayuki Ohnishi¹, Norimasa Shimobayashi², Daisuke Nishio-Hamane³, Shinoda Keiji⁴, Satomi Enju⁵ (1. Non, 2. Kyoto Univ., 3. ISSP, Univ. of Tokyo, 4. Osaka Metro. Univ., 5. Ehime Univ.)

Keywords:Proto-ferro-anthophyllite, Amphibole, Granitic pegmatite, Nomi Island, Etajima

Proto-ferro-anthophyllite was found in a granitic pegmatite from Nomi Island (Ogaki-cho Okimi, Etajima City), Hiroshima Prefecture, Japan. The mineral occurs as fibrous crystals up to 4 mm in length, in association with fayalite, magnetite, “biotite”, and quartz. It is white to colorless or pale brown, transparent with a vitreous luster. The refractive indices are α = 1.670(3), β = n.d., and γ = 1.704(3). SEM-EDS analyses gave the empirical formula ((Vacancy)_0.89Na_0.10K_0.01)_Σ1.00(Fe_6.24Mg_0.41Mn_0.36Ca_0.02Cr_0.01Ti_0.01)_Σ7.05(Si_7.90Al_0.10)_Σ8.00O₂₂(OH)₂ on the basis of O = 22 (anhydrous part). Selected area electron diffraction pattern indicated the Pnmn structure type. The unit cell parameters calculated from powder X-ray diffraction data are a = 9.43(3), b = 18.14(3), and c = 5.34(1) Å. It is likely that proto-ferro-anthophyllite from Nomi Island was formed by a late-hydrothermal alteration of fayalite.