Potassic-ferropargasite, a new member of the amphibole group, from Kabutoichiba, Mie Prefecture, central Japan

抄録

Potassic-ferropargasite, ideally KCa₂(Fe₄²⁺Al)Si₆Al₂O₂₂(OH)₂, is a new member of the amphibole group occurring in a schistose calcareous hornfels from Kabutoichiba, Kameyama, Mie Prefecture, central Japan. The amphibole occurs as black subhedral to anhedral crystals, up to about 700 μm long, associated with calcite, biotite, K-feldspar, plagioclase, scapolite, chlorite and titanite. Potassic-ferropargasite is optically biaxial negative with α = 1.680(2), β = 1.690 (calc.), γ = 1.698(2) and 2V = 80-90°. The mineral is monoclinic, space group C2/m, with refined unit-cell parameters a = 9.937(5), b = 18.108(5), c = 5.335(4) Å, β = 105.30(3)°, V = 926.0(9) ų, Z = 2. The three strongest lines in the X-ray powder-diffraction pattern [d in Å (I) hkl] are 2.72 (100) 151, 8.48 (81) 110 and 2.61 (59) 061. Electron-microprobe gave SiO₂ 38.08, TiO₂ 1.92, Al₂O₃ 15.12, Cr₂O₃ 0.44, FeO 19.96, MnO 0.22, MgO 6.22, CaO 11.73, Na₂O 1.57, K₂O 2.74, F 0.13, Cl not detected, O = F + Cl -0.05, total 98.08 wt%, corresponding to (K_0.54Na_0.42)_Σ0.96(Ca_1.95Na_0.05)_Σ2.00(Fe²⁺_2.59Mg_1.44Al_0.67Ti_0.22Cr_0.05Mn_0.03)_Σ5.00(Si_5.91Al_2.09)_Σ8.00O₂₂(OH_1.94F_0.06)_Σ2.00 on the basis of O = 23, assuming OH + F + Cl = 2. The crystal structure of the clinoamphibole type was refined to an R of 6.5%.