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

Abstract

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) Å3, 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 (K0.54Na0.42)Σ0.96(Ca1.95Na0.05)Σ2.00(Fe2+2.59Mg1.44Al0.67Ti0.22Cr0.05Mn0.03)Σ5.00(Si5.91Al2.09)Σ8.00O22 (OH1.94F0.06)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%.