Epidote-(Sr), CaSrAl₂Fe³⁺(Si₂O₇)(SiO₄)(OH), a new mineral from the Ananai mine, Kochi Prefecture, Japan

Abstract

Epidote-(Sr), CaSrAl₂Fe³⁺(Si₂O₇)(SiO₄)(OH), the Sr-analog of epidote, was found in the Nagakawara and Hohnomori deposits at the Ananai mine, Kochi Prefecture, Japan. It occurs in the form of prismatic crystals up to 1 cm in length in the tinzenite veins or the fine crystal aggregates in piemontite breccia. Epidote-(Sr) is optically biaxial negative, α = 1.737(2), β = 1.780 (2), γ = 1.792 (2), and 2V_calc = 62° and has perfect cleavage parallel to {001}. It exhibits pleochroism—X: pale greenish yellow, and Y and Z: pale reddish brown to brownish pink. Its calculated density is 3.74 g/cm³, and it has a Mohs' hardness of 6.5. The representative empirical formula of epidote-(Sr) from the Nagakawara deposit is (Ca_1.10Sr_0.90)_Σ2.00 (Al_1.92Fe³⁺_0.87Mn³⁺_0.20)_Σ2.99Si_3.01O₁₂(OH) on the basis of OH = 1 and O = 12 per formula unit. The mineral is monoclinic with a space group of P2₁/m, a = 8.928 (5), b = 5.652 (1), c = 10.244 (5) Å, β = 114.46 (4)°, V = 470.5 (3) ų, and Z = 2. The strongest seven lines in the X-ray powder diffraction pattern [d in Å (I/I₀) (hkl)] were 2.92 (100) (11-3), 2.58 (49) (202), 3.50 (42) (21-1), 2.61 (42) (31-1), 2.72 (41) (013), 2.83 (32) (020), and 3.26 (23) (201). Epidote-(Sr) from the Nagakawara deposit may have been formed from the residual hydrothermal fluid after the crystallization of tinzenite.