Epidote-(Sr), CaSrAl_2Fe^<3+>(Si_2O_7)(SiO_4)(OH), a new mineral from the Ananai mine, Kochi Prefecture, Japan

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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 2Vcalc = 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 (Ca1.10Sr0.90)Σ2.00 (Al1.92Fe3+0.87Mn3+0.20)Σ2.99Si3.01O12(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) Å3, 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.
2008-12-01