Ferrorhodonite, CaMn₃Fe[Si₅O₁₅], a new mineral species from Broken Hill, New South Wales, Australia

Abstract

The new mineral ferrorhodonite, a Mn²⁺–Fe²⁺ ordered analogue of rhodonite with the idealized formula CaMn₃Fe[Si₅O₁₅], was found in the manganese-rich metamorphic rocks of the Broken Hill Pb–Zn–Ag deposit, Yancowinna Co., New South Wales, Australia. Ferrorhodonite occurs as brownish red coarsely crystalline aggregates in association with galena, chalcopyrite, spessartine, and quartz. The mineral is brittle. Its Mohs hardness is 6. Cleavage is perfect on {201} and good on {021} and {210}. The measured and calculated values of density are 3.71 (2) and 3.701 g cm⁻³, respectively. Ferrorhodonite is optically biaxial positive, with α = 1.731 (4), β = 1.736 (4), γ = 1.745 (5) and 2 V (meas.) = 80 (10)°. The average chemical composition of ferrorhodonite is (electron-microprobe data, wt%): CaO 7.09, MgO 0.24, MnO 32.32, FeO 14.46, ZnO 0.36, SiO₂ 46.48, and total 100.95. The empirical formula calculated on 15 O apfu (Z = 2) is Ca_0.81Mn_2.92Fe_1.29Mg_0.04Zn_0.03Si_4.96O₁₅. The Mössbauer and IR spectra are reported. The strongest reflections in the powder X-ray diffraction pattern [(d, Å (I, %) (hkl)] are: 3.337 (32) (−1–13), 3.132 (54) (−210), 3.091 (41) (0–23), 2.968 (100) (−2–11), 2.770 (91) (022), 2.223 (34) (−204), 2.173 (30) (−310). Ferrorhodonite is isostructural with rhodonite. The crystal structure was solved based on single-crystal X-ray diffraction data and refined to R ₁ = 4.02% [for 3114 reflections with I > 2σ(I)]. The mineral is triclinic, space group P $\bar{1}$, a = 6.6766 (5), b = 7.6754 (6), c = 11.803 (1) Å, α = 105.501 (1)°, β = 92.275 (1)°, γ = 93.919 (1)°; V = 580.44 (1). The crystal-chemical formula of ferrorhodonite inferred to be: ^M5(Ca_0.81Mn_0.19) ^M1−3(Mn_2.52Fe_0.48) ^M4(Fe ²⁺_0.81 Mn_0.12Mg_0.04Zn_0.03) [Si₅O₁₅]. .