Ryabchikovite, CuMg(Si2O6), a new pyroxene group mineral, and some genetic features of natural anhydrous copper silicates

Ryabchikovite, ideally CuMg(Si2O6), a new pyroxene-group mineral (IMA No. 2021-011) was discovered in exhalations of the active Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. The associated minerals are diopside, hematite, cuprospinel, fluorophlogopite, anhydrite, johillerite, tilasite, and aphthitalite-group sulfates. Ryabchikovite forms thin (up to 25 μm), light brown to reddish-brown epitactic crusts on short-prismatic brownish-gray crystals of diopside (up to 0.5 mm). The new mineral is optically biaxial (+), α = 1.685(5), β = 1.690(5), γ = 1.703(4), and 2V(meas) = 60(15)°. The average chemical composition (wt%, electron microprobe data) is: MgO 18.05, CaO 0.77, CuO 26.46, ZnO 2.23, Al2O30.93, Fe2O31.89, SiO250.10, total 100.43. The empirical formula calculated based on 6 O atoms per formulas unit is Mg1.05Cu0.78Zn0.06Fe0.063+Ca0.03Si1.96Al0.04O6$\left(\mathrm{Mg}_{1.05} \mathrm{Cu}_{0.78} \mathrm{Zn}_{0.06} \mathrm{Fe}_{0.06}^{3+} \mathrm{Ca}_{0.03}\right)\left(\mathrm{Si}_{1.96} \mathrm{Al}_{0.04} \mathrm{O}_{6}\right)$Electron backscattered diffraction and powder X-ray difraction show that ryabchikovite is a Cu,Mg-ordered analog of clinoenstatite. Ryabchikovite adopts the space group P21/сand has the following unit-cell parameters: a= 9.731(9), b= 8.929(8), c= 5.221(4) Å, β = 110.00(6)°, V= 426.3(7) Å3, and Z= 4. Ryabchikovite is named in honor of the outstanding Russian geochemist and petrologist Igor Dmitrievich Ryabchikov (1937–2017). Our studies reveal that copper analogs of rock-forming minerals could be found in fumarolic systems. Their crystallization does not require high temperatures or/and pressures (below 500 °C/Pa).