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Ryabchikovite, ideally CuMg(Si 2 O 6 ), 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 2 V  (meas) = 60(15)°. The average chemical composition (wt%, electron microprobe data) is: MgO 18.05, CaO 0.77, CuO 26.46, ZnO 2.23, Al 2 O 3  0.93, Fe 2 O 3  1.89, SiO 2  50.10, total 100.43. The empirical formula calculated based on 6 O atoms per formulas unit isM   g1.05C   u0.78Z   n0.06F   e0.063   +C   a0.03S   i1.96A   l0.04O6$\left(\mathrm{Mg}_{1.05} \mathrm{Cu}_{0.78} \mathrm{Zn}_{0.06} \mathrm{Fe}_{0.06}^{3+} \mathrm{Ca}_{0.03}\right)\left(\mathrm{Si}_{1.96} \mathrm{Al}_{0.04} \mathrm{O}_{6}\right)$   Electron backscattered diffraction and powder X-ray difraction show that ryabchikovite is a Cu,Mg-ordered analog of clinoenstatite. Ryabchikovite adopts the space group  P 21/ с  and 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) Å 3 , 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).

Ryabchikovite, CuMg(Si 2 O 6 ), a new pyroxene group mineral, and some genetic features of natural anhydrous copper silicates