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-grey 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, Al2O3 0.93, Fe2O3 1.89, SiO2 50.10, total 100.43. The empirical formula calculated based on 6 O atoms per formulae unit is (Mg1.05Cu0.78Zn0.06Fe 3+ 0.06Ca0.03)(Si1.96Al0.04O6). Electron backscattered diffraction and powder X-This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.