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 mu m), 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 (+), alpha = 1.685(5), beta = 1.690(5), gamma = 1.703(4), and 2V (meas) = 60(15)degrees. 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 formulas unit is (Mg1.05Cu0.78Zn0.06Fe0.063+Ca0.03)(Si1.96Al0.04O6). Electron backscattered diffraction and powder X-ray diffraction show that ryabchikovite is a Cu,Mg-ordered analog of clinoenstatite. Ryabchikovite adopts the space group P2(1)/c and has the following unit-cell parameters: a = 9.731(9), b = 8.929(8), c = 5.221(4) angstrom, beta = 110.00(6)degrees, V = 426.3(7) angstrom(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 degrees C/Pa).