Cobalt atoms at M(2) site in C2/c clinopyroxenes of the system CaMgSi₂O₆ (Di)–CaCoSi₂O₆ (CaCoPx)

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Single crystals of clinopyroxenes with two different compositions in the system CaMgSi₂O₆–CaCoSi₂O₆ were synthesized under the condition where the crystals were coexiting with the melts. The compositions of the obtained crystals are [Ca_0.970Co_0.030][Mg_0.831Co_0.169]Si₂O₆ (abbreviated CaCoPx20) and [Ca_0.951Co_0.049][Mg_0.486Co_0.514]Si₂O₆ (abbreviated CaCoPx70). The crystals have the same structure as diopside (space group C2/c) with the cell dimensions a=0.97527(10), b=0.89261(8), c=0.52486(7) nm, β=105.856(9)°, Z=4, D_x=3.370(1) g·cm⁻³ in CaCoPx20, and a=0.97700(10), b=0.89395(8), c=0.52451(7)nm, β=105.720(9)°, Z=4, D_x=3.544(1) g·cm⁻³ in CaCoPx70.
The X-ray structure analyses gave unusually high residual electron densities up to about 6.9×10³ e·nm⁻³ for CaCoPx20 and 1.3×10⁴ e·nm⁻³ for CaCoPx70 at the position apart from the M(2) sites by about 0.04 nm along the b-axis in the difference Fourier maps based on the diopside model, where all of the Co²⁺ ions were located at the M(1) sites. These residual electron densities are due to partial replacement of Ca²⁺ by Co²⁺ at the M(2) sites, which amount to 3.0(4)% in CaCoPx20 and 4.9(4)% in CaCoPx70. The replacement was qualitatively confirmed by the ALCHEMI method under the condition of the (020) planar channeling.