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

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 [Ca0.970Co0.030][Mg0.831Co0.169]Si2O6 (abbreviated CaCoPx20) and [Ca0.951Co0.049][Mg0.486Co0.514]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, Dx=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, Dx=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.