Crystal-chemical studies on the immiscibility phenomena in the NaCrSi2O6-CaMgSi2O6 pyroxene system

Abstract

The crystal structure of (Na.45Ca.55) (Cr.45Mg.55) Si2O6 pyroxene synthesized at 1200°C and 2.2GPa has been refined by the least squares method using 751 reflections measured with an automatic single crystal diffractometer. The weighted R factor is 0.030 for 642 observed reflections. The cell dimensions are: a=9.645A, b=8.821A, c=5.254A, β=106.47°, and V=428.67A3; space group C2/c. Comparison of a new crystal structure refinement for the (Na.45Ca.55) (Cr.45Mg.55)Si2O6 pyroxene with the published refinements for five clinopyroxenes in the system NaCrSi2O6-CaMgSi2O6 indicates that the O1A1-M1-O1B2 angles correlate with the M1-O1A1 distance as two different trends reflecting the repulsion effect between the O1 oxygens and the non-bonding 3d electrons of the Cr3+ ion. The repulsion effect exists in the NaCrSi2O6 side of the miscibility gap in the system NaCrSi2O6-CaMgSi2O6, in which the Cr3+ ion occupies a smaller octahedral site. On the other hand, the repulsion effect does not exist in the CaMgSi2O6 side, in which the Cr3+ ion occupies a larger octahedral site.