The crystal chemical configuration of an experimental subcalcic augite sample crystallized from an alkali olivine basalt at 18 kbar and 1320°C was studied by means of single-crystal X-ray diffractometry and electron microprobe analysis. The investigated sample shows an exceptionally low Ca content (0.487 atoms per formula unit, or apfu), the lowest ever recorded for a quadrilateral pyroxene of ascertained C2/c space-group symmetry. Low Ca is associated with relatively high Al (0.232 apfu) and Al (0.207 apfu) and moderate Na (0.080 apfu) contents. Thus, the cation substitution mechanism appears to be quite different from that found in natural clinopyroxene phenocrysts from volcanic rocks, as well as in megacrysts from alkaline volcanics. The uncommon chemical composition requires peculiar structural arrangements that produce an exceptionally small and distorted M2 polyhedron and an uncommonly regular tetrahedron and that involve strong cell volume reduction.
The structural variations in C2/c pyroxenes with decreasing Ca + Na content are described in detail and provide evidence of the role of structural constraints in determining the presence of the miscibility gap. High pressure and temperature conditions and rapid quenching appear to be crucial for the stability of such a distorted M2 site configuration as that shown by the studied sample. The relatively high Al and, despite the high-pressure origin, moderate Na contents arise from specific structural stability requirements.