CaMgFeAlSi2O6

CaMgFeAlSi2O6 is a calcic aluminous clinopyroxene composition within the pyroxene group. It is best described as a solid-solution member of the diopside–hedenbergite–augite series, with calcium as the dominant large cation and with magnesium, iron, and aluminum occupying the octahedral M sites. Aluminum can substitute for silicon in the SiO4 tetrahedra, giving a (Si,Al)2O6 unit that accompanies the M-site cations Ca, Mg, and Fe.

In pyroxenes, the general formula is XY(Si,Al)2O6, where X is a large cation such as Ca and Y represents the octahedral sites that host Mg, Fe, and sometimes Al. The composition CaMgFeAlSi2O6 thus reflects a mixed diopside-hedenbergite-augite character, lying between the endmembers CaMgSi2O6 (diopside) and CaFeSi2O6 (hedenbergite), with aluminum contributing to tetrahedral substitution.

Geologically, CaMgFeAlSi2O6 can occur in a range of rocks. It is commonly associated with calcic plagioclase-bearing igneous rocks such as diorites, granodiorites, and basalts, and can also appear in metamorphic rocks where pyroxenes crystallize under varying temperature and pressure. It provides petrological information about crystallization histories and the oxidation state of the rock.

Physical properties of pyroxenes in this composition class are typical of clinopyroxenes: prismatic, often green to brown or black coloring depending on Fe content; vitreous luster; hardness around 5–6 on the Mohs scale; two good directions of cleavage. As a mineralogical composition, it is primarily of interest for petrology and crystallography rather than as a distinct mineral species.