Pyroxene

The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks.

Pyroxenes have the general formula XY(Si,Al)2O6, where X represents calcium (Ca), sodium (Na), iron (Fe(II)) or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron (Fe(II) or Fe(III)).

The name pyroxene is derived from the Ancient Greek words for 'fire' (πυρ, pur) and 'stranger' (ξένος, xénos).

Their structure consists of parallel chains of negatively-charged silica tetrahedra bonded together by metal cations.

The enstatite-ferrosilite series ([Mg,Fe]SiO3) includes the common rock-forming mineral hypersthene, contains up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite (and the ferrosilite equivalents).

A related mineral wollastonite has the formula of the hypothetical calcium end member (Ca2Si2O6) but important structural differences mean that it is instead classified as a pyroxenoid.

In jadeite and aegirine this is added by the inclusion of a +3 cation (aluminium and iron(III) respectively) on the Y site.

Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, and there are several alternative schemes: In nature, more than one substitution may be found in the same mineral.