Eclogitic pyroxenes, ordered with P2 symmetry

X-ray diffraction crystal-structure analysis of omphacite from eclogite, Tiburon Peninsula, Marin County, California, shows that this clinopyroxene has P2 symmetry with a nearly ordered distribution of the multiple cation content defined by its approximate formula: (Na_0.5Ca_0.5) (Mg_0.4Fe²+_0.1Al_0.4Fe³+_0.1)Si₂O₆. Na+ and Ca²⁺ tend to assume alternate locations in the structure, and (Mg,Fe²⁺) octahedra alternate with Al³⁺ or (Al,F³⁺) octahedra in chains along c.