Eclogitic Pyroxenes, Ordered with P2 Symmetry

Abstract

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_o.5 Ca_o.5) (Mg_o.4Fe²+ _0.1 Al_0.4 Fe³ +_0.1) Si₂0₆. 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.