Eclogitic Pyroxenes, Ordered with P2 Symmetry

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Science  25 Nov 1966:
Vol. 154, Issue 3752, pp. 1003-1004
DOI: 10.1126/science.154.3752.1003


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: (Nao.5 Cao.5) (Mgo.4Fe2+ 0.1 Al0.4 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.