Pyroxene crystals in almost monomineralic veins cutting a blueschist rock from California show sector zoning. Portions of each crystal which grew at faces cutting the c axis are sodic augite (e.g. Jd11Ac20Aug64) whereas omphacite (e.g. Jd35Ac18Aug47) formed at faces parallel to the c axis (probably at {100}, {010}, {110}). Examination by transmission electron microscopy reveals that the omphacite has a primitive lattice with antiphase domains on a scale of less than ~50Å and, in a few areas, exsolution lamellae approximately parallel to (100) of a disordered pyroxene. The sodic augite contains fine lamellae of amphibole (?) approximately parallel to (010).

I suggest that the activities of ions in the solutions from which crystallization occurred dictated a pyroxene composition between omphacite and augite but that this lay within a broad, low-temperature miscibility gap and two pyroxenes grew simultaneously. The sectoral arrangement developed during rapid crystal growth; local charge balance effects in each new layer of material controlled the composition distribution. Short-range ordering occurred during growth in directions normal to the pyroxene chains (giving omphacite) but was restricted during growth parallel to the chains (giving sodic augite).

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