Average structure of an An₄₈ plagioclase from the Hogarth Ranges.

The average structure (C1) of a volcanic plagioclase megacryst with composition An₄₈ from the Hogarth Ranges, Australia, has been determined using three-dimensional, single-crystal neutron and X-ray diffraction data. Least-squares refinements, incorporating anisotropic thermal motion of all atoms and an extinction correction, resulted in weighted R factors (based on intensities) of 0.076 and 0.056, respectively, for the neutron and X-ray data. The results indicate that restrictive assumptions about M sites, on which previous plagioclase refinements have been based, are not applicable to this An₄₈ and possibly not to the entire compositional range. -J.A.Z.