Abstract
The paper reports the refinement of five plagioclase structures from different geological settings. Compositions and space groups of the samples are An0 (C-1), An66 (I-1), An91 (I-1), An94 (I-1), and An94 (P-1). Emphasis is on calcic plagioclases with highly significant results for 'a', 'b', 'c', and 'd' type reflections. The structural contribution to 'b', 'c', and 'd' type superstructure reflections is investigated with Fourier analysis. The resolution of the X-ray determination of the Al, Si tetrahedral site occupancies is explored. The Na, Ca positions are refined as split sites and found to be differently occupied by Na and Ca atoms, respectively. Individual tetrahedral bond lengths are shown to be highly correlated to their structural environment. Average tetrahedral bond lengths indicate that those four tetrahedra which represent an "albite-like" structural unit in a four-ring chain extending along z in I-1 anorthite maintain their identity also at intermediate compositions while the other four non-equivalent tetrahedra gradually change their Al content from an "anorthite-like" to an "albite-like" ordering pattern.
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