Abstract

The crystal structures of seven members of the graftonite-beusite series, ideally (Fe2+,Mn2+,Ca)3(PO4)2, monoclinic P21/c, a 8.77–8.81, b 11.43–11.58, c 6.13–6.17 Å, β 99.19–99.32°, V 607.5–617.7 Å3, have been refined to R1 indices of 2.1–3.7% using ~1300–1600 unique observed reflections (|Fo| > 5σF) collected using a single-crystal diffractometer equipped with MoKα X-radiation. The crystals used in the collection of the X-ray data were subsequently analyzed with an electron microprobe and the structural and microprobe results were used to assign site populations. The refined site-scattering values and linear variation in mean bond-length as a function of aggregate-cation radius indicate that Ca is completely ordered at the M(1) site. Similarly Mn is ordered at the M(1) and M(3) sites, with any excess Mn occurring at M(2), and Mg is completely ordered at M(2). Detailed consideration of incident bond-valence sums at the three M sites indicates that the coordination numbers of the M(1), M(2), and M(3) sites are [8], [5], and [6], respectively, although the differences between these and [7], [5], and [5] are very small. Ca is dominant at the M(1) site in a previously refined beusite structure, and there are compositions reported here and elsewhere in which Ca is dominant at M(1) in graftonite-like compositions, indicating the potential for new mineral species in this group.

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