The crystal chemistry of the gedrite-group amphiboles. I. Crystal structure and site populations.

The crystal structures of twenty-five orthorhombic Fe-Mg-Mn amphiboles, a = 18.525–18.620, b = 17.806–18.034, c = 5.264–5.303 Å, V = 1737.6–1776.7, space group = Pnma, Z = 4, have been refined to R indices in the range 2.1–7.8% using 790–1804 unique observed reflections measured with Mo-Kα X-radiation on a Bruker P4 automated four-circle diffractometer equipped with a 1K CCD detector. The quality of the refinements is strongly a function of the [4]Al content of the crystals because of unmixing in the central part of the series due to the presence of a low-temperature solvus. The amphibole crystals were analysed by electron microprobe subsequent to collection of the X-ray intensity data and span the anthophyllite–gedrite series from 0.17–1.82 [4]Al a.p.f.u. Mössbauer spectroscopy shows that the amphiboles of this series commonly contain small but significant amounts of Fe3+. The amount of [4]Al is linearly related to the grand <T–O> distance by the equation <T–O> = 1.6214 + 0.171 [4]Al, R = 0.980; the slope of this relation is not significantly different fromthat characteristic of a hard-sphere model. The <T–O> distances indicate the following site preference for [4]Al: T1B > T2B > T1A ≫ T2A. The <M2–O> distances are compatible with all [6]Al and Fe3+ ordered at the M2 site. The grand <M1,2,3−O> distance is related to the mean radius of the constituent cations, <rM1,2,3>, by the equation ≪M1,2,3−O≫ = 1.4684 + 0.8553(7) <rM1,2,3>. [ABSTRACT FROM AUTHOR]