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

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
² Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA
³ Department of Evolution of Earth Environments, Graduate School of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chu-ku, Fukuoko 810-8560, Japan

^* E-mail: frank_hawthorne{at}umanitoba.ca

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-K 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 from that 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 <M^1,2,3–O> distance is related to the mean radius of the constituent cations, <r^M1,2,3>, by the equation <<^M1,2,3–O>> = 1.4684 + 0.8553(7) <r^M1,2,3>.

KEYWORDS: anthophyllite, gedrite, orthorhombic amphibole, crystal-structure refinement, electron-microprobe analysis, Mössbauer spectroscopy, site populations

This article has been cited by other articles:

				F. C. Hawthorne, M. Schindler, Y. Abdu, E. Sokolova, B. W. Evans, and K. Ishida The crystal chemistry of the gedrite-group amphiboles. II. Stereochemistry and chemical relations Mineralogical Magazine, June 1, 2008; 72(3): 731 - 745. [Abstract] [Full Text] [PDF]