Unusual M3+ cations in synthetic amphiboles with nominal fluoro-eckermannite composition: Deviations from stoichiometry and structural effects of the cummingtonite component

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Unusual M3+ cations in synthetic amphiboles with nominal fluoro-eckermannite composition: Deviations from stoichiometry and structural effects of the cummingtonite component

机译：Unusual M3+ cations in synthetic amphiboles with nominal fluoro-eckermannite composition: Deviations from stoichiometry and structural effects of the cummingtonite component

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Single-crystal structure-refinement and electron-microprobe analysis of synthetic amphiboles with nominal fluoro-eckermannite composition and different trivalent cations (Al, Sc, Ti 3+ , V 3+ , Cr 3+ , Ga,) show significant deviations from nominal stoichiometry. Synthetic Sc- and Cr-bearing fluoro-eckermannite are close to nominal stoichiometry, whereas the corresponding nominal Al-, V 3+ -, Ga-, and Ti-bearing species contain very few trivalent cations and approximate Na(NaMg)Mg 5 Si 8 O 22 F 2 , a composition that has not been found in natural systems. The presence of a significant cummingtonite component strongly affects unit-cell parameters, coordination geometry around the B-, C-, and T-group sites, and cation ordering at the A-group sites. The high-charge cations are completely ordered at the M2 site, and there is a well-developed linear relationship between M2-O and the constituent-cation radius at the M2 site. The synthetic fluoro-eckermannite structure is stabilized by large spherically symmetric trivalent cations at the M2 site; for small spherically symmetric trivalent cations, it is not stable (at least at the synthesis conditions used here). Synthetic chromium-fluoro-eckermannite is stabilized by the non-spherically symmetric 3d 2 electronic arrangement, whereas Ga and V 3+ (3d 2 ) do not stabilize the fluoroeckermannite structure.

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《The American mineralogist》 |1999年第2期|102-111|共10页
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CNR-Centro di Studio per la Cristallochimica e la Cristallografia (CSCC) via Ferrata 1, I-27100 Pavia, Italy;

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.;

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia Canada V6T 1Z4;

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Unusual M3+ cations in synthetic amphiboles with nominal fluoro-eckermannite composition: Deviations from stoichiometry and structural effects of the cummingtonite component

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