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

Single-crystal structure-refinement and electron-microprobe analysis of synthetic amphiboles with nominal fluoro-eckermannite composition and different trivalent cations (Al, Sc, Ti3+, V3+, Cr3+, Ga), show significant deviations from nominal stoichiometry. Synthetic Sc- and Cr-bearing fluoro-eckermannite are close to nominal stoichiometry, whereas the corresponding nominal Al-, V3+-, Ga-, and Ti-bearing species contain very few trivalent cations and approximate Na(NaMg)Mg5Si8O22F2, 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 3d3 electronic arrangement, whereas Ga and V3+ (3d2) do not stabilize the fluoro-eckermannite structure.