Synthetic fluoro-amphiboles : site preferences of Al, Ga, Sc and inductive effects on mean bond-lengths of octahedra

The crystal structure and site populations of four synthetic amphiboles of nominal M3+-substituted fluoro-magnesiokatophorite (NaCaNaMg4M3+Si7AlO22F2, M3+ = Al, Ga, Sc) and Ga-substituted fluoropargasite (NaCa2Mg4GaSi6Al2O22F2) compositions have been refined to R indices of 1-3% using intensity data collected with MoK alpha X-radiation. The crystals used in the collection of the X-ray data were subsequently analyzed with an electron microprobe. Both techniques confirm significant deviations from the expected stoichiometry. Octahedrally coordinated trivalent cations are totally ordered at the M(2) site. In the Ga-bearing crystals, Ga and Al occur in both octahedral and tetrahedral coordination (i. e., there is Ga-Al disorder between the M and T sites); in Ga-bearing fluoro-magnesiokatophorite, tetrahedrally coordinated Ga and Al are completely ordered at the T(I) site, whereas in Ga-bearing fluoropargasite, Ga is partly disordered over T(1) and T(2), and Al is ordered at T(1). Therefore, only the fraction of Ga occurring in octahedral coordination in amphiboles should be used to calculate partition coefficients to be compared to those of the other M-[6](3+) lithophile elements of interest in geochemical studies. In the M3+-substituted fluoro-magnesiokatophorite crystals, Na-A is ordered at the A(m) site, whereas in Ga-substituted fluoropargasite, Na-A and Ca-A are disordered between A(m) and A(2), in accord with the model of Hawthorne et at. (1996). Examination of the available data on synthetic fluoro-amphiboles shows that there are significant and systematic variations in <Mg-O,F> both at M(1) and M(3) as a function of bulk composition. These variations can be rationalized as structural strain in response to misfit between the strip of octahedra and the chain of tetrahedra.