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 (NaCa2Mg4GaSi6Al2O22F 2) 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. B oth techniques confirm significant deviations from the expected stoichiomet ry. Octahedrally coordinated trivalent cations are totally ordered at the M (2) site. In the Ga-bearing crystals, Ga and Al occur in both octahedral an d tetrahedral coordination (i. e., there is Ga-Al disorder between the M an d T sites); in Ga-bearing fluoro-magnesiokatophorite, tetrahedrally coordin ated Ga and Al are completely ordered at the T(I) site, whereas in Ga-beari ng fluoropargasite, Ga is partly disordered over T(1) and T(2), and Al is o rdered at T(1). Therefore, only the fraction of Ga occurring in octahedral coordination in amphiboles should be used to calculate partition coefficien ts to be compared to those of the other M-[6](3+) lithophile elements of in terest in geochemical studies. In the M3+-substituted fluoro-magnesiokatoph orite 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 ac cord with the model of Hawthorne et at. (1996). Examination of the availabl e 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 bu lk composition. These variations can be rationalized as structural strain i n response to misfit between the strip of octahedra and the chain of tetrah edra.