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 synttretic amphiboles of nominal lf*-substituted fluoro-magnesiokatophorite (1.{aCaNaMgaM3*Si7AlO22F2, I'i|* = N, Ga, Sc) and Ga-substituted fluoropargasite (NaCazMg+GaSia\7zonF) compositions have been refined to R indices of 1-37o using intensity data collected with MoKo X-radiation. The crystals used in the collection of the X-ray data were subsequently analyzed with an electron microprobe. Borh tschniques 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 disorderbetween theMand Zsites); in Ga-bearing fluoro-magresiokatophorite, tefahedrally coordinated Ga and Al are completely ordered at the I(1) site, whereas in Ga-bearing fluoropargasite, Ga is partly disordered over (1) and f(2), and Al is ordered at Z(l). 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 t61ll* lithophile elements of interest in geochemical studies. ln the /1y'3+-substitutedfl uoro-magnesiokatophorite crystals, ANa is ordere.datilreA(m) site, whereas in Ga-substituted fluoropargasite, ANa andACa are disordered between A(m)and A(2), in accord with the model of Hawllome et al. (1996). Examination of the available data on synthetic fluoro-amphiboles shows that there are signiflcant and systematic variations in <Mg-O,F> both at M(1) and M(3) as a function of brrlk composition. These variations can be rationalized as structural strail in response [o misfit between the strip of octahedra and the chain of tetrahedru.