AL-[6] DISORDER IN AMPHIBOLES FROM MANTLE PERIDOTITES

The crystal structures of 15 amphiboles (pargasite and pargasitic horn blende) from the Finero mafic-ultramafic complex, Ivrea-Verbano Format ion, Italy, have been refined to R indices of similar to 1.5% using Mo K alpha X-ray data. Site populations were assigned from the results of site-scattering refinement and electron-microprobe analysis, combined with crystal-chemical analysis. Consideration of mean bend-lengths an d chemical composition shows that these amphiboles have significant Al (up to 0.32 apfu) at the M(3) site, as well as considerable Al at the M(2) site. This is the first time that significant C-group Al has bee n observed at octahedrally coordinated sites other than M(2). There is no Al at the M(1) site; the amphibole structure seems to exert very s tringent crystal-chemical constraints to prevent this particular occup ancy, resulting from the inability of the structure to relax so as to accommodate the local bond-valence arrangement necessary for the occur rence of Al at the M(1) site. This finding of significant Al disorder over the M(2) and M(3) sites is in accord with infrared and H-2 MAS NM R spectra of synthetic pargasite in the principal OH-stretching region . The results in the case of both natural and synthetic pargasite indi cate that the main control on the degree of disorder is composition ra ther than conditions of crystallization. The significant (up to 11.5%) solid solution of a ferromagnesian amphibole component in the Finero amphiboles strongly correlates with the observed parageneses.