[6]Al DISORDER IN AMPHIBOLES FROM MANTLE PERIDOTITES
ROBERTA OBERTI, FRANK C. HAWTHORNE*, LUCIANO UNGARETTI and ELIO CANNILLO
CNR Centro di Studio per la Cristallochimica e la Cristallografia, via Abbiategrasso 209, I-27100 Pavia, Italy
* Permanent address: Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2.
Abstract
The crystal structures of 15 amphiboles (pargasite and pargasitic hornblende) from the
Finero mafic-ultramafic complex, Ivrea-Verbano Formation, Italy, have been refined to R indices of 1.5%
using MoK 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 bond-lengths
and 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 been 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 stringent crystal-chemical
constraints to prevent this particular occupancy, resulting from the inability of the structure to
relax so as to accommodate the local bond-valence arrangement necessary for the occurrence 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 2H MAS NMR spectra of synthetic pargasite in the principal
OH-stretching region. The results in the case of both natural and synthetic pargasite indicate that the main
control on the degree of disorder is composition rather 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.
Keywords: amphibole, crystal-structure refinement, Al-disorder, peridotite, pargasite, Finero mafic-ultramafic
complex, Ivrea- Verbano Zone, Italy.