The behaviour of Mn in amphiboles: Mn in richterite
Oberti, Roberta; Hawthorne, Frank C.; Ungaretti, Luciano; Cnr, Cannillo
published: Mar 4, 1993
manuscript accepted: Aug 31, 1992
manuscript received: Apr 4, 1992
ArtNo. ESP147050501006, Price: 29.00 €
Abstract The crystal structures and site occupancies of 5 manganoan richterites have been refined to R indices ≤2% using single-crystal MoKα X-ray diffraction data. For all these amphiboles, EMP analyses indicate significant occupancy of the M(4) site by (Mn2+, Fe2+, Mg). Difference-Fourier maps show the presence of electron density within the M(4) cavity but displaced along the 2-fold axis towards the octahedral strip ; this position is designated M(4'). The local stereochemistry about the M(4') site is very similar to that around the M(4) site in the monoclinic ferromagnesian amphiboles, suggesting that the M(4') site is occupied by (Mn2+ Fe 2+, Mg). Comparison between the results of the structure refinements and of EMP analyses of the same richterite crystals indicate that the M(4') site is dominantly occupied by Mn2+. At the octahedrally-coordinated M( 1,2,3) sites, Mn is present as Mn2+ and is strongly ordered at the M(2) site. The and distances in richterites are linear functions of the amount of F- ⇄ OH- substitution at the O(3) site. The need for dimensional matching among the three independent octahedral sites suggests that extensive F" substitution is inhibited in manganoan richterites, in which the M(2) site is particularly large.