The behaviour of Ti in amphiboles: I. Four- and six-coordinate Ti in richterite

Abstract

The crystal structures and site-populations of 12 richterites have been refined to R indices of 1-2% using single-crystal MoKα X-ray diffraction data. Potassium-fluor-richterites from lamproites are characterized by [4] (Si+Al) sums less than 8.0 apfu (atoms per formula unit), suggesting the presence of other isomorphous tetrahedral substituents; richterites from other environments do not show this feature. The results of the structure refinements (mean bond lengths and mean atomic numbers) and crystal-chemical arguments show [4] i 4+ to occur at the t52) site in lamproitic richterites. Some richterites also show significant amounts of Ti 4+ in octahedral coordination; combination of site occupancy refinement and stereochemical data show [6] Ti 4+ to occur almost entirely at the M(1) site in these amphiboles, locally associated with the occurrence of O 2− at the adjacent O(3) site(s)