The behaviour of Ti in amphiboles: I. Four- and six-coordinate Ti in richterite
Oberti, Roberta; Ungaretti, Luciano; Cannillo, Elio; Hawthorne, Frank C.
European Journal of Mineralogy Volume 4 Number 3 (1992), p. 425 - 440
published: Jun 11, 1992
manuscript accepted: Mar 7, 1991
manuscript received: Sep 2, 1991
ArtNo. ESP147050403003, Price: 29.00 €
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 (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 Ti+ to occur at the T(2) site in lamproitic richterites. Some richterites also show significant amounts of Ti4+ in octahedral coordination; combination of site occupancy refinement and stereochemical data show Ti+ to occur almost entirely at the M(l) site in these amphiboles, locally associated with the occurrence of O2- at the adjacent 0(3) site(s)