Detection, crystal-chemical mechanisms and petrological implications of Ti-[6](4+) partitioning in pargasite and kaersutite

Fe-free kaersutites with different titanium contents were synthesised from oxide mixtures of "Fe-free basanite" and "Fe-free alkali-olivine basalt" compositions by equilibration at upper-mantle conditions (T = 1070 degrees C, P = 1.4 GPa) and quenching. Single-crystal structure-refinement and EMP + SIMS analyses performed on the same crystals show that Ti-[6](4+) enters all the three independent octahedral sites. The resulting site-population can be related to two different crystal-chemical mechanisms, which can be described by means of the (M(1))Ti4+ (O(3))O-2(2- M(1))Mg--1(2+) (O(3))OH2-- and M-(2,M-3)Ti4+ (T(1,2))Al-2(3+) (M(2,3))Mg--1(2+) (T(1,2))Si--2(4+) exchange vectors. They both increase amphibole stability at high temperatures. The presence of Ti4+ at M(3) can be now straightforwardly detected from a significant increase of the equivalent-isotropic atomic displacement-parameter and confirmed by inspection of the M(3)-O bond lengths; when Ti4+ occupies M(3), short [M(3)-O] distances couple with shorter M(3)-O(I) and longer M(3)-O(3) than expected in the presence of (M(3))Fe3+ balancing for dehydrogenation. Re-consideration of previous refinements shows that (M(3))Ti4+ occurs only in a few titanian pargasite and kaersutite among the similar to 200 in the CSCC database, and is always related to low amounts of Al-[6]; ironically: it was most important in a kaersutite from the type locality. The entrance of Ti4+ at the M(3) site is thus far less favoured than that of Al and Fe3+; this fact is most probably a consequence of steric constraints and electrostatic repulsion due to the simultaneous presence of a small tetravalent cation in all the three independent sites of the octahedral strip.