Detection, crystal-chemical mechanisms and petrological implications of Ti4+ partitioning in pargasite and kaersutite
Tiepolo, Massimo; Zanetti, Alberto; Oberti, Roberta
European Journal of Mineralogy Volume 11 Number 2 (1999), p. 345 - 354
published: Apr 19, 1999
manuscript accepted: Nov 13, 1998
manuscript received: Jul 6, 1998
ArtNo. ESP147051102016, Price: 29.00 €
Abstract 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°C, P = 1.4 GPa) and quenching. Single-crystal structure-refinement and EMP + SIMS analyses performed on the same crystals show that Ti4+ 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)O2-2M(1)Mg2+-1O(3)OH--2 and M(2,3) Ti+4T(1,2)Al3+M(2,3)Mg2+-1T(1,2)Si4+-2 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 distances couple with shorter M(3)-O(1) 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 ~200 in the CSCC database, and is always related to low amounts of Al; 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.