Distribution of titanium between silicates and oxides in igneous rocks

Abstract

Ti commonly occurs in igneous rocks as sphene (or perovskite), in solution in silicates (olivines, pyroxenes), and as iron-titanium oxides. Rocks are known in which there is more Ti in pyroxenes than in the oxides. From thermodynamic considerations it appears that when silicates and iron oxides coexist Ti will occur preferentially in the oxide phases at low partial pressure of O. Entry of Ti into pyroxenes is determined by the free energy of mixing and is therefore possible only at high temperature; the higher t temperature of crystallization of the pyroxene, the more Ti it is likely to contain. Al in 4-coordination (z-sites) in pyroxenes favors introduction of Ti; Al in 6-coordination (y-sites) does not. Other things being equal, more Ti is expected in pyroxenes in undersaturated magmas (low silica activity), and in those containing but little Fe. The assemblage hypersthene-rutile is less likely than enstatite (or clinoenstatite)-ilmenite; magnetite-sphene (or magnetite-perovskite) is more likely than diopside-ilmenite at low temperature and high partial pressure of O.