CONDITIONS FOR CRYSTALLIZATION AND CONCENTRATION OF PEROVSKITE-TYPE MINERALS IN ALKALINE MAGMAS

In order to determine the conditions for magmatic crystallization of perovskite-type minerals, several phase diagrams have been studied with pure perovskite and complex natural loparite solid solutions. Crystallization of perovskite is limited by perovskite-sphene (titanite-titanosilicate) equilibrium. Sphene was shown to react with nepheline melt in the pseudobinary nephline-sphene system and this results in perovskite crystallization. Such reaction reveals itself in more complex systems containing the nepheline-sphene section. A large field for perovskite liquidus crystallization exists on the nepheline-diopside-sphene join and perovskite is present in the subsolidus of this pseudo-system. Addition of anorthite to the nepheline-diopside-sphene join decreases the stability of perovskite and leads to its dissolution by the melt. Experimental data on the solubility of loparite in nepheline and multicomponent lujavrite melts suggest the possibility of early magmatic crystallization of this REE and Nb-bearing mineral from some natural agpaitic magmas.