ЭКАНДРЮСИТ - ZNTIO3 - В ПЕГМАТИТАХ ЩЕЛОЧНО-ГРАНИТНОЙ ФОРМАЦИИ КОЛЬСКОГО ПОЛУОСТРОВА

Abstract

Dctaile studies of the ilmcnitc-group minerals in intragranite pegmatites of the alkaline-granitic formation at Kola Peninsula revealed the presence of ecandrcwsite, ZnTiOs - the rare zinc member of this group. Ecandrew-sitc occurs there in two assemblages: ecandrcwsitc-pyrophanitc and ecandrcwsitc-ilmenite ones. Irregular-shaped grains of the ecandrewsitc-pyrophanite assemblage, up to lOOmkm in size, form segregations inside quartz-zircon aggregates and along their boundaries. Those aggregates are considered as the metasomatic pseudo-morphs after some primary Zr-silicates. Individuals of the ecandrcwsite-pyrophanite assemblage are characterized by an intraphase heterogeneity reflecting the change in composition: it is asymmetrically smooth in direction «edge - center - edge», across the grain. In the whole, these variations are rather weak: from Mn-ccandrewsite to Zn-pyrophanitc, with low Fc contents. Asymmetry of distribution is the general pattern for these elements, i. e. some edges arc enriched in Zn, with a smooth reduce of its abundance, together with increase of the Mn content to the opposite edge. The subhedral tabular crystals and irregular flattened grains of the ecandrcwsitc-ilmenite assemblage, up to 15-20 mm in size, are enclosed within aggregates of hydroastrophylli-te and quartz or grouped into veinlcts. Such vcinlcts frame quite often the quartz-zircon aggregates, or, conversely, the quartz-zircon aggregates envelop the ecandrcwsitc-ilmenite segregations. Individuals of the assemblage show two types of intraphase heterogeneity. The composition changes smoothly and asymmetrically from the border toward the centre and the opposite border of the individual from ecandrcwsite to Zn-ilmenitc; but sharply and symmetrically from ilmenitc in the rims to ccandrewsitc in the narrow central zones of the tabular crystals. In the ecandrewsite-ilmcnitc assemblage compositional variations arc considerable: from Fe-ecandrewsitc to almost pure ilmenitc. Data on morphology, composition and character of mineral associations allow to propose the mechanism of their formation. The ecandrcwsitc-pyrophanitc assemblages arc considered as the earlier ones, being a product of replacement after the primary minerals (eudialytc, astrophyllitc) formed in situ. The ecandrewsi-tc-ilmcnitc assemblage is a product of later crystallization reflecting the accumulation of zinc and iron at the late hydrothcrmal stages, partly due to destruction of initial minerals.