Manganilminite and pyrophanite from syenites of Ilmeno-Vischnevogorsky alkaline complex (South Ural)

For the first time, ilmenites with maximum Mn contents were found in the alkaline rocks of the Ilmeny-Vishnevogorsk complex. Previously, these minerals were revealed in pegmatites without description of their relationships and chemical composition. The formation of manganilmenite and pyrophanite is caused by isomorphic substitution of Fe2+ by Mn due to the mobility of these elements under alkaline conditions. The results of microprobe analyses showed that ilmenites from miaskites and pyroxene-syenite pegmatoic rocks are characterized by broadly variable Mn contents (12.5 - 33.5 wt. %). Along with early crystals of Mn-bearing ilmenite the rocks host high-Mn ilmenites - manganilmenites (Ilm54-73Pph27-46) and pyrophanites (Pph48-70Ilm30-52), which form anhedral aggregates and lamellas after magnetite. The manganilmenites-pyrophanites are the latest minerals, which were formed during metasomatic substitution of Fe2+ by Ti and Mn. The primary Mn-bearing ilmenites were not altered during later metasomatic processes.