Osumilite occurs as rare, transparent, flattened, hexagonal light-blue prisms in vugs of a volcanic ejectum from Fosso Ricomero (Viterbo province, Latium, Italy) inside the basal trachytic flow of the Vico Volcanic Complex. The mineral is intimately associated with a V-rich mineral of the pseudobrookite group and hematite. The measured cell parameters are (in Å) a = 10.095(5) and c = 14.352(1). The formula derived by WD-EMPA is (Ca,Na,K)0.91(Mg,Fe,Mn)2.04 (Al,Mg)3(Si,Al)12O30. The pseudobrookite-group mineral was indexed assuming a space group of Bbmm. The cell parameters are a = 9.751(2), b = 9.956(1), and c = 3.720(1). The proposed general formula is (Fe,Mg)1±x-(Fe,Al)2±x(Ti,V,Hf)3O10.

Secondary pseudobrookite was also found in the groundmass together with sanidine, Ti-rich phlogopite, Fe-Ti oxides (titanomagnetite and titanohematite), and hercynite-rich spinel.

High oxygen fugacity (above that of hematite-magnetite), rapid crystallization, low P (less than 0.8 kbar), and high T (720-850°C) may be inferred as the conditions of formation of the osumilite + pseudobrookitea ssemblageT. he crystallization of theseu nusual minerals is ascribed to accumulation of the fluid phase in the vesicles after the crystallization of the ejectum.

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