Pink osumilite occurs with quartz, hypersthene, orthoclase, cordierite, plagioclase, graphite, and pyrrhotite in a granulite in the contact aureole of the Precambrian Nain complex. It occurs as a discrete phase as well as with quartz and hypersthene in symplectites. The osumilite is uniaxial positive (locally anomalously biaxial), e = 1.5473±0.0002, ω = 1.5406±0.0002, a = 10.117(2) Å, c = 14.255(6) Å, and density is 2.62-2.64 g/cm3. Its composition, determined by electron probe analyses, is □IX(K0.88Na0.07Ba0.02Ca0.01)XII(Mg1.46Fe0.55Mn0.01)VI(Al2.59Mg0.39Ti0.02)IV(Si10.35Al1.65)IVO30. When its composition is compared with other osumilites and related minerals, the following prominent substitutions are apparent: Al3+⇋Fe3+; Mg2+⇋Fe2+; Na+⇋K+; Si4+ + (Mg,Fe)2+⇋2(Al,Fe)3+; and (Na,K)+ + (Al,Fe)3+⇋□ + Si4+.

Prior experimental and natural evidence has indicated that osumilite is formed only at very low pressures and probably metastably. However, in the Nain specimen textural and chemical data suggest osumilite was a stable phase and, based on P-T data for the metamorphic aureole of the Nain complex, the osumilite formed at PTotal ~ 5 kbar and T = 700°-900°C, but PH2O<< PTotal. We conclude that osumilite is stable at moderate pressure under anhydrous conditions.

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