Late-magmatic biotites in nephelinites from Oahu, Hawaii contain unusually high amounts of BaO (up to 20 weight percent) and TiO2 (up to 14 weight percent) and are markedly low in SiO2 (21 to 28 weight percent) and K2O (2 to 6 weight percent). As BaO content increases, SiO2, K20, and MgO decrease and FeO and Al2O3 increase. Atomic substitution schemes indicate that Ba substitutes entirely for K, Na, and Ca in interlayer sites; Al compensates for tetrahedral Si deficiencies; and Ti is probably limited to octahedral substitution for Mg and Fe. Tetrahedral occupancy by Ti appears unlikely, although undetermined Fe3+ may compensate for tetrahedral deficiencies. A substitution scheme of the type K + 3(Mg,Fe) + 3Si = Ba + 2Ti + 3Al is suggested.

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