Summary: A 230 g kaersutite xenocryst from basalt at San Carlos, Arizona, has the composition SiO₂ 40·10 %, TiO₂ 4·44, Al₂O₃ 14·53, Fe₂O₃ 3·24, FeO 10·05, MnO 0·14, MgO 11·00, CaO 11·06, Na₂O 2·99, K₂O 1·62, H₂O + 0·73, H₂O − 0·10; D = 3·28; α 1·680, β 1·700, γ 1·715; a 9·858 Å, b 18·063 Å, c 5·315 Å, β 105° 14′. Kaersutite occurring as xenocrysts in basic volcanic rocks and tuffs, and as a phase in garnet-pyroxene xenoliths from such rocks, is probably of upper mantle origin, and may be the important potassium-bearing phase in this region.