Pseudoleucite in a Tinguaite from the Bearpaw Mountains, Montana

The article presents chemical analyses of the pseudoleuciteand groundmass of a pseudoleucite tinguaite, chemical analysesof the principal minerals, chemical estimates of the mode ofthe groundmass, and both chemical and micrometric modes of thepseudoleucite. The principal minerals are hedenbergitic acmite,a nepheline containing 2.3 per cent Fe₂O₃ and 7.9 per cent K₂O,and K-feldspar containing a little BaO and very little Na₂O.The micrometric mode of the pseudoleucite, in satisfactory agreementwith the chemical mode, is nepheline 29.8 per cent, K-feldspar66.2 per cent, acmite 3.0 per cent; the chemical mode of thegroundmass is nepheline 26.8 per cent, K-feldspar 46.6 per cent,and acmite 26.7 per cent. A new analysis of a nepheline fromthe Bancroft area, made to test the analytical procedure, isalso included.