Pseudoleucite in a Tinguaite from the Bearpaw Mountains, Montana

The article presents chemical analyses of the pseudoleucite and groundmass of a pseudoleucite tinguaite, chemical analyses of the principal minerals, chemical estimates of the mode of the groundmass, and both chemical and micrometric modes of the pseudoleucite. 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 agreement with the chemical mode, is nepheline 29.8 per cent, K-feldspar 66.2 per cent, acmite 3.0 per cent; the chemical mode of the groundmass is nepheline 26.8 per cent, K-feldspar 46.6 per cent, and acmite 26.7 per cent. A new analysis of a nepheline from the Bancroft area, made to test the analytical procedure, is also included.