Abstract

Lamproite dikes and plugs occurring near Jordan, Montana, contain highly unusual mineral assemblages of three major types: (I) sanidine, armalcolite, and alkali amphiboles (potassium richterite and potassium riebeckite); (II) armalcolite, Ti-phlogopite, diopside, olivine (altered), and sanidine; (III) armalcolite, Ti-phlogopite, diopside, olivine (altered), analcite, and glass. These rocks are low in Al2O3, rich in TiO2, and extremely rich in K2O. They may be related in origin to the TiO2- and K2O-rich ultramafic rocks associated with the Montana diatremes.

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