Unusual nepheline syenite pegmatites, mineralogically resembling in part those of the Kola Peninsula, are found in a composite stock of Tertiary alkalic rocks in the Bearpaw Mountains, Montana. The pegmatites occur in the field as small segregation knots in nepheline syenite and as lens-shaped (compact or clefted) bodies, dikelets, and dikes in nepheline syenite or contiguous monzonites and shonkinites. The pegmatitic bodies are very numerous in an area of a few square miles.

Two partial chemical analyses of sanidine, and complete analyses of potash nepheline, lamprophyllite, and fibrous aegirite are presented with descriptions of the physical properties of these and associated minerals. An analysis is also included of a potash-barium rich nepheline syenite, interpreted to have given rise by fractionation to the pegmatitic magma.

The distribution and utilization of the rare constituents in the formation of the minerals in the pegmatites are discussed. The pegmatitic magma probably had a low volatile content, high concentration of rare constituents (Ti, Zr, Sr), and high initial temperature; and it probably crystallized rapidly at a shallow depth. These pegmatites are the first expression of the difierentiation history of the nepheline syenites, and are followed by more volatile-rich pegmatites and complex veins.

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