Trace amounts of ammonium ion (0.7 mol% NH4+ substitution) have been identified from the infrared spectra of alkali feldspars from pegmatites occurring in the southern Black Hills district, South Dakota. The spectroscopic data indicate that the ammonium is structurally bound, most likely in the M site of the microcline. These results indicate that ammonium was likely a component of metamorphic fluids present during anatexis of metasedimentary rocks in the southern Black Hills district.

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