This paper describes a recently opened soda-rich feldspar deposit, the Big Tom, five miles south of Custer, South Dakota. The associated rocks consist of pre-Cambrian schists cut by pegmatite and granite dikes. The minerals found are albite, cleave-landite, microcline-perthite, quartz, muscovite, tourmaline, beryl, amblygonite and columbite. Other commercial feldspar deposits in the vicinity are Tin Mountain, Tricky Claims, and the White Elephant. The dikes in the region are probably genetically related to the Harney Peak granite batholith. Evidence for a hydrothermal phase appears absent, and therefore the pegmatite, as exposed, may have originated from the fractionation of a soda-rich magma.

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