Abstract: An exceptionally pure illitic clay is present in outcrops of the Blaylock Formation (Silurian) in the Ouachita Mountains of southeastern Oklahoma. Unweathered claystone samples from this unit in Beavers Bend State Park have been found to consist of a well-crystallized illite or hydrous mica. Impurities commonly found in illites such as non-clay minerals and non-micaceous clay minerals are conspicuously absent. The sample cannot be classified as a mica because it is hydrated and the particle-size distribution is in the clay size range. A detailed examination of the deposit suggests that it may have value as a reference illitic clay mineral.