A middle Tertiary rhyolite tuff on the northeast side of the Patagonia Mountains in Santa Cruz County, southeastern Arizona contains lenses of calcareous low-swelling montmorillonite clay, as much as 10 to 15 m thick. The presence of the tuff has been known for years, but the clay has not been described previously. The clay lenses, which are virtually silt- and sand-free, were probably formed by diagenetic alteration of fairly clean ash-fall-tuff beds. In preliminary tests, the clay exhibited only about 9 percent shrinkage on drying and about 1 percent shrinkage on firing. Cracking and distortion were minimal in both drying and firing. Further testing needs to be done on the clay to determine its suitability as a specialty clay or as an additive to other clays.