Sepiolite (2MgO.3SiO 2 .4H 2 O) occurrences in 4 mining districts were investigated by field and laboratory techniques to determine the geological environment. In the Maxfield Mine, Big Cottonwood Canyon, Salt Lake County, Utah, sepiolite occurs in carbonates of the Madison Formation (Mississippian) as paper-thin, white veinlets in a larger ore vein containing pyrite, galena, sphalerite, calcite, and quartz. White to tan sepiolite, calcite, and chalcedony, in veins up to 6 in. wide, occur in an unnamed Permian carbonate formation in the Ferber Mining district, Elko County, Nevada. In the Gold Hill, Utah, mining district, the sepiolite occurs within the Ochre Mountain Limestone [Mississippian] in the Cane Springs Mine, where the sepiolite is associated with sulfide ore minerals in a silicated limestone matrix. In the Mineral Range, Beaver County, Utah, white sepiolite, in veins up to 1 in. wide, is found with calcite and quartz in a carbonate bed of the Kaibab Formation (Permian). The analytical data were obtained on the sepiolite specimens by chemical, X-ray diffraction, infrared-absorption, and optical analyses. Similarities in the sepiolite occurrences, widely separated geographically, indicate a common mode of genesis. In all cases where field relations could be studied, the following similarities were noted: 1) the host rock is a carbonate, partly or wholly dolomite; 2) calcite and quartz and/or opal occur with the sepiolite veins; 3) intrusive rocks are found in the vicinity; 4) sulfide minerals are present with the sepiolite or are found in the area. It is believed the sepiolites were formed by low-temperature hydrothermal solutions during the last stages of the sulfide-mineral deposition in carbonate beds.