Satin spar (fibrous gypsum) veins occur in rocks overlying evaporites in the Amadeus Basin, Australia; Appalachian Basin, USA; Cheshire Basin, England; Elk Point Basin, Canada; Palo Duro Basin, USA; Paradox Basin, USA, and Zechstein Basin, England. These antitaxial veins, which are characterized by central partings and near-vertical fibers, fill horizontal and inclined fractures. Most satin spar veins occur in highly fractured or brecciated clastic strata that overlie or are associated with bedded halite, anhydrite, and gypsum. Saline springs are commonly present, indicating that halite dissolution is active. The similarities in morphology and occurrences of these veins suggest a common genesis. Satin spar veins result from several simultaneously active processes. Recharge of low-salinity surface water results in dissolution of shallow ( nearly equal 200 m to nearly equal 750 m deep) halite, development of cavernous porosity, and formation of extensional fractures in the rock column overlying salt-dissolution zones. Greater solubility of anhydrite than gypsum at low temperatures and at salinities below halite saturation causes hydration of anhydrite to gypsum, which takes place without significant volume change. Once groundwater is saturated with respect to gypsum, any further anhydrite hydration and solution must result in supersaturation, and the excess CaSO 4 carried by ground water flowing from dissolution zones precipitates as gypsum in open, high-permeability fractures.