ABSTRACT Large crystal pseudomorphs, composed of limestone and dolomite, that radiate upward to form centimeter- to meter-tall fans are known from every well-preserved Late Archean carbonate platform on earth. In many cases these crystal fans are an important facies, constituting as much as 50% of the observed volume of carbonate rock. Texturally, the fans are composed of elongate blades consisting of a mosaic of crystals with randomly oriented optic axes. In some pseudomorphs, trains of inclusions define the fibrous character of the precursor mineral, and the blades exhibit blunt terminations when draped by micrilic sediment. Some of the pseudomorphs contain strontium concentrations of up to 3700 ppm. Associated facies include strongly elongate giant stromatolites, hummocky cross-stratified sandstones, ooid-intraclast packstone to grainstone, small domal stromatolites, and several thinly laminated micritic facies that may display desiccation cracks. Previously, some of these crystal fans have been interpreted as calcite-and dolomite-replaced pseudomorphs after gypsum, formed under restricted conditions resulting from evaporative concentration of seawater. However, replacement textures and elevated strontium concentrations suggest that the crystal fans are more likely the result of neomorphism of large botryoids of aragonite that formed thick crusts directly on the sea floor. Furthermore, occurrence of the crystal fans in direct association with strongly elongate giant stromatolites and hummocky cross-stratified sediments suggests precipitation of the fans in open marine, wave- and current-swept environments. Although evaporation of seawater may have contributed to the growth of fans in some peritidal environments, most occurrences are not associated with any other indicators of evaporitic conditions such as halite or gypsum pseudomorphs. The reinterpretation of most reported occurrences of Late Archean gypsum pseudomorphs as aragonite pseudomorphs indicates that calcium sulfate precipitation from Late Archean seawater was rare, and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth history. Rapid aragonite precipitation rates and the paucity of calcium sulfate precipitation can be accounted for in a model for Late Archean seawater featuring, relative to present-day seawater, higher supersaturation with respect to calcium carbonate and high HC0 3 concentrations.