The mineral weddellite (calcium oxalate dihydrate) is rarely reported in sediments although it is common in biologic materials including plants, forest soils, and the urinary stones of mammals. Research, designed to evaluate the factors influencing the fuel-grade potential of peat deposits in Florida, has now documented the occurrence of weddellite as a primary constituent in the inorganic (ash) fraction of peat and other organic-rich soils. Weddellite is believed to originate within the peat-forming basin rather than as a product of sample storage or laboratory preparations. The mineral's origin is genetically related to the metabolic activity of abundant plant life at the depositional site, and its concentration in the basin is directly related to the percentage of organic matter preserved in the sediments. Scanning electron micrographs reveal that the mineral's characteristic prismatic needle shape is preserved in peat. Differential thermal analysis shows initial-water-loss endotherms at approximately 100 degrees C and 200 degrees C, followed by exothermic transformations to CaCO 3 at 300 degrees C and to CaO near 780 degrees C.