Water-insoluble material is present mainly as thin stratigraphic layers throughout the potash ore zone(s) of the Prairie Evaporite. These clay seams constitute about 6% of the ore as mined. After clay minerals, which make up about one third of the total, the main water-insoluble constituents, in approximate order of decreasing abundance, are anhydrite, dolomite, hematite, quartz, potassium feldspar, hydrocarbon, and sporadic non-diagnostic palynomorphs.Clay mineralogy in the following mines has been studied: Cominco (Vanscoy); Central Canada Potash Co. Ltd. (Colonsay); and Potash Corporation of Saskatchewan (Allan and Lanigan). A total of 49 samples has been examined. In each sample, following concentration of clay minerals by centrifuging, X-ray diffractograms were obtained for untreated, glycolated, and heat-treated material (300, 450, 580, 650, and 725 °C). Additional runs were made on several samples under conditions of controlled humidity.The main clay minerals are Fe–Mg chlorite (14 Å (1.4 nm)), illite, and Mg-septechlorite (7 Å (0.7 nm)). Of the two chlorites, septechlorite is the more thermally stable. One or more of sepiolite, smectite, mixed layer (chlorite–smectite), and possibly traces of vermiculite are also present in some samples. The septechlorite, sepiolite, and vermiculite very likely originated as direct products of evaporation under hypersaline conditions, or are the result of diagenesis. Absence of otherwise ubiquitous septechlorite in a sample from Second Red Beds west of the 0-edge of the evaporite basin supports this concept. The proportions and kinds of clay minerals present in the ore zone(s) seem to reflect the extent to which hypersaline conditions were developed. The illite and 14 Å (1.4 nm) chlorite are of regional (detrital) origin.