Six cores from the Sigsbee deep in the Western Gulf of Mexico were obtained from the Lamont Geological Observatory for mineralogical studies. Two cores came from a low seaknoll that rises approximately 200 fathoms above the bottom of the deep. Two other cores are from the edge of the seaknoll, and the remaining 2 cores are from the abyssal plain surrounding the seaknoll. The cores from the abyssal plain are characterized by a series of graded layers of silt and clay, whereas those on the seaknoll do not show grading. Mineralogically these cores contain variable proportions of quartz, calcite, montmorillonite, illite, chlorite, and possibly a minor amount of kaolinite. Montmorillonite is the most abundant clay mineral present, which was unexpected because previous knowledge of the clay minerals in marine sediments indicated that illite and chlorite are the dominant clay minerals in a marine environment. The presence of large quantities of montmorillonite in the abyssal sediments may indicate that much of the montmorillonite carried into the Gulf of Mexico by rivers is not deposited on the continental shelf, but is held in suspension and deposited in deep and quiet water far from shore. Previous reports indicate that the shallow shelf areas are characterized by the presence of illite and chlorite. In general the illite and chlorite in the cores are better developed in sediments that lie below the present surface, even at a depth of a few centimeters, indicating that some crystal structure reorganization has taken place during diagenesis.