ORIGIN AND OCCURRENCE OF SMECTITIC CLAYS

Because most swelling clays that cause engineering problems are composed largely, or in part, of smectite-group clay minerals, special attention is given to their origin and occurrence.

Clays, including smectites, are formed mostly by alteration of other minerals and rocks, but the physical and chemical environmental requirements for alteration to form clay are only approximately known. Evidence for the conditions necessary for the formation of smectite has been obtained by studying the geologic settings of their occurrence and by their synthesis in the laboratory. Smectites are formed by alteration of silica-bearing rocks; the altering solutions are alkaline (pH above 7), magnesium-rich, and stagnant, (Keller, 1956,1957; Grim, 1953, 316-323). Silica-bearing rocks as varied as granite, basalt, serpentine, and graywacke sandstone may alter to smectite under the appropriate conditions. The smectite species formed depends on the conditions of alteration and the chemistry of the parent rock and altering solutions. Glassy rhyolitic volcanic ash is especially susceptible to alteration and is known to be the parent material of many smectite deposits.

Smectite in sedimentary clays may have formed in place, or it may have been derived from the reworking of older deposits. However, as noted by Tourtelot (1974, p. 269) "... the accumulation of nearly all the highly montmorillonitic, thick, and widespread shale units that are of recognized engineering significance.. " has resulted from deposition of volcanic ash in ocean basins. Marine smectitic clays are more abundant and extensive than those of fresh-water origin because throughout geologic history ocean basins have been the largest repositories of detrital sediments and probably have been the largest areas in which the environment was favorable for the formation of smectite. Smectitic sedimentary beds also have formed in saline alkaline lakes and playas in the deserts of the western United States (Droste, 1961; Deike and Jones, 1980). Those deposits generally are not as widespread as marine bentonite beds, but the smectite in them may be highly expansive.

The occurrence of smectite in the stratigraphic column is distinctly related to geologic age. Most clays of pre-Late Mississippian age are composed predominantly of illite and chlorite and contain only small amounts of smectite, whereas younger clays "... have a complex clay-mineral suite with montmorillonite, mixed-layer clay, and kaolinite increasing in importance" (Weaver, 1967, p. 2185). Most clays in which smectite is a major constituent are of Mesozoic and Cenozoic ages. The comparatively smaller amounts of smectite in older clays may be due to smectite having been converted to illite, or to environmental conditions unfavorable for its development.

Clay below a depth of 14,000 ft contains little or no smectite. As observed by Burst (1959) and Weaver (1959), smectitic clay minerals decrease in abundance with depth of burial, and illite increases. Evidence suggests that in the process of deep burial smectite is converted to illite. This may explain the scarcity of expandable clay minerals in formerly deeply buried shales that are now exposed in mountainous regions.