Saponite from near Ballarat, California

White saponite occurs in joints and open fracture zones in metamorphosed dolomitic limestone near Ballarat, California. The saponite appears to have formed by hydrothermal alteration, possibly during Pliocene times. The material shows a 06l X-ray powder diffraction peak at 1.529 ]k, A1203 and MgO contents of about 4.4 and 23.5%, respectively, and a half-cell octahedral atoms summation value of about 2.82. The saponite appears to consist of a single, uniform clay species; the main impurities are fine shards of diopside and tremolite. The infrared and thermal properties of the Ballarat saponite are similar to those of the Allt Ribhein saponite. It has a lower water-holding capacity than montmorillonite and is characterized by lower Atterberg limits and expansion pressures and higher compaction densities. The apparent density of the saponite, 2.865 g/cm 3, is greater than that of montmorillonite. This saponite is available from the Source Clays Repository of The Clay Minerals Society. Key Words--Engineering properties, Hydrothermal alteration, Saponite, Sepiolite, Source clay, X-ray powder diffraction. I N T R O D U C T I O N While investigating a deposit of white diopside in Panamint Valley near Ballarat, Inyo County, California, adjacent bodies of white clay were observed. The material in a nearby clay pit was moist at shallow depth under extremely hot, dry conditions. The purpose of this investigation is to describe the properties of this clay, including its physical, chemical, and engineering features. The clay was described as saponite by Post (1981). Representative quantities of this clay has been added to the Source Clays Repository of The Clay Minerals Society. LOCATION A N D GEOLOGY The saponite is found about 3 miles northeast of Ballarat (Figure 1) in Sec. 26, T21S, R44E, on a steep hillside at an elevation of about 1600 ft (488 m) in South Panamint Valley. Ballarat is an abandoned gold mining center near the base of the west flank of the Panamint Range on the east side of South Panamint Playa at an elevation of ab0ut 1060 ft (323 m) in the lower Sonoran desert physiographic region (U.S.G.S. Telescope Peak Quandrangle topographic map, scale 1:62,500, 1952). Dual units are given on Figure 1 for use in conjunction with the topographic map. The geology of the area was first described by Murphy (1932); a more detailed study of the area was made by Labotka (1978). The remnants of Pleistocene Lake Panamint, which covered the clay deposits, were investigated by Smith (1976), and the clay mineral content of the bed of South Panamint Playa was discussed by Droste (1961). The saponite occurs in joints and open fracture zones in metamorphosed dolomitic limestone in the Late Cambrian Noonday Dolomite Formation near the base Copyright 9 1984, The Clay Minerals Society of the Panamint Range. The highly brecciated and crumpled limestone is light brown to buffon weathered surfaces and gray where fresh. Lenses of bladed tremolite are common. The saponite occurs chiefly in bedding plane joints, and thin, fibrous sheets of sepiolite locally occur against the upper fracture and joint surfaces. The limestone structure containing the clay minerals was submerged in Lake Panamint at least five times in the last 100,000 years (Smith, 1976), and during evaporation of the lake the limestone was subjected to surface erosion. South Panamint playa, two miles to the west, is a bolson structure; hence, all erosion products in the watershed are carried toward the lake bed. The clays in the lake bed are montmorillonite, illite, chlorite, and kaolinite (Droste, 1961); magnesium salts are extremely rare. The larger deposits of saponite are in white, surface halo zones of limestone that appear to have been leached by hydrothermal action. The clay pit is moist; there is a flowing warm spring near the deposit. There appears to have been hydrothermal replacement of metamorphosed dolomitic limestone below the weathering profile during the numerous, recent faulting and flexing episodes in the area. If the Mg source for the saponite and sepiolite were the dolomite, the alteration products should not be limited to the fracture zones but also found in the nearby playa. Magnesian clays and salts are extremely rare in the playa sediments, hence, the Mg must have been supplied by phreatic ground water. Thin sheets ofsepiolite appear to have formed at the expense of the saponite in the lake during its periods of evaporation and increased salinity between PlioPleistocene glacial episodes. Some flexible sheets extend into the saponite bodies. The saponite may have formed earlier, possibly in Pliocene times during pe-