The Edwin clay forms a commercially important deposit 3 miles west of Ione, California. It is a sedimentary clay of the Ione formation of Eocene age.
In the type locality the Ione sediments lie in a narrow trough between the Sierra Nevada source rocks on the east and an outlying ridge of greenstone on the west. Investigation of the clay and of accompanying quartz grains shows that the Ione clays are separable into two mineralogical groups, the Cheney Hill clays along the western side of the trough, and the Eastern Ione clays in the center and on the eastern side.
Detailed studies of inclusions in the quartz grains place the Edwin clay in the Cheney Hill group, although it contains less quartz than other clays of this type.
Electron micrographs, X-ray-and electron-diffraction patterns, chemical analyses, differential thermal analyses, firing tests, and fluorescent tests with morin dye show that the Edwin clay consists of kaolinite with appreciable amounts of gibbsite, hematite, and authigenic anatase, in addition to quartz grains and a suite of heavy minerals.
The rocks of the Sierra Nevada provided the clay, quartz, and heavy minerals. Ferruginous laterite, residual upon the greenstone, furnished the gibbsite and hematite. Much of the hematite stain in the clay was removed by organic solutions produced under stagnant-water conditions. The presence of CO2 in the solutions caused much of the iron to be precipitated in the lower part of the clay as siderite.