English: Hematitic kaolinite from the Eocene of California, USA (4.3 cm across along the base).

Kaolinite is a clay mineral (Al2Si2O5(OH)4 - aluminum hydroxysilicate). It forms by chemical decomposition (weathering) of potassium feldspar, a common mineral in granite and granite-like igneous rocks. Pure, massive kaolinite rock is soft, powdery, and bright white-colored. A famous high quality, white kaolinite deposit occurs in the Paleocene to Eocene of the Macon, Georgia area.

The rock shown above is a deep brownish-red kaolinite from the Eocene Ione Formation of California. The red coloration is due to hematite (Fe2O3 - iron oxide). The Ione Formation contains various siliciclastic sedimentary rocks such as conglomerate, sandstone, siltstone, and claystone. This hematitic kaolinite is from the claystone portion of the unit. Light-colored (non-reddish) kaolinite also occurs in the Ione Formation. California's Ione Fm. kaolinite has long been mined as a source of clay for use in making ceramics, brick, tiles, pipes, cement, etc.

The kaolinite clays of the Ione Formation were deposited in ancient deltaic and intertidal estuarine environments. The deposits are a mixture of detrital kaolinite, broken down from originally sand-sized agglomerates of microcrystalline kaolinite, and kaolinite cement that formed between the detrital grains after deposition. The kaolinite sediments formed by erosion of and fluvial delivery from widespread lateritic paleo-oxisols (fossil soil horizons).

Stratigraphy: Ione Formation, Middle Eocene

Locality: clay pit near the town of Ione in the western foothills of the Sierra Nevada Mountains, central California, USA