GEOLOGY OF NATURALLY OCCURRING ASBESTOS IN CALIFORNIA

California has experienced a rising concern over potential public exposure to naturally occurring asbestos (NOA) in recent years. Consequently, geologists in California are increasingly being called upon to evaluate the NOA potential of property prior to land-use decisions, land acquisition, or property development. While economic deposits of the asbestos minerals are rare, small non-economic occurrences of chrysotile or amphibole asbestos, which may be of environmental concern, may be present in a variety of rock types and geologic environments around the State.

In California, NOA is most commonly associated with serpentinite and serpentinized ultramafic rocks. Other rock types in which NOA has been reported to occur in California include: gabbro, granite, schist, limestone, dolomite, shale, slate, amphibolite, albitite, and nickel laterite. The occurrence of asbestos in these rock types is less common than the occurrence in serpentinite/ultramafic rocks.

Chrysotile occurs most commonly in serpentinites and serpentinized ultramafic rocks but may also occur in altered or metamorphosed mafic rocks or in metamorphosed carbonate rocks. The amphibole asbestos minerals (tremolite, actinolite, anthophyllite, riebeckite, and cummingtonite-grunerite) also occur in and immediately adjacent to serpentinites and serpentinized ultramafic rocks and in a variety of other metamorphosed rocks including metamorphosed mafic plutonic rocks, mafic volcanic rocks, ironstones, iron-rich cherts, carbonate rocks, and granitic rocks. In many of these occurrences, NOA may be more likely to found at geologic boundaries or in fault or shear zones where fluid flow has been enhanced. Any rock that has a chemical composition that would allow the formation of amphibole or serpentine minerals may contain asbestos if physical conditions have been favorable at some point in the rock's history.

In addition to occurring in a variety of metamorphic rocks, asbestos minerals may also occur in sedimentary rocks, soils, or sediments derived from asbestos containing parent materials.