Asbestos is the commercial and industrial term for a group of six naturally occurring, fibrous silicate minerals. These minerals are found in rock and soil across the globe, and their unique fiber-like structure historically made them desirable for commercial use. The natural presence of these fibers is now the focus of health concerns, as disturbance can release microscopic fibers into the air.
Geological Formation of Asbestos
Asbestos minerals are formed deep within the Earth through complex geological processes, primarily the metamorphism of rock. Metamorphism involves the alteration of existing rocks by intense heat and pressure, causing the minerals within the rock to recrystallize into new, fibrous structures.
The fundamental chemical makeup of all asbestos minerals is a hydrated silicate structure, meaning they are composed primarily of silica, oxygen, and water, with other elements like magnesium, iron, and calcium. Water plays a significant part in this formation, as hot, mineral-rich fluids circulate through cracks in the rock, aiding in the development of the thin, fibrous crystal shape.
The parent rocks for most asbestos deposits are rich in magnesium, such as ultramafic rocks or altered limestone deposits. The transformation of these magnesium-rich materials into serpentine or amphibole minerals, which are the two main groups of asbestos, often occurs near tectonic plate boundaries or fault zones.
The Six Mineral Varieties
The six minerals classified as asbestos are divided into two distinct structural groups: serpentine and amphibole. This classification is based on the arrangement of the silicate chains within the crystal lattice, which profoundly affects the physical shape of the fibers.
The serpentine group contains only one type, chrysotile, which is the most common form of asbestos used historically. Chrysotile fibers have a layered or sheet-like structure that causes them to curl, resulting in long, flexible, and often white fibers. This flexibility is why chrysotile was frequently woven into fabrics and used in a wide range of products.
The amphibole group includes the other five regulated types: crocidolite, amosite, anthophyllite, tremolite, and actinolite. Amphibole minerals feature a double-chain silicate structure that crystallizes into straight, needle-like fibers. These fibers are generally more brittle than chrysotile.
Locations of Natural Deposits
Naturally Occurring Asbestos (NOA) is a term used to distinguish these natural deposits from manufactured asbestos-containing products. NOA is commonly found in specific geological settings, particularly in areas rich in ultramafic and serpentine rock. Serpentine rock, in particular, often contains chrysotile asbestos and is abundant in mountainous regions and near fault zones around the world.
The percentage of asbestos in these rocks can vary widely, ranging from less than one percent up to 25 percent or more. When this host rock is intact and undisturbed, the risk of exposure is generally low. However, NOA can be present in soil, gravel, and crushed rock, especially in areas where the parent rock has been weathered or eroded.
Exposure can occur when these natural deposits are disturbed by both human activities and natural processes. Construction, grading land for development, or driving on unpaved roads surfaced with asbestos-containing rock can crush the material and release microscopic fibers into the air. Natural weathering and erosion also contribute to the release of these fibers.