The Geology of Asbestos, Part I

If you have spent much time on this website and have read a fair amount of the content here, you know that asbestos is a generic term for several different silicate minerals–essentially rock–that have unique qualities that allow it to be woven into fabric and used in many applications in which using other types of solid, rigid stone (granite, etc.) would be impractical. But have you ever wondered where it actually comes from and how it is formed? If you remember science class in middle school or took geology in college, you know that all rocks fall into three classifications: igneous, or “fire rock,” such as granite and basalt, made from volcanic processes; sedimentary, made from deposits of material, of which sandstone and shale are examples; and metamorphic, which starts as either igneous or sedimentary, but are changed by intense heat and/or pressure (precious gems are classified as metamorphic, as is quartz). The two most commonly used forms of asbestos are called amphibole and chrysotile. Most asbestos products in the U.S. were made from the latter. Amphiboles, which include amosite (brown asbestos) and crocidolite (blue asbestos) are what chemists and geologists call chain silicates. This means that the charged molecules, or ions, that make up its structure bond together in long chains of material.

The individual chains are quite strong, but the bonds between them are very weak. This kind of molecular structure is what creates fibrous material that is both strong and flexible. The fibers that make up amphiboles are long and straight, while those of chrysotile are curly. To understand why this is, it helps to review the molecular structure of rock and the role of ions. All minerals are made up of ions, which are positively or negatively charged molecules. These ions are attracted to ions with an opposite charge, which is what causes them to bond to one another. Chrysotile is a “sheet silicate.” With most sheet silicates, these ions are evenly spaced from one layer to the next, allowing these sheets to mesh and lie flat and rigid. With chrysotile however, the spacing on one side of the sheet is too wide to fit evenly to the ions on the other side. Therefore, when these ions are drawn together, it causes the molecular sheet to curl up, resulting in the curly structure of chrysotile fibers.