A general name for the useful, fibrous varieties of a number of rock-forming silicate minerals that are heat-resistant and chemically inert; two varieties exist: amphibole asbestos, the best grade of which approaches the composition Ca₂Mg₅(OH)₂Si₈O₂₂ (tremolite), and serpentine asbestos, usually chrysotile, Mg₃Si₂(OH)₄O₅.