serpentine (sûrˈpəntēn, –tĪn) [key], hydrous silicate of magnesium. It occurs in crystalline form only as a pseudomorph having the form of some other mineral and is generally found in the form of chrysotile (silky fibers) and antigorite and lizardite (which are both tabular). Chrysotile is also known as commercial asbestos which has been used for fireproofing and insulating material. It is commonly some shade of green, but may also be reddish, yellowish, black, or nearly white. It has a greasy or silky luster and is often translucent, even in large masses. Serpentine can be found in igneous rock, but it is more often a secondary mineral, usually resulting from the alteration of minerals or rocks containing magnesium, and it occurs very widely throughout the world, mainly as a product of metamorphism in rocks rich in olivine, pyroxene and amphibole. Serpentine rocks and basalts in the central zone of the Appalachian Mts of the eastern United States represent sutures of old oceanic crust, known as ophiolites, crushed between colliding continents. Essentially all lower crust and upper mantle rocks recovered from the mid-ocean ridges have been serpentinized to some degree by reaction with seawater. Serpentine rocks are classified as common serpentine and precious serpentine, the common serpentine being darker, less translucent, and sometimes impure. Serpentine is sometimes used as a gem and the massive varieties are quarried. Used like marble for decorative purposes, when serpentine is mixed with calcite, dolomite, or magnesite, a mottled or veined rock called verd antique is produced, although the masses are frequently jointed and only small slabs can be secured. Serpentine takes a beautiful high polish, but it is easily cracked and discolored by exposure to the weather and is consequently of little value for exterior use. Serpentine deposits are found in Canada, S Africa, and Vermont and Arizona in the United States.