Serpentine

Serpentine, a hydrous silicate of magnesia and iron, 3(MgFe)O,2SiO₂,2H₂O, of a dull green, reddish or brownish colour, with specific gravity 2.5 to 2.7, and hardness 3 to 4. It occurs as an alteration-product in olivine (q.v.), or, less frequently, of hornblende and augite; and, as the serpentinisation of these minerals proceeds along their cleavage planes, it presents distinctive structures - that from olivine being irregularly-meshed; that from augite, rectangularly-netted or bladed, and that from hornblende, latticed, with blades intersecting at angles of 124°. Serpentinite, or massive serpentine occurring as a rock, is dull green and red, mottled and veined with fibrous chrysotile and white steatite, and is easily scratched with a knife. Many serpentinites occur in dykes and veins, and are undoubtedly formed from the hydration of olivine-basalts (peridotites), and others from diabase, gabbro, or hornblendic rocks. Serpentine also occurs, however, disseminated through limestones forming ophicalcites (q.v.), which have been supposed to be altered dolomites, or marine deposits, but are probably neither. Serpentinite occurs at the Lizard in Cornwall, in Anglesea, Aberdeenshire, and Connemara, and is used for ornamental purposes.