Topic summary
Lawsonite
Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to translucent colorless, white, pink, and bluish to pinkish grey glassy to greasy crystals. Refractive indices are nα= 1.665, nβ= 1.672 – 1.676, and nγ= 1.684 – 1.686. It is typically almost colorless in thin section, but some lawsonite is pleochroic from colorless to pale yellow to pale blue, depending on orientation. The mineral has a Mohs hardness of 7.5 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture. Not to be confused with Larsonite, a fossiliferous jasper mined in Nevada.
Lawsonite is a metamorphic mineral typical of the blueschist facies. It also occurs as a secondary mineral in altered gabbro and diorite. Associate minerals include epidote, titanite, glaucophane, garnet and quartz. It is an uncommon constituent of eclogite. Its scarcity in eclogite that has been exhumed to the Earth's surface does not reflect its abundance at depth in subduction zones but rather the fact that lawsonite is easily replaced by other minerals.
Lawsonite was first described in 1895 for occurrences on Ring Mountain of the Tiburon peninsula, Marin County, California and was named after geologistAndrew Lawson (1861–1952) of the University of California by two of Lawson's graduate students, Charles Palache and Frederick Leslie Ransome.