Lawsonite
Lawsonite is a hydrous calcium aluminium sorosilicate mineral with the chemical formula CaAl2Si2O7(OH)2·H2O. It is a metamorphic mineral characteristic of blueschist facies environments, formed under conditions of high pressure and relatively low temperature.
Occurrence and Formation:
Lawsonite typically forms in areas of subduction zones, where oceanic crust is thrust beneath continental crust. The high pressure and low temperature conditions in these settings favor the formation of lawsonite from pre-existing minerals such as plagioclase feldspar. It is often found associated with other blueschist facies minerals such as glaucophane, jadeite, and pumpellyite. Lawsonite can also occur in eclogites, another type of high-pressure metamorphic rock.
Physical Properties:
Lawsonite is typically colorless to bluish-gray or white, but can also be shades of pink, purple, or even yellow. It forms as prismatic or tabular crystals, often elongated and striated. The mineral has a vitreous to greasy luster and a Mohs hardness of 6. Its specific gravity ranges from 3.09 to 3.13. Lawsonite exhibits perfect cleavage on {100} and {010}, and is brittle with a conchoidal fracture. It is biaxial (+), with refractive indices ranging from approximately 1.665 to 1.686.
Optical Properties:
Lawsonite is optically biaxial positive. Its birefringence is relatively low, resulting in low-order interference colors under crossed polarizers. Its optical orientation is X = b, Y = c, and Z = a.
Uses:
Lawsonite itself doesn't have significant industrial uses. Its primary importance lies in its value as an indicator mineral of high-pressure, low-temperature metamorphic conditions. The presence of lawsonite can provide valuable information about the tectonic history of a region and the conditions under which the rocks formed.
Alteration:
Lawsonite is often subject to alteration under lower pressure and higher temperature conditions, typically transforming into minerals such as zoisite, epidote, albite, and quartz. This alteration is indicative of a change in the metamorphic environment.
Identification:
Lawsonite can be identified based on its characteristic occurrence in blueschist facies rocks, its crystal habit, cleavage, hardness, and optical properties. X-ray diffraction analysis can provide definitive confirmation of its presence.