What is Lawsonite?
Lawsonite is a water-bearing calcium aluminium silicate that records high-pressure, low-temperature metamorphism in subduction zones.
Mineralogy and crystal structure
Reference formula: CaAl2Si2O7(OH)2·H2O. Lawsonite is an orthorhombic hydrous silicate whose crystal structure stores water in a metamorphic mineral assemblage. Its stability field makes it an important indicator of subduction-related pressure-temperature conditions. Lawsonite is classified with silicates and has a orthorhombic crystal system. Typical hardness is 6.5–7 on the Mohs scale and specific gravity is approximately 3.0–3.1.
Colour, lustre and habit
Typical colours include colourless, white, blue, grey. The usual prismatic, tabular, granular habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Lawsonite forms in or is associated with blueschists, high-pressure metamorphic rocks and altered basaltic oceanic crust. Important documented localities include California, Japan, New Caledonia, Greece, Turkey and Italy.
A clear crystal that records subduction
Lawsonite offers more than a pale colour: it is a compact record of water entering a descending slab. A good specimen connects clean crystal form with one of the most important geological processes on Earth, making it valuable for both display and study.
Collecting and care
Protect clear crystals and cleavage surfaces from impact; keep locality and metamorphic association with the specimen.
