What is lawsonite?

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, 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 8 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture.

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Some articles on lawsonite:

Eclogite Facies
... Eclogites containing lawsonite (a hydrous calcium-aluminium silicate) are rarely exposed at Earth's surface, although they are predicted from experiments and thermal models to form ... The rarity of lawsonite eclogites therefore does not reflect unusual formation conditions but unusual exhumation processes ... Lawsonite eclogite is known from the U.S ...
Significance of Lawsonite
... Lawsonite is a significant metamorphic mineral as it can be used as an index mineral for high pressure conditions ... Lawsonite is known to form in high pressure, low temperature conditions, most commonly found in subduction zones where cold oceanic crust subducts down ...