lawsonite

{{Infobox mineral | name = Lawsonite | image = Lawsonite-169898.jpg | imagesize = 260px | category = Sorosilicate | alt = | caption = Sample from the type locality in California with two elongated, lustrous and translucent crystals of pastel pink, lawsonite in mica schist. Size: | formula = CaAl2Si2O7(OH)2·H2O | IMAsymbol = Lws | molweight = | strunz = 9.BE.05 | dana = | system = Orthorhombic | class = Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) | symmetry = Cmcm | unit cell = a = 5.847, b = 8.79, c = 13.128 [Å]; Z = 4 | color = Colorless, white, pale blue to grayish blue | colour = | habit = Commonly prismatic, tabular; also granular, massive | twinning = Common on {101} lamellar | cleavage = Perfect on {100} and {010}, imperfect on {101} | fracture = | tenacity = Brittle | mohs = 7.5 | luster = Vitreous, greasy | streak = White | diaphaneity = Translucent | gravity = 3.05–3.12 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.665 nβ = 1.672–1.676 nγ = 1.684–1.686 | birefringence = δ = 0.019–0.021 | pleochroism = Weak; X = blue, pale brownish yellow; Y = deep bluish green, yellowish green; Z = colorless, yellowish | 2V = Measured: 84°–85° | dispersion = Strong, r > v | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }}

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.