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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General Information:
 Chemical Formula:
CaAl2Si2O7(OH)2·(H2O)
 Composition:
(Molecular Weight = 314.24 gm)
Calcium 12.75 % Ca
Aluminum 17.17 % Al
Silicon 17.88 % Si
Hydrogen 1.28 % H 
Oxygen 50.91 % O 
 Empirical Formula:
CaAl2Si2O7(OH)2·(H2O)
 Environment:
Originally described from a crystalline schist associated with serpentine. Also found as a secondary mineral in altered gabbros and diorites.
 Locality:
Reed Station, Tiburon peninsula, Marin County, California, USA.
Physical Properties:
 Cleavage:
[010] Perfect, [001] Perfect, [110] Poor
 Color:
colorless, white, gray, blue, or pinkish.
 Density:
3.09
 Diaphaniety:
Translucent to transparent
 Habits:
Prismatic - Crystals Shaped like Slender Prisms (e.g. tourmaline)., Tabular - Form dimensions are thin in one direction.,
 Hardness:
7.5 - Garnet
 Luminescence:
None.
 Luster:
Vitreous - Greasy
 Streak:
white
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