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The Mineral lawsonite

Lawsonite Crystals in Matrix

Lawsonite is a rare mineral, and is a hydrated variant of the feldspar Anorthite. It is named after Andrew Cowper Lawson (1861-1952), a professor of geology at the University of California at Berkeley. The Bay Area of California is the type locality and primary region of this mineral, and this is where Lawson taught. Lawson's main claim to fame is his identification and naming of the San Andreas Fault.

Chemical Formula

CaAl₂Si₂O₇(OH)₂· H₂O

Color

Light blue, light pink, light purple, gray, tan, or white. May also be multicolored with color zoning of light blue, light pink, or light purple. Rarely bright green or bluish-green.

Crystal System

Orthorhombic

Properties

Streak
White

Hardness
7.5

Transparency
Transparent to translucent

Specific Gravity
3.1

Luster
Vitreous

Cleavage
1,2;3,3

Fracture
Uneven

Tenacity
Brittle

Crystal Habits

Crystals are usually prismatic or tabular, and they are often crude and of grainy appearance. Also in groupings of stubby or tabular crystals. May also be granular and massive. Crystals are sometimes twinned.

Additional Information

Composition
Hydrous basic calcium aluminum silicate

In Group
Silicates; Sorosilicates

Striking Features
Localities, perfect cleavage, color, and hardness.

Environment
In metamorphosed blue schists, as well as altered gabbros and basalt.

Rock Type
Metamorphic

Varieties

Chromian Lawsonite

- A bright green, chromium-rich variety of Lawsonite, described from Syros Island, Greece.

Uses

Lawsonite is used in geological studies to determine the formation and metamorphism of certain rock formations.

Noteworthy Localities

Lawsonite is a not commonly represented in collections, though this mineral is probably more widespread than perceived. In the U.S., the only significant localities are in the Coast Ranges area of Central California. Large, pale blue crystals of Lawsonite were found at the type locality near the Reed Station in the Tiburon Peninsula, Marin Co., California. This locality has since been built over and is no longer in existence. Light tan and pink crystals have come from Valley Ford, Sonoma Co., California. A deep green, chromium-rich form of Lawsonite was reported as newly discovered material from Cape Marmari, Syros Island, Greece, in 2010.