Smith College - Geology 222b - Petrology

Petrographic Data File

Lawsonite
Property
Value
Comments
Formula CaAl2(Si2O7)(OH)2 H2O

Same composition as anorthite, with (AlO.OH) octahedra linked by Si2O7 groups.

Ca2+ and H2O molecules are located between these polyhedra.

Crystal System Orthorhombic  
Crystal Habit Tabular or prismatic crystals Frequently twinned polysynthetically on {110}
Cleavage

Perfect cleavage {010} and {001}

Poor cleavage {110}

 
Color/Pleochroism Colorless, pale blue to bluish-grey. Vitreous to greacy luster. Transluscent. Pleochroism includes colorless, blue, and yellow.
Optic Sign Biaxial (+)   
2V

76-84

 

Optic Orientation

X = a

Z = c

 
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 

1.665
1.674
1.684
0.0130-0.020
 
Max Birefringence 0.0190-0.0210  
Elongation Elongated along c  
Extinction Lengthfast extinction parallel with elongation direction If measred against cleavage, may become symmetrical depending on orientation.
Dispersion r > v  
Distinguishing Features

High hardness (8).

Occurrence

Lawsonite is a typical mineral of the glaucophane schist facies associated with chlorite, titanite, glaucophane, quartz, epidote, and garnet. Also found as a secondary mineral in altered gabbros and diorates.Generally located on the Tiburon Peninsula in San Francisco Bay, California.

Lawsonite is a common constuent of schists and gneisses formed under high pressure and low temperature. Common in low grade mafic metamorphic rocks.

Editors Julianne Zimmerman ('06)

 
Lawsonite specimen in XPL
Lawsonite pseudomorph in PPL and XPL


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