Smith College - Geology 222b - Petrology

Petrographic Data File

Serpentine
Hydrous Magnesium Silicate
Property
Value
Comments
Formula Mg3Si2O5(OH)4 3 types: chrysotile, lizardite, and antigorite
Crystal System Monoclinic Beta = 93°
Crystal Habit Crysotile: Fibrous, elongated, and parallel to  crystallographic axis a Lizardite and antigorite: flat, tabular crystals A mesh-like texture can be viewed with polarized light.
Cleavage Chrysotile: fibrous 
Lizardite: basal cleavage
Antigortie: perfect {001}

Color/Pleochroism Green in thin section
Optic Sign Biaxial (-)
2V highly variable, may be sensibly uniaxial
Optic Orientation Slow ray vibration direction is typically parallel to the length of fibers in chrysotile -parallel extinction
Antigorite -Optic plane is perp or parallel to (010).  X=c, Y=b, Z=a or X=c, Y=a, Z=b

Refractive Indices
alpha = 
beta = 
gamma = 

1.529-1.595
1.530-1.603
1.537-1.604

Max Birefringence .001-.010 first order colors
Elongation  parallel to  crystallographic axis a  
Extinction Straight on fibres, cleavage or crystal edge.  Chrysotile is length-slow  
Dispersion  r > v or r<v
Distinguishing Features With the exception of cross-fibers of chrystolite in veins, the varietites of serpentine cannot be distinguished without X-ray diffraction or other techniques.
Occurrence Commonly formed by hydrothermal alteration of mafic and ultramafic rocks- peridotite and pyroxenite.  These contain olivine and pyroxene.  Associated minerals :talc, calcite, brucite, chlorite, and chromite.  Also can be found in contact metamorphosed carbonate rocks, associated with calcite, forsteritre, dolomite, magnesite, and calc-silicate.
Editors Emilie Flemer (01), Jennifer Unis (01), Rebecca-Ellen Farrell (03), Liz Hogan (04)

 
A thin section of serpentine under cross polorized light.  Note the orthopyroxene and its exsolution lamellae and the high birofrengence of the olivine minerals.  The fibrous serpentine is the vein like mineral between the orthopyroxene and olivine.  
The same thin section of serpentine under unpolorized light.  Note the greenish, yellowish color of the serpentine mineral.  



Here the mesh-like texture can be viewed through the cross-polarized light (x5) Size=2000microns



Serpentine in plane-polarized light (x5) Size= 2000microns
Serpentine in plane-polarized light. Click for rollover image.
WWW Images  UCLA Serpintine Thin Section and Information 


Return to Petrography Index