Smith College - Geology 222b - Petrology

Petrographic Data File

Serpentine
Hydrous Magnesium Silicate
Property
Value
Comments
Formula

Mg3Si2O5(OH)4

Very minor substitution of Al for Si, and of Fe and Al for Mg.

 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 giving it parallel extinction.
For antigorite - Optic plane is perp to (010).  X=c, Y=b, Z=a
Refractive Indices
alpha = 
beta = 
gamma = 
1.538-1.567
~1.566
1.545-1.574
Max Birefringence .001-.010 first order colors
Elongation Chrysotile is length-slow  
Extinction Parallel to fibres, cleavage or crystal edge.   
Dispersion  r > v for antigorite
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 olivine and orthopyroxene in mafic and ultramafic rocks (peridotite, dunite, and pyroxenite).  Serpentine can also be found as an alteration of forsterite in metamorphosed carbonate rocks.
Associated Minerals talc, calcite, brucite, chlorite, and chromite. 
Editors Emilie Flemer ('01), Jennifer Unis ('01), Rebecca-Ellen Farrell ('03), Liz Hogan ('04), Sofia Johnson ('19)

 
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 altered peridotite. Much of the field of view is colorless serpentine with low (1st order) retardationin XPL. The opaque minerals are magnetite or chromite from the Fe in the original olivine and orthopyroxene. Click on the thumbnail image to see a larger version that rolls over to XPL when you mouseover the image.
WWW Images

Union College [under amphiboles]
Oxford University [image 113,114]

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