Smith College - Geology 222b - Petrology

Petrographic Data File

Formula (Cl,La,Th)PO4  
Crystal System monoclinic  
Crystal Habit small equant or elongate crystals simple twins on {001} are common

{100} is distinct

conchoidal fracture, brittle
Color/Pleochroism Olive or yellowish-green in hand samples.  Colorless to pale green in thin section.  Weak, pale green pleochroism in thin section.  
Optic Sign Biaxial (+)   
2V 7-19  
Optic Orientation X=b 
O.A.P. = (010)
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 

Max Birefringence 0.045-0.075  
Extinction parallel   
Distinguishing Features In thin section: high birefringence, high relief, and pale yellow color. Staurolite is more distinctly pleochroic and has much lower birefringence. Some grains may resemble zircon, titanite, xenotime, or members of the epidote family.
Occurrence Monazite is an accesory mineral in granite, granitic pegmatite, syenite, and carbonatites. Because it is stable in the weathering environment, monazite may be part of the heavy mineral fraction of the clastic sediments. It is infrequently found in hydrothermal vein deposits. In metamorphic rocks it is found in metamorphosed dolostone and in mica schist, gneiss, and granulites. (Nesse, W.D. 2000. Introduction to Mineralogy. New York, NY: Oxford University Press, Inc. pp. 349-50.)
Editors Merilie Reynolds '08

Photomicrograph of monazite (small equant grain at center) in plane polarized light. Select image and then move cursor over image to view in crossed polarized light. Double click to see larger photo. Magnification is 10x.

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