Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Ca5(PO4)3(F, OH,Cl)  
Crystal System Hexagonal  
Hardness 5  
Crystal Habit Elongated, small, hexagonal  
Cleavage {0001}  and
{10-10} both poor
Color/Pleochroism Colorless in thin section  
Optic Sign Uniaxial (-)  
2V N/A Biaxial carbonate-apatite has a 2V of about 200
Optic Orientation N/A  
Refractive Indices
omega = 
delta = 

Max Birefringence .007 First order gray.  However, apatite can vary greatly in composition. Due to this fact the birefringence may be much lower than .007.
Elongation Length fast  
Extinction Parallel  
Dispersion Moderate  
Distinguishing Features Low birefringent hexagonal prisms with high relief. Apatite doesn't have cleavage, but the prisms can have widely spaced cross fracture. Don't depend on the cross fracture and mistake it for sillimanite! Sillimanite has a higher birefringence, is length slow, and is biaxial.
Composition The variation of apatite minerals is in the proportions of F, OH, and CL.
Morphology Stubby hexagonal prisms in mafic igneous rocks, slender prisms in felsic igneous rocks, or anhedral grains in metamorphic rocks.
Occurrence Found mainly in various igneous and metamorphic rocks, however they are found in carbonatites and sedimentary rocks.  Due to the phosphate content, apatite crystals are usually small and can be hard to detect in thin section. 
Editors Anna Marchefka ('02), Emmy Smith ('08), Jen Axler ('11)

The apatite crystal is very small and elongated. Its high relief allows it to stick out from the other colorless minerals. Under cross-polars apatite exhibits a first order gray color. Click on the image to see it in both plain and (rotated and magnified) in cross polarized light.
Here small elongate apatite crystals are found within a hornblende crystal. 
This is a photo of an apatite crystal in plane polarized light. The outline of the apatite is barely visible because of the moderate relief. Click on the image to see a larger ppl/xpl rollover version.
A single apatite crystal in plane polarized light. Click on the image to see it in both plain and cross polarized light.
A number of apatite crystals found as inclusions within a larger mineral. Click on the image to see it in both plane and cross polarized light.
WWW Images U.C.L.A. Petrographic Workshop

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