Chlorite
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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Plane-Polarized Light
Very common mineral in low-grade metamorphic rocks and as an alteration product of ferromagnesian minerals. Low relief, colorless to green. Light green is most common shade. Micaceous cleavage, often matted or feltlike appearance.
Crossed Polarizers
Low birefringence results in first-order whites. Anomalous interference colors very common (A). Dark blue, brown, purple and green are possible. Fine-grained, matted-looking varieties often show undulose extinction.
The light green mass in the center of this plane-polarized view is chlorite. Most of the brownish micaceous mineral around it is biotite.
The crossed-polarizer view below shows dark purple anomalous extinction colors. Chlorite is one of the most common minerals to show anomalous extinction.
Another common appearance of chlorite. In this crossed-polarizer view we see parallel blades of chlorite, very similar in texture to mica, but distinguished by first-order white interference colors. Again, we also see dark purple anomalous extinction.
The plane-polarizer view below shows a field mostly of light green chlorite, with two opaque magnetite crystals at the top.
The same field in crossed polarizers. The chlorite is actually a mass of criss-crossing small plates, giving the material a felt-like texture. This is a very common texture in chlorite. Note that these extinction colors are normal. Not all chlorite shows anomalous extinction.
The plane-polarizer view below is pretty typical of greenschist facies metamorphic rocks. The brown is biotite, the light green is mostly chlorite, and the darker green includes hornblende (note one grain with 56-124 cleavage just below left center).
In the crossed-polarizer view below, the chlorite stands out because of its anomalous extinction colors.
In the impure quartzite below, the clay filling between the quartz grains has been metamorphosed to bright green chlorite.
The same field in crossed polarizers. The chlorite is a fine-grained felt-like mass showing first-ordred whites and normal extinction colors.
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Created 10 Oct 1997, Last Update 17 Nov 2011
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