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.
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The light
green mass in the center of this plane-polarized view is chlorite. Most of the
brownish micaceous mineral around it is biotite.
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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.
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The
plane-polarizer view below shows a field mostly of light green chlorite, with
two opaque magnetite crystals at the top.
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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.
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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).
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In the
crossed-polarizer view below, the chlorite stands out because of its anomalous
extinction colors.
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In the
impure quartzite below, the clay filling between the quartz grains has been
metamorphosed to bright green chlorite.
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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 14 Dec 2009
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