Plagioclase
Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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Plane-Polarized Light
- Low relief
- Clear
- Cleavage often visible (A)
- Usually numerous inclusions, giving it a much dustier look than quartz. The inclusions are due to microscopic alteration (B)
Crossed Polarizers
- Gray to white interference colors, slightly lower than quartz
- The tiger-stripe twinning is far and away the most distinctive feature, but beware! Not all plagioclase shows twinning.
- Euhedral crystals are common and frequently zoned (A). Zoning is a valuable tracer of magmatic history.
- Corroded crystals (B) are also common due to alteration or reaction with magma or other minerals
- Alteration products often have high interference colors (C). Tiny euhedral epidote crystals are common as alteration products.
Quartz, the feldspars, cordierite and nepheline all have low relief and gray to white interference colors. Beginners find them hard to tell apart. View
Quartz and its Look-Alikes. The photos below show the
typical appearance of plagioclase in thin section.
Return to Thin-Section Index
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Created 22 Sept 1997, Last Update 20 May 2002
Not an official UW Green Bay site