Plagioclase

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof 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 thetypical appearance of plagioclase in thin section.

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Created 22 Sept 1997, Last Update