Cordierite

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Plane-Polarized Light

A ferromagnesian metamorphic mineral that can easily be confused with quartz and feldspar. Occurs in low-pressure metamorphic rocks and contact metamorphic rocks. Distinguishing characteristics:

• Commonly shows feathery alteration veinlets (pinite - A).
• Pleochroic yellow halos around inclusions of zircon are common (B).
• Iron-staining alteration along cracks (C).
• Often has numerous spinel inclusions (D).

Crossed Polarizers

In relief and interference color, looks much like quartz and feldspar. However:

• Quartz lacks alteration
• Plagioclase can be distinguished by its lamellar twinning (A).
• Microcline can be distinguished by its tartan twinning (B).
• Feldspars often have perthitic texture (C).

Pinite is mostly muscovite and shows high interference colors.

Cordierite in plane-polarized light. Note the low relief, the abundant opaque inclusions and the orange alteration.

In crossed polarizers we see the corierite is riddled with small quartz inclusions. They don't show up well in plane polarized light because quartz and cordierite have nearly the same refractive index. Note the first-order grayish white, very similar to the interference color of quartz.

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Created 10 Oct 1997, Last Update 20 May 2002

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