A ferromagnesian metamorphic mineral that can easily be confused with quartz and feldspar. Occurs in low-pressure metamorphic rocks and contact metamorphic rocks. Also occurs in granulite facies rocks. Most references cite one of the two occurrences and seem blissfully unaware of the other. It appears that dry conditions rather than any specific pressure favors the formation of cordierite. Since it's a magnesium aluminum silicate, we might expect water to favor biotite instead. Distinguishing characteristics:
In relief and interference color, looks much like quartz and feldspar. However:
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 along cracks.
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.
Created 10 Oct 1997, Last Update 14 Dec 2009
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