Occurrence and Compositon
Cordierite is a metamorphic mineral found in high grade pelitic metamorphic rocks. Its composition is (Mg,Fe)2Al4Si5O18.

Cordierite can be difficult to identify if untwinned, inclusion free, and unaltered. It can be clear with birefringence equivalent to quartz and plagioclase. When twinned, the twins may be distinctive truncating polysyntheitc twins, but may also appear very much like typical plagioclase twins. Cordierite often alters partially or whooly to a fine grained sericite, termed pinite. Inclusions of zircons may give rise to burn marks around included grains, appearing sometimes as brown rings or pleochroic halos.

Important properties
·Appearance and habit - generally blocky and anhedral, more rarely having a pseudohexagonal appearance
·Color - clear
·Interference colors - first order grays and whites
·Twinning common - Polysynthetic twins may look like plagioclase but frequently pinch out in one direction.
·Inclusions - Cordierite may contain any of a number of minerals as inclusions. Included zircons often have burn marks or halos around them due to radiation damage.

Similar minerals
·Cordierite is easily confused with plagioclase in some rocks but cordierite twins are often imperfectly formed and cordierite often contains burn marks around zircons.
·Quartz is similar to untwinned cordierite but quartz is uniaxial (+). Cordierite often is slightly to completely altered, giving it a hazy or scuzzy appearrance; quartz does not alter.

Cordierite, Orthopyroxene and Biotite from near Kazabazua, Quebec

The four photos above are of a spectacular rock from near Kazabazua, Quebec. It contains primarily cordierite and orthopyroxene with lesser amounts of biotite. The cordierite is twinned (and in XP looks a lot like plagioclase) and contains pleochoric halos around zircon inclusions. The halos appear as "burn" marks in PP light. The orthopyroxene is blocky and fractured, shows high relief (PP), and has first order interference colors (XP). It is pleochroic, with color ranging from light pink to light green. Biotite is also pleochroic and here exists as flakes in various shades of brown (PP) and has upper 2nd order interference colors (XP).

Cordierite, Biotite and Quartz from near Ft. Coulonge, Quebec

Imperfectly twinned cordierite makes up most of this view (large grain at center of photo). A number of flakes of biotite and a few grains of quartz surround the cordierite. The small high-relief material in the center of the cordierite grain are needles of sillimanite.

The field of view is about 2.5 mm across.

Cordierite, Garnet and Biotite

This sample contains large twinned cordierite (left side) and a highly fracture garnet with some quartz inclusions (right side). Minor biotite and quartz are also present. The cordierite interference colors and twins, visible in the XP view, make this cordierite appear similar to some plagioclase. The garnet is isotropic and so is black in the XP view.

These photos are from the same thin section as the preceding ones.