Cordierite is a common mineral in contact and medium to high grade metamorphic rocks, but has many optical similarities to quartz and feldspars that can lead to misidentification.  It has negative relief in A, relative to coexisting garnet.  Unlike quartz, which is uniaxial, cordierite (biaxial) alters along fractures and grain boundaries (A) to a chlorite-dominated assemblage referred to as pinite.  Maximum interference colours of first order yellow are just slightly above those shown in B (grey), in which polysynthetic twinning is also evident.  Though not typically pleochroic, pleochroic halos (C and D, rotated 90 degrees to each other) can occur around zircon inclusions.  Samples are from unknown localities.  All views are 2.2 mm across.  A, C, D ppl, B x-nicols.