Talc is colourless in plane polarized light, and its high birefringence causes its relief to change with rotation of the stage (compare A and B which are rotated 90 degrees relative to one another), a feature that can be mistaken for faint pleochroism.  Maximum interference colours are in the third order (C).  Extinction is parallel to cleavage traces and, like the micas, the grains have a mottled or “crinkly” appearance at and near extinction (D).  Talc and muscovite appear very similar in thin section and usually cannot be reliably distinguished based on optical characteristics alone.  Paragenesis, or mineral assemblage, can be a useful guide.  Although muscovite occurs in a wide variety of igneous and metamorphic rocks, talc is rather restricted in occurrence, typically to high magnesium rocks (mafic or ultramafic) in which is it associated with tremolite, dolomite, serpentine, magnesite and other Mg-rich minerals.  Sample is from a talc-tremolite schist from St. Lawrence County, New York.  All views are 2.2 mm across. A and B ppl, C and D x-nicols.