Apatite is a common accessory mineral in many igneous and metamorphic rocks.  It is colourless and has moderate positive relief relative to quartz and feldspars.  It typically occurs as euhedral to subhedral hexagonal prisms. In A, euhedral apatite grains stand out in moderate positive relief relative to enclosing feldspar, whereas the negative relief of the apatite in biotite is more difficult to discern due to the dark colour of the biotite. In C, the relief of the large grain appears positive relative to adjacent altered feldspar, but the small hexagonal prisms at top centre have distinctly negative relief relative to enclosing titanite. Apatite has maximum interference colours of first order grey, visible in grains with the c axis horizontal (especially obvious in the large grain in D), and hexagonal cross sections, with the c axis vertical, appear black. Extinction is parallel to the c axis (parallel extinction).  Apatite does not have cleavage, but a parting perpendicular to the c axis is common (C and D).  A and B are from a tonalite from the Hepburn Batholith in the Wopmay Orogen, NWT and C and D are from an essexite from Mt. Johnson, Quebec. All views are 2.2 mm across.  A and C ppl, B and D x-nicols.