Orthopyroxene occurs commonly in tholeiitic basalts and in plutonic mafic and ultramafic igneous rocks, and in granulite facies meta-pelites and meta-basic rocks.  In views A and B, the distinct pink to blue/green pleochroism of hypersthene, a moderately Fe-rich orthopyroxene, is illustrated (B is rotated 90 degrees relative to A).  Several orthopyroxene grains are present in these views, all showing these pleochroic colours.  Grains that have the same shade of green in both images are augite, a clinopyroxene, and both varieties of pyroxene have high positive relief relative to enclosing plagioclase.  In C, a virtually monomineralic orthopyroxene rock in which the pyroxene is relatively Mg-rich, pleochroism is very weak, as indicated by only slight colour variations between grains of various orientations.  In D, interference colours range up to first order red, the maximum for orthopyroxene.  The grain at the centre of the left margin shows that orthopyroxene extinction is parallel.  Bright streaks in the extinct grains are fine exsolution lamellae of clinopyroxene.  A and B are from a two-pyroxene garnet granulite from Algonquin Park, Ontario, and C and D are from an unknown location.  Images A and B are 2.2 mm across and C and D 5.5 mm across.  A and C ppl, B and D x-nicols.