Riebeckite is the Fe-rich and oxidized equivalent of the sodic amphibole glaucophane.  It commonly occurs in alkaline granites, syenites and similar rocks, and some metamorphosed iron formations.  It has moderate to high positive relief relative to quartz and feldspars, is strongly coloured and pleochroic in shades of yellow, blue and green (A-C).  Absorption can be so strong as to give the mineral the appearance of being opaque in some orientations.  Sections showing the characteristic prismatic {110} amphibole cleavage are shown in A and B, and C illustrates the appearance of longitudinal sections.  The extinction angle is small in longitudinal sections and symmetric in basal sections.  Maximum interference colours range to upper first order, but are masked by the strong absorption (D).  Sample is from a riebeckite syenite from an unknown location.  All images are 2.2 mm across.  A, B and C ppl, D x-nicols.