Olivine rich in the Fe end-member, fayalite, is much less common than the magnesian variety.  Fayalitic olivine does occur in some iron-rich gabbros and syenites, in some evolved volcanic rocks such as Fe-rich dacites, and in some iron-rich metasedimentary rocks.  Many of the optical characteristics described for magnesian olivine also apply to Fe-rich varieties, such as high positive relief (relative to feldspar, for example), high interference colours (upper third order, just slightly higher than Mg-olivine), lack of cleavage and presence of irregular fractures.  An important difference is that Fe-rich olivine has a much lower 2V (approximately 46 degrees in pure fayalite).  Unlike magnesian olivine, fayalite can coexist with quartz, and the two fayalite grains (Mg/(Mg+Fe) = 0.01) in image A stand out in high positive relief relative to the quartz in that sample.  The low to moderate positive relief of the fayalite relative to the pale-green pyroxenes is more difficult to observe due to the Fe-oxide alteration of the fayalite.  The pyroxenes show extensive exsolution of clinopyroxene from orthopyroxene (B), and a single grain of garnet occurs at the pyroxene-quartz contact in the SE quadrant.  Sample is a garnet-bearing quartz fayalite pyroxenite from Baffin Island, Nunavut, Canada.  Both images are 2.2 mm across.  A ppl, B x-nicols.