Olivine is a common mineral in mafic and ultramafic igneous rocks (e.g., basalts, cumulate peridotites and mantle xenoliths) and some high-grade metamorphic rocks, including marbles.  The Mg/(Mg+Fe) value of olivines in most such rocks is in the range 0.65 - 0.95, and the optical characteristics of magnesian olivine described here apply to these compositions.  Mg-rich olivine does not occur in quartz-bearing rocks. Fayalite, the iron-rich end-member of the olivine solid solution series, is described on a separate page.  In A, the high positive relief of olivine relative to surrounding plagioclase is evident.  Typical of most olivine in plutonic and many volcanic rocks, the grains are anhedral.  Olivine lacks cleavage, but commonly has a characteristic network pattern of fractures.  In B grains show a variety of interference colours, up to the maximum of third order.  In such anhedral olivine grains, there is no crystallographic reference for extinction angle designation.  Images C and D illustrate euhedral to subhedral olivine phenocrysts in basalt.  Note the parallel extinction of the euhedral grain in the SE quadrant. Mg-rich olivine has a 2V of 90 +/- 10 degrees.  Images A and B are from a troctolite (olivine - plagioclase rock) with small euhedra of chromite from an unknown location and C and D are from a basalt from an unknown location. All views are 5.5 mm across.  A and C ppl, B and D x-nicols.