A | B |
Fe2SiO4 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. |
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