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A ferro-magnesian mineral. Light green, glassy, often granular, although this piece is massive (i.e. no visible structure), no cleavage. In mafic igneous rocks the olivine is found as isolated grains surrounded by pyroxene and Ca plagioclase. It may be a phenocryst in basalt.
This is a relatively rare mineral in most igneous rocks, or one that is hard to see without a microscope. Large accumulations such as this form the igneous rock dunite, or when mixed with pyroxene, peridotite.