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A ferro-magnesian mineral. Light green, glassy, often granular, as is this specimen, no cleavage. Constructed of isolated silica tetrahedra held together ionically by iron and magnesium ions. With no planes of weakness there is no cleavage, and because the crystals grow outward in all directions from a tetrahedral seed they are granular.
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.