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A ferro-magnesian mineral. Dark green to black, although weathered specimens may look faded. Dull luster; blocky appearance; 87-93 degree cleavage, although it is typically hard to find or see. Constructed of single chains of silica tetrahedra, stacked in alternating rows, ionically held together by iron and magnesium ions. The cleavage forms from the way the cleavage skips through the single chains.
    This is a common mineral in mafic igneous rocks usually mixed with Ca Plagioclase.

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Last Update: 8/17/00

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