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PLAGIOCLASE #2   Sample in gabbro
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A feldspar mineral, typically light to dark gray, with two directions of cleavage at 90 degrees. Cleavages are well developed and commonly seen in specimens in rocks. The third direction fractures, however, and has an irregular, broken appearance. A common property is irridescence (play of colors) as seen in this specimen, and striations (fine, very even grooves on the cleavage surface, not obviously present here.) The surface is typically transluscent, that is, light seems to penetrate below the surface so you can see into the mineral.
In mafic igneous rocks Ca Plagioclase is common although its colors can range to light gray. In intermediate igneous rocks plagioclase becomes more sodium rich and the color can go more white.