PLAGIOCLASE #1
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 are striations (fine, very even grooves on the cleavage surface, seen here.), and irridescence (play of colors, not seen here, but seen on specimen 2). The surface is typically transluscent, that is, light seems to penetrate below the surface so you can see into the mineral.
Feldspar crystal structure is complex but consists of rings of four tetrahedra (resulting in the 4-sided 90o cleavage), strung into warped chains, held together ionically by metallic cations. The specific cations determine the particular feldspar obtained (see link).
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.