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A feldspar mineral, typically white to light 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. The top shiny face and lower right surface (in shadow) are the cleavage faces in this picture. The fracture face is the upper right end of the crystal. A common property is striations (fine, very even grooves on the cleavage surface, well developed on this specimen.) The surface is typically transluscent, that is, light seems to penetrate below the surface so you can see into the mineral.
In felsic igneous rocks Na Plagioclase is common. In intermediate igneous rocks plagioclase becomes more calcium rich but the color tends to remain white or light gray.