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modification of granite, an intrusive igneous rock; it consists essentially of quartz and white mica (muscovite) and is characterized by the absence of feldspar and biotite. The rock usually has a silvery, glittering appearance from the abundance of layered muscovite crystals, but many greisens resemble a pale granite. The white mica mostly forms large plates with imperfect crystalline outlines. The quartz is rich in fluid enclosures. The most common accessory minerals are tourmaline, topaz, apatite, fluorite, and iron oxides; altered feldspar and brown mica also may be present.