The mica servesĪs a filler, improves the workability of the compound, and reduces cracking in the finished product. The primary use of ground mica is in joint compound used to finish seams and blemishes in gypsum wallboard. Ground mica, mostly muscovite, is used in the United States to manufacture a variety of products. The transparent nature of muscovite is clearly seen in this photo. Specimen is approximately 3 inches (7.6 centimeters) across. Muscovite: Muscovite from Mitchell County, North Carolina.
Ground muscovite: Photograph of ground muscovite from Mt. It is evidence that these sediments and rocks have not been subjected to severe weathering. Tiny flakes of muscovite sometimes survive long enough to be incorporated into sediments and immature sedimentary rocks. It is quickly transformed into clay minerals. Muscovite is not especially resistant to chemical weathering. Muscovite can occur as isolated grains in schist and gneiss, or it can be abundant enough that the rocks are called "mica schist" or "micaceous gneiss." The heat and pressure of metamorphism transforms clay minerals into tiny grains of mica which enlarge as metamorphism progresses. Muscovite can form during the regional metamorphism of argillaceous rocks. Muscovite rarely occurs in igneous rocks of intermediate, mafic, and ultramafic composition. These crystals are called "books" because they can be split into paper-thin sheets. In granite pegmatites, muscovite is often found in large crystals with a pseudohexagonal outline. In igneous rocks, it is a primary mineral that is especially common in granitic rocks. Muscovite is found in igneous, metamorphic, and sedimentary rocks.
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