Muscovite and Fuchsite
Muscovite is a common rock-forming mineral. It is a type of mica found in many types of igneous and metamorphic rock. It is easily recognized because of its perfect cleavage that allows it to separate into thin, transparent, flexible sheets.
Muscovite has a high resistance to heat and in the form of thin transparent sheets it has been used as windows on high-temperature furnaces and ovens. It is an insulator and was used in the past to make circuit boards and as an early window glass. In fact the name muscovite comes from Muscovy-glass, a name formerly used for the mineral because of its use in Russia for windows.
By chemical composition, muscovite is a potassium aluminum silicate hydroxide fluoride. It has a specific gravity of 2.76 to 3.00, a refractive index of 1.552 to 1.618, and forms in the monoclinic crystal system. Like kyanite, it has a variable hardness, with a Mohs hardness of 2 in one direction and 4 in the other. Muscovite can be colorless or show tints of gray, brown, green, yellow or (rarely) violet or red. It is translucent to transparent.
Ruby in Fuchsite Cabochon
The green chromium-rich variety of muscovite is known as fuchsite (pronounced FOOK-site). It can have an attractive blue-green to emerald green color and display a sparkly shine if the crystals are small. Due to its attractive color, fuchsite is often used as an ornamental stone. Fuchsite is sometimes confused with zoisite, since both green minerals can sometimes be found interspersed with ruby. However, zoisite is a distinct mineral with a different chemical composition (calcium aluminum silicate). Zoisite is also a much harder material with a Mohs hardness score of 6.5 to 7.
Muscovite is found all over the world but the largest and most important deposits are found in Russia, Pakistan, India, and Brazil. Muscovite is characteristic of granite and granite pegmatites, associated with quartz and feldspars. In pegmatites, it is often found in immense sheets that are commercially valuable. The United States produces only a small amount of high yield sheet mica. However, the US leads the world in the production of scrap mica. The major locations of scrap mica production in the US are North and South Carolina, Connecticut, New Mexico, Georgia and South Dakota.