Mica is a naturally occurring mineral that typically forms in pseudo-hexagonal or rhombic plate-like, sheet-like, or columnar crystal structures. Its color varies depending on its chemical composition, with higher iron content resulting in darker shades. Mica is known for its excellent insulation properties, heat resistance, luster, and stability under various physical and chemical conditions. It is also flexible, tough, and thermally insulating. The most commonly used types in industry are muscovite and phlogopite. Mica finds wide application in construction materials, fire protection systems, welding electrodes, plastics, electrical insulation, paper, asphalt paper, rubber, pearlescent pigments, and other chemical industries. Interestingly, mica has even made appearances as a character name in anime.
Mica, also known as mica, is one of the most abundant rock-forming minerals, belonging to the aluminosilicate family with a layered structure. It includes minerals like potassium, aluminum, magnesium, iron, and lithium. Mica comes in many varieties, with monoclinic crystals being the most common, followed by trigonal and others. Common types include biotite, muscovite, phlogopite, lepidolite, and sericite. Mica usually appears in plate-like or columnar shapes, with colors ranging from light to dark depending on the iron content. Muscovite is typically clear or light-colored, while biotite tends to be black or dark brown. Phlogopite can be yellow, green, or colorless, and lepidolite often shows lavender or pink hues. Mica has a glassy luster and a pearly sheen on its cleavage surfaces. Its hardness ranges from 2 to 3.5, and its specific gravity is between 2.7 and 3.5. Mica exhibits perfect cleavage along its basal planes, allowing it to be split into thin sheets.
Muscovite and phlogopite are particularly valued for their electrical insulation, non-conductivity, acid and alkali resistance, and pressure resistance. They are widely used in electronic and electrical applications. In the gas industry, mica chips and powders serve as fillers. Lithium mica is also a key source of lithium. Historically, mica has been used by humans due to its large crystal size and vibrant colors. Ancient Chinese texts refer to it by names such as "Huapilian," "Tianbing," and "Xionghei." The pigment used in Dunhuang’s Mogao Grottoes was found to be finely ground natural muscovite, which provided excellent color performance.
China has long recognized and utilized muscovite and phlogopite, but systematic exploration of mica deposits began after the founding of the People's Republic of China. Significant discoveries were made in places like Danba, Sichuan, and Tuguiwula, Inner Mongolia. The 1958 National Mica Conference in Shanxi marked a turning point in mica resource development. The Altai mica deposit in Xinjiang played a crucial role in expanding China’s mica reserves. Subsequent discoveries in Jiangsu, Hebei, and other regions further increased domestic production. Today, Lubaishan Village in Hebei is the largest producer of crushed mica and biotite, with ongoing research into industrial applications.
In the 1950s and mid-1970s, muscovite and phlogopite were classified as strategic resources. Major mining operations took place in Sichuan, Xinjiang, Inner Mongolia, and other provinces. State-owned mines like those in Altay and Danba were key producers. Before 1978, annual output was around 1,700 to 2,500 tons, but production declined afterward due to changes in demand.
The main minerals in muscovite pegmatite ores include micro-plagioclase, feldspar, quartz, and muscovite. Secondary minerals include biotite, garnet, and apatite. In phlogopite deposits, apatite, tremolite, diopside, calcite, and microcline are commonly found. Muscovite in fine-grained deposits makes up 50–70%, with potassium feldspar and quartz accounting for about 10%. Trace minerals include magnetite and limonite.
Mica belongs to the aluminosilicate group and has a layered structure. It is divided into three main types: muscovite, biotite, and lepidolite. Muscovite includes sericite and rare sodium mica, while biotite includes phlogopite, biotite, and manganese biotite. Lepidolite is rich in lithium. Industrial use mainly focuses on muscovite and phlogopite.
Muscovite has the chemical formula KAlâ‚‚[AlSi₃Oâ‚â‚€][OH]â‚‚, containing 45.2% SiOâ‚‚, 38.5% Alâ‚‚O₃, 11.8% Kâ‚‚O, and 4.5% Hâ‚‚O, along with trace elements like Na, Ca, Mg, Ti, Cr, Mn, Fe, and F. Phlogopite has the formula KMg₃[AlSi₃Oâ‚â‚€][F,OH]â‚‚, with 7–10.3% Kâ‚‚O, 21.4–29.4% MgO, 10.8–17% Alâ‚‚O₃, 38.7–45% SiOâ‚‚, and 0.3–4.5% Hâ‚‚O, along with small amounts of Fe, Ti, Mn, Na, and F.
Mica typically crystallizes in the monoclinic system, forming laminated or book-like crystals. Well-developed mica may form pseudo-hexagonal prisms. It has perfect cleavage, allowing it to be split into very thin layers. Muscovite can be split into about 10 layers, while phlogopite can be split into 5–10. The cleavage surface exhibits a glassy or pearly luster.
Muscovite is generally colorless and transparent, though it may show green, brown, yellow, or pink tints. Phlogopite is typically yellow, brown, or black, with a glassy sheen and a pearly or semi-metallic luster on the cleavage surface. Muscovite has a transparency of 71.7–87.5%, while phlogopite has lower transparency, ranging from 0 to 25.2%.
Muscovite has a hardness of 2–2.5, while phlogopite measures 2.78–2.85. The elastic modulus of muscovite is (1475.9–2092.7) × 10ⶠPa (15050–21340 kg/cm²), and that of phlogopite is (1394.5–1874.05) × 10ⶠPa (14220–19110 kg/cm²).
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