[Chemistry Class Notes] Mica Pdf for Exam

Micas are a category of minerals with one distinguishing physical property: individual mica crystals can be easily broken into incredibly thin elastic plates. This characteristic can be defined as a perfect basal cleavage. It is common in metamorphic and igneous rock and is occasionally found as small flakes in sedimentary rock. It is specifically prominent in several pegmatites, schists, and granites of mica many feet across have been found in some of the pegmatites.

General Considerations

Out of 28 known species of the mica group, only 6 of them are common rock-forming minerals. Muscovite, which is the common biotite and light-coloured mica, typically black or nearly so, are the abundant ones. Phlogopite, which is typically paragonite, and brown is macroscopically indistinguishable from the muscovite. They are also fairly common. Lepidolite, in general, pinkish to lilac in colour, takes place in lithium-bearing pegmatites.

Glauconite, which is a green species that doesn’t contain similar general macroscopic characteristics to other micas, takes palace sporadically in several marine sedimentary sequences. Except for glauconite, all of these micas easily exhibit observable perfect cleavage into the flexible sheets. Glauconite occurs most often as pellets like grains, containing zero apparent cleavage.

Chemical Compositions

Few of the natural micas contain end-member compositions. For example, most of the muscovites have sodium substituting for some potassium, and diverse varieties contain vanadium or chromium or a combination of both aluminium’s replacing parts; furthermore, the Si: Al ratio can range from the indicated 3:1 up to up to 7:1.

The same variations in composition are well-known for other micas. As a result, much like other mineral groups (for example, garnets), different individual parts of naturally occurring mica specimens have different amounts of perfect end-member compositions.

Crystal Structure

Micas contain sheet structures whose basic units have two polymerized sheets of silica (SiO₄) tetrahedrons. Two of these sheets can be placed next to each other with their tetrahedron vertices pointed in the same direction; the sheets are cross-linked by the cations. For example, aluminium in hydroxyl and muscovite pairs complete the coordination of these cations. 

As a result, the cross-linked double layer can be tightly bound, has the bases of the silica tetrahedron on each of its outer faces, and is negatively charged. This charge is balanced by the singly charged large cations. For example, potassium, present in muscovite, which joins the cross-linked double layers to produce the complete structure. The differences among the mica species are based upon differences in the X and Y cations.

Origin

Micas can originate as the result of diverse processes under many various conditions. Their occurrences include crystallization from the consolidating magmas, deposition by the fluids, which are derived either from or directly associated with the magmatic activities, deposition by fluids circulating during both regional and contact metamorphism, and formation as the result of the processes of alteration, perhaps even those, which are caused by the weathering, that involve minerals like feldspars.

Micas’ stability ranges have been studied in the field, and in some cases, their presence (rather than their absence) or some part of their chemical structure may function as geobarometers or geothermometers.

Occurrence of Mica

Mica can be distributed widely and takes place in metamorphic, sedimentary, and igneous regimes. Large crystals of mica, which are used for multiple applications, are typically mined from the granitic pegmatites.

The single crystal of mica (phlogopite), which is the largest documented, was found in Lacey Mine, Canada, Ontario; it is measured as 10 m × 4.3 m × 4.3 m and weighing up to 330 tonnes. The same-sized crystals were also found in Russia and Karelia.

Flake and scrap mica can be produced all over the world. The primary producers of mica as of 2010 were found to be: Finland (68,000 tons), Russia (100,000 tons), South Korea (50,000 tons), United States (53,000 tons), Canada (15,000 tons), and France (20,000 tons). The total production was 350,000 tons globally, although there is no reliable data available for China. Most of the sheet mica was formed in Russia (1,500 tons) and India (3,500 tons).

Flake mica is found in a variety of places, including metamorphic rock known as schist, as a byproduct of the mining of kaolin and feldspar resources, placer deposits, and pegmatites. Considerably, sheet mica is less abundant than scrap and flake mica, and it is recovered occasionally from the mining flake and scrap mica. The important sources of the sheet mica are given as pegmatite deposits. Sheet mica prices differ by grade, ranging from under $1 per kilogramme for low-quality mica to ＄2,000 or more per kilogram for high-quality mica.

In India and Madagascar, it is also artisanally mined in poor working conditions and with child labour help.

Use of Mica

Micas can be used in a wide range of products ranging from paints, drywalls, fillers, especially in automobile parts, shingles and roofing, electronics, and more. The mineral can also be used in cosmetics to add “frost” or “shimmer.”