mica

Rock with mica

Mica sheet

Mica flakes

The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition. The highly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

The word "mica" is thought to be derived from the Latin word micare, meaning "to glitter", in reference to the brilliant appearance of this mineral (especially when in small scales).

Mica classification

Chemically, micas can be given the general formula^[1]

X₂Y_4–6Z₈O₂₀(OH,F)₄

in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs;

Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.;

Z is chiefly Si or Al but also may include Fe³⁺ or Ti.

Structurally, micas can be classed as dioctahedral (Y = 4) and trioctahedral (Y = 6). If the X ion is K or Na the mica is a common mica whereas if the X ion is Ca the mica is classed as a brittle mica.

Trioctahedral micas

Common micas:

• Phlogopite
• Biotite
• Zinnwaldite
• Lepidolite
• Muscovite

Brittle micas:

• Clintonite

Interlayer deficient micas

Very fine-grained micas with typically more variation in ion and water content are informally termed clay micas. They include

• Hydro-muscovite with H₃O⁺ along with K in the X site;
• Illite with a K deficiency in the X site and correspondingly more Si in the Z site;
• Phengite with Mg or Fe²⁺ substituting for Al in the Y site and a corresponding increase in Si in the Z site.

Occurrence

Mica output in 2005

The British Geological Survey reports that as of 2005, India had the largest deposits of mica in world. China was the top producer of mica with almost a third of the global share, closely followed by the USA, South Korea and Canada. Large deposits of sheet Mica were mined in New England from the 19th Century to the 1960s. Large mines existed in Connecticut, New Hampshire, and Maine.

Mica is widely distributed and occurs in igneous, metamorphic and sedimentary regimes. Large crystals of mica used for various applications are typically mined from granitic pegmatites.

Until the 19th century, large crystals of mica were quite rare and expensive as a result of the limited supply in Europe. However, its price dramatically dropped when large reserves were found and mined in Africa and South America during the early 1800s. The largest sheet of mica ever mined in the world came from a mine in Denholm, Quebec, Canada.^[2]

Scrap and flake mica is produced all over the world. Flake mica comes from several sources: the metamorphic rock called schist as a by-product of processing feldspar and kaolin resources, from placer deposits, and from pegmatites. Sheet mica is considerably less abundant than flake and scrap mica. Sheet mica is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are pegmatite deposits.

Properties and uses

Mica has a high dielectric strength and excellent chemical stability, making it a favoured material for manufacturing capacitors for radio frequency applications. It has also been used as an insulator in high voltage electrical equipment, and between the bars of commutators in Direct Current motors and generators. It is also birefringent and is commonly used to make quarter and half wave plates.

Illites or clay micas have a low cation exchange capacity for 2:1 clays. K+ ions between layers of mica prevent swelling by blocking water molecules.

Because mica can be pressed into a thin film, it is often used on Geiger-Müller tubes to detect low penetrating alpha particles.

Some brands of toothpaste include powdered white mica. This acts as a mild abrasive to aid polishing of the tooth surface, and also adds a cosmetically-pleasing glittery shimmer to the paste. The shimmer from mica is also used in makeup, as it gives a translucent "glow" to the skin or helps to mask imperfections.

Mica is used in the production of pearlescent pigments. Many metallic looking pigments are composed of a substrate of mica coated with another mineral, usually titanium dioxide (TiO₂). The resultant pigment produces a reflective color depending on the thickness of the coating. These products are used to produce automobile paint, shimmery plastic containers, high quality inks used in advertising and security applications.

Mica sheets are used to provide structure for heating wire (such as in Kanthal or Nichrome) in heating elements and can withstand up to 900 °C (1,650 °F).

Another use of mica is in the production of ultraflat thin film surfaces (e.g. gold surfaces) using mica as substrate. Although the deposited film surface is still rough due to deposition kinetics, the back side of the film at mica-film interface provides ultraflatness, when the film is removed from the substrate.

Muscovite mica is the most common substrate for sample preparation for the atomic force microscope. Freshly-cleaved mica surfaces have been used as clean imaging substrates in atomic force microscopy, enabling for example the imaging of bismuth films,^[3] plasma glycoproteins,^[4] membrane bilayers,^[5] and DNA molecules.^[6]

Greases used for axles are composed of a compound of fatty oils to which mica, tar or graphite is added to increase the durability of the grease and give it a better surface.

Isinglass

Because mica is resistant to heat it is used instead of glass in windows for stoves and kerosene heaters. It is also used to separate electrical conductors in cables that are designed to have a fire-resistance rating in order to provide circuit integrity. The idea is to keep the metal conductors from fusing in order to prevent a short-circuit so that the cables remain operational during a fire, which can be important for applications such as emergency lighting.

Thin transparent sheets of mica called "isinglass" were used for peepholes in boilers and lanterns because they were less likely to shatter compared to glass when exposed to extreme temperature gradients. Such peepholes were also used in "isinglass curtains" in horse-drawn carriages^[7] and early 20th century cars. A book about a journey in a Model T Ford car describes isinglass curtains as follows:

"Oiled canvas side curtains were put up over the windows for wind, rain, and cold (there were no heaters) and were held in place with rods that fit into the doors and twisting button snaps around the perimeter... 'Isinglass' peepholes in the curtains allowed limited visibility. Isinglass was made of thin sheets of cracked mica."^[8]

Mica in ancient times

Hand carved from mica from the Hopewell tradition

Human use of mica dates back to pre-historic times. Mica was known to ancient Egyptian, Greek and Roman civilizations, Chinese civilization, as well as the Aztec civilization of the New World.

The earliest use of mica has been found in cave paintings created during the Upper Paleolithic period (40,000 BC to 10,000 BC). The first hues were red (iron oxide, hematite, or red ochre) and black (manganese dioxide, pyrolusite), though black from juniper or pine carbons has also been discovered. White from kaolin or mica was used occasionally.

A few kilometeres northeast of Mexico City stands the ancient site of Teotihuacan. The most striking visual and striking structure of Teotihuacan is the towering pyramid of the sun. The pyramid contained considerable amounts of locally mined mica in layers up to 30 cm (12 in) thick.^[9]

Throughout the ages, fine powders of mica have been used for various purposes, including decorative purposes. The colored Gulal and Abeer used by Hindus of north India during holi festival contain fine small crystals of mica. The majestic Padmanabhapuram palace, 65 km (40 mi) from Trivandrum in India, has colored mica windows.

See also

• Mica insulator

References

External links

• Mineral Galleries data
• Mindat

Wikimedia Commons has media related to: Mica