Heat resistance study of mineral mica species

Mica are a group of minerals with a perfect cleavage. They can split into very thin leaves with equivalent surfaces. Among all known mineral mica species, muscovite and phlogopite are highly important industrially because can be easily split into thin leaves and simultaneously have very high electrical characteristics, incombustibility and a great chemical fastness. Additionally, they are thermally and chemically resistant, poorly hygroscopic, as well as flexible, resilient and transparent as thin leaves. Not only muscovite and phlogopite but also biotite and vermiculite are of practical importance. Biotite is sometimes used to replace muscovite or phlogopite. Vermiculite is a hydrated biotite; it can be hardly split into thin leaves, has low electrical properties and is poorly thermally-resistant. Affected by high temperatures, mica releases water and so gradually loses its brightness and transparency, strongly swells, splits and becomes fragile. Thus, the properties of mica affected by high temperatures largely reduce. Therefore, in order to apply the mineral in materials and devices operating at a very high temperature, we should decide for a relevant type of mica which thermal resistance meets the operating conditions. By the results of the differential-thermal analysis, the mica crystal structure increases in swelling. The authors have identified a temperature range in frames of which mica retains its working properties. Visual changes in swelling in the temperature range of 20-1200 °C have been identified and analyzed. The questions of mica thermal stability for its long-term use in the electric and radio industry have been solved. The technical use of mica in more critical cases has a temperature limit of 600-650 °C. The significant residual swelling allows for the practical use of mica as a heat insulating material.