Some physical properties of mica

Abstract

This paper gives data on the linear thermal expansion, changes in structure, power factors, and effects of heat treatments on the thickness, opacity, and color of micas (muscovite, phlogopite, biotite, ripidolite, and zinnwaldite)from different domestic and foreign sources. Tremendous linear thermal expansion was noted for some samples of phlogopite and biotite micas in a direction perpendicular to the cleavage plane. The transitions shown in the expansion curves of these samples at elevated temperatures appear to be related to the structural changes indicated in the X-ray diffraction patterns. Heat treatment of two phlogopite micas and two muscovite micas to 600°C, with or without a load, caused con- siderable increases in the power factors of the former and only slight changes in the latter. Nearly all the muscovite samples showed the greatest increases in thickness (up to 155 percent) after heat treatment at 8000 C. The large increases in thickness of nearly all the muscovite samples accompanied changes from clear or translucent to opaque, or from polychrome to metallic color. None of the species of mica can be considered as a substance or material of fixed and re- producible properties. The physical properties of mica depend largely upon the chemical composition, the nature of the crystals, their magnitude and orientation, the presence of impurities, the way in which these enter the structure, the heat treatment, etc. Some of the phlogopite and biotite micas that possess tremen- dously high thermal expansion in a direction perpendicular to the cleavage plane may be used for high-expanding elements in temperature-responsive devices but may be unsatisfactory for use in devices in which large changes in dimensions with changes in temperature are not desirable.