Electrical properties of mica

Clear ruby muscovite mica is classified as a low loss insulating material and many values for dielectric loss and dielectric constant are given in the literature. Such values presumably are based on measurements made in the direction perpendicular to the plane of basal cleavage. We find that mica inherently has a much higher dielectric loss parallel to the plane of cleavage and that impurities such as entrapped water, and semi-conducting mineral inclusions further exaggerate this condition. Such excessive parallel loss, particularly that due to entrapped moisture, may have some bearing on the anamolous behavior of some mica capacitors, with respect to poor life, excessive r-f heating, silver ion migration, low Q of small size capacitors, excessive temperature coefficient and other adverse effects.