MICA The commercial importance of mica is due to its highly devel oped cleavage, which permits the manufacture of dimensioned parts with great accuracy; its transparency; flexibility; stability at high temperatures ; and insulating qualities. It is an essential constituent in the ignition system of the internal combustion motor and therefore of great importance in the automobile and aeroplane industries. The rapid development of wireless communication has also been dependent upon abundant supply of high grade mica. Mica production is restricted to the ancient, stable, land masses such as the peninsula of India, central and southern Africa, Mada gascar, the Appalachian area in North America and the eastern areas in South America.
The micas of economic importance are of two varieties : musco tite, found in many localities; and phlogopite found in Madagascar and Canada. India, the United States, Canada and Madagas car produce over 90% of the world's mica. In none of these countries is there any immediate indication of exhaustion of the deposits. Production at recent levels may be maintained for an
indefinite period, and continued search for new deposits may be depended upon to add to known resources.
For years the chief sustaining feature of the mica industry was the fact that there was no satisfactory substitute known, except for a few minor applications. In its most essential uses, as an insu lator in electrical machinery and appliances, and in high grade condensers (especially for radio work), mica had no competition.
In 1938 a possible substitute was announced, in the product known as "Alsifilm," made from bentonite, a type of clay of which plentiful supplies are available. Favourable development of this product will greatly affect the mica industry, in that it promises to supply a satisfactory substitute for the ordinary electrical uses of mica. Up to mid-1939, however, no way had been found to impart to Alsifilm the peculiar combination of properties re quired for high grade condenser work. (G. A. Ro.)