Chapter XXIII: Mica

BY DURAND A. HALL

USES OF MICA

Two varieties of mica are of particular economic importance: muscovite or white mica, and phlogopite or amber mica. Three other varieties, lepidolite, zinnwaldite and biotite, find occasional commercial use.

Mica is marketed as sheet or block mica, mica splittings, thin sheets split chiefly from smaller sizes of block mica, and scrap or ground mica. The uses to which sheet or block mica may be put depend upon the size, thickness and shape of the piece which can be cut from it and the quality of the material itself. Factors entering into the quality of mica are: presence or absence of stains, spots, inclusions, cracks or pin holes; flexibility and elasticity; hardness; degree of distortion of the sheets; transparency; and dielectric strength.

An essential use of sheet or block muscovite is in electrical work; from the mica are made condensers for radio equipment, magnetos and certain telephone equipment; also to a less extent for resonators in sounding boxes. This mica is also used in making spark plugs, particularly plugs in high-compression engines, for winding cores and as washers in place of porcelain. These uses were widely extended by the war to meet requirements for motor transport, airplanes and radio equipment.

A great deal of this variety of mica is used for other insulating purposes. There are a vast variety of uses, such as for sheets, washers and disks in dynamos, electric-light sockets, guards in rheostats, fuse boxes, telephones, etc.

Sheet or block phlogopite is used for general electrical insulation--particularly where mica softer than muscovite is required.

Mica splittings of both muscovite and phlogopite are employed for the manufacture of “built-up mica,” which is used widely for electric insulation in many different forms, such as sheets, tubes, cups, etc. Mica board built up from phlogopite splittings is used extensively for insulation between the copper segments of commutators.

Among the less essential uses of sheet or block mica (mainly muscovite) are in windows for stove fronts and ovens; chimneys and shades for lamps and lanterns; and for many other purposes where a transparent non-inflammable, non-shattering material is required. It is also used for heat insulation, in various electric heating devices.

Ground mica is also used for heat insulation, as in pipe and boiler covering, etc.; and as a patent roofing, both as a coating to prevent sticking when rolled, and as a filler in the roofing itself. It is also used in annealing steel, and as a lubricant for wooden bearings.

Among the non-essential uses of mica, those for which a satisfactory substitute is known, are the uses of sheet or block mica for phonograph diaphragms, and for decorative purposes, chiefly in India. On a similar basis are the uses of ground mica (mixed with oil) as a lubricant for metal bearings; as a filler for rubber goods, etc.; and for decorative purposes--in wall paper, decorative paints, ornamental stone, etc.

=Substitutes.=--No other substance possesses the combination of elasticity, toughness, flexibility, transparency, ability to withstand excessive heat and sudden changes in temperature, high dielectric strength, flatness and amenability to splitting into thin films, which belongs to mica.

For the vast variety of electrical equipment in which mica is used, no satisfactory substitute has been found. In the manufacture of certain low-tension condensers sheets of oiled paper have been used instead of mica films, but attempts to substitute this material more widely have met with little success. According to one report a compressed paper product called “Pertinax” was developed in Germany during the war which is claimed to be “most satisfactory” for all electrical purposes, even for the manufacture of high-tension condensers. The fact, however, that Germany was paying $75 a pound for mica from Norway, and continued to use mica in the manufacture of condensers for airplane magnetos, indicates that complete substitution was not possible.

For a great many glazing purposes it is possible to substitute heat-withstanding or non-shattering varieties of glass.

CHANGES IN PRACTICE

From the nature of mica deposits there has been little encouragement for the application of any but rather crude and simple methods of mining. These methods proved sufficient as long as there remained new and easily accessible deposits. There is at present a tendency in India, Canada, and the United States to apply more scientific methods to exploration and extraction, where the exhaustion of the easily worked deposits is threatened.

Sorting, cleaning, grading, trimming and cutting of mica for the market are all essentially hand processes, and from the nature of the product will continue to be. For this reason producing localities possessing abundant cheap labor have a distinct advantage over those where labor is high and scarce. In one important direction attempts have been made to apply a mechanical device to a process which has always required hand labor. This is in the manufacture of mica splittings, widely used in the manufacture of built-up mica. These inventions have not yet been demonstrated as commercially successful on a broad scale.

Mica is being used to an increasing extent for electrical purposes. The war created a large demand, particularly for the better grades of material for the manufacture of magnetos and radio condensers and spark plugs, and many of these uses will continue to require much mica. The use of mica for glazing purposes, however, particularly in the fronts of stoves, is diminishing. This is due to the decreasing manufacture of the type of stove in which mica is used.

GEOLOGICAL DISTRIBUTION

It has been estimated that mica constitutes about 4 per cent. of the igneous rocks of the world. Segregation into deposits of workable size containing mica of commercial size and quality is comparatively rare. Mica mines are worked for the small percentage of sheet or block mica they contain, the large amount of waste mica being utilized only where a ready market warrants the grinding.

Individual deposits of both muscovite and phlogopite are characterized by their extreme irregularity, so that any prediction as to reserve is uncertain. This fact is responsible for the crude methods of mining which have for so long been almost universally employed. The output of a district as a whole is from many small mines rather than from any single large one.

An important consideration in the geological distribution of mica from an economic standpoint is the degree of dynamic metamorphism to which the region has been subjected during or subsequent to the formation of the mica-bearing deposits. This is due to the fact that the value of sheet mica depends, among many other factors, upon the freedom from distortion of the sheets.

Muscovite mica in commercial quantities invariably occurs in dikes and lenses of pegmatite which are considered to represent segregations of certain portions of granitic intrusions. The principal associated minerals are quartz and feldspar, both of which are usually considerably in excess of the mica. The dikes or lenses of pegmatite may be within the granite itself or in other inclosing rocks that may seem unrelated to a parent intrusive. Schists or gneisses form the inclosing rock of the pegmatite in most commercial deposits.

Phlogopite is much rarer than muscovite and occurs in deposits structurally similar to granitic pegmatites. The important associated minerals are pyroxene, apatite and calcite, although in certain deposits the mass is almost entirely of mica. The Canadian deposits, which are best known, consist of veins and pockets in pyroxenite dikes and the inclosing rock is usually limestone, a significant fact as regards the origin of the deposits.

GEOGRAPHICAL DISTRIBUTION

India, Canada, and the United States produce about 98 per cent. of the sheet mica of the world. The output of India and of the United States is entirely muscovite, whereas the Canadian production is almost entirely phlogopite. Brazil and Argentina have become important producers in the last two years, and German East Africa was becoming of considerable importance immediately before the war. South Africa, Guatemala, Ceylon, Madagascar and Australia have produced small amounts of mica and may be considered as potential sources of supply.

_India_ produces about 65 per cent. of the sheet mica of the world, and is the most important source of high quality muscovite. The two principal producing regions are the Hazaribagh district, in the Province of Behar, and Orissa, in Bengal, and the Nellore district, Madras Presidency.

The output of _Canada_ is practically all phlogopite or amber mica, of which that country is the world’s principal source. The important producing districts are north of the city of Ottawa, in the Province of Quebec, and in the central part of the Province of Ontario. The Lacey mine, in Ontario, has been the largest producer of amber mica in Canada for many years.

The _United States_ produces about 15 per cent. of the sheet mica muscovite of the world. In terms of value, about 70 per cent. of the American product comes from North Carolina, and 23 from New Hampshire. Other districts are insignificant: 3 per cent. comes from Virginia, 1 from South Dakota, and 3 from Georgia, Alabama, Idaho, and Colorado taken together.

The principal sources of mica in _Brazil_ are the states of Bahia, Sao Paulo, Goyaz and Minas Geraes. The two last mentioned are particularly important. The deposits are widely scattered over a large area.

Production of mica in _Argentina_ has been incidental until very recently and the development has been slight. Deposits are numerous but are of special importance in the provinces of Cordoba, San Luis and San Juan. As far as is known muscovite is the only mica produced.

Considerable activity has been shown in mica mining in _Guatemala_ and shipments have been made quite regularly to this country. The mica, however, is of inferior grade and has not found favor in American markets.

The production of the former _German East Africa_ is confined entirely to muscovite mica found in the districts of the Ulguru Mountains and neighboring ranges. Considerable quantities of high-grade material were exported to Germany before the war. The mica mines in German East Africa were worked during the war under the direction of the British Ministry of Munitions. The district is of importance as a future source of high-grade muscovite.

The principal locality in _South Africa_ is near Leydersdorp, in the Northern Transvaal. Both muscovite and amber mica are reported from this area.

Small amounts of high-quality amber mica have been shipped from _Ceylon_ to England.

Deposits of excellent quality mica are known to exist in the McDonald range, in _South Australia_, from which small shipments have been made sporadically. This is the only district from which lepidolite or lithia mica has been noted in sheets of commercial size.

In _China_, large deposits of muscovite mica are reported from the vicinity of Kiao-Chau bay and also from Shantung, but no commercial development has taken place, and it is believed that the deposits are worthless as a source of sheet mica.

Prior to 1914 efforts to produce mica in _Norway_ on a commercial scale met with no success and little is known of the deposits. The great need of mica in Germany during the war stimulated production in Norway, tremendous prices being paid for the material.

CHANGES IN KNOWN GEOGRAPHICAL DISTRIBUTION IN THE NEAR FUTURE

The most important change in known geographic distribution of mica for the near future concerns the increased development of the South American fields. During the war Brazilian mica found considerable favor both in this country and Europe, and the better grades are considered equal to the best India mica. Importations of mica from _Brazil_ were of much importance in meeting the large demands of the United States for mica of high quality to fill government needs. Owing to the nature of mica deposits in general, it is not safe to make estimates concerning reserves. This is particularly true of Brazil, where the industry is young, and careful prospecting and development have not been carried far. From the large number of known deposits and the rapidity with which the industry responded to war demands, despite difficulties in transportation from the mines to the seaboard, it would appear that Brazilian mica will play an increasingly important part in the mica markets of the world, particularly of the United States. No matter how great the merit of the mines of a district may be, however, unless the mica is carefully prepared, graded as to size and quality, and shipments are standardized, it cannot expect to attain permanent favor among consumers in this country or in Europe.

Less is known concerning the future possibilities of _Argentina_ as a mica-producing country of importance. Many deposits are known and considerable shipments have been made to the United States and to Europe. The material received here has not been equal to Indian or Brazilian mica.

There is every reason to expect that _India_ will retain its position as the most important mica-producing country. The great number of deposits offer almost every grade of muscovite required in the trade. The industry is well-established, and labor conditions are extremely favorable for the production of a commodity requiring such a large amount of hand labor before being ready for market. It is true, however, that the richest and most easily accessible deposits have been mined out, and scientific methods must be applied to mining if production is to retain its former place.

It is doubtful whether the immediate future will see any important change in the position of the _United States_ as a mica producer. Although the reserves of the country as a whole are probably large, the deposits are small and the percentage of high-quality mica is not great. Production of important amounts of medium-grade mica will continue. Labor conditions are not favorable to producing cheaply an article that has a large part of its value determined by preparation and careful grading.

Scientific methods have been applied to the mining and preparation of a considerable part of the Canadian output. Reserves are large and _Canada_ will undoubtedly retain for many years its position as the principal source of amber mica.

POLITICAL AND COMMERCIAL CONTROL

The British Empire, through state sovereignty, controls 75 per cent. of the present sheet-mica production of the world. In Brazil and Argentina, which have important potential resources of mica, and in the United States, political control is determined by state sovereignty.

For the most part, the ownership of mines and concessions in India has been in the hands of natives. In the Hazaribagh district, F. F. Christen & Co., an English firm, owns rights over large areas of land outside the government forest and has mined on a considerable scale. At the outbreak of the war, Germany, through commercial interests, had obtained a large measure of control over many Indian mines. S. O. Fillion & Co., of New York, is the one American firm known to be working mines in India.

The most important producing phlogopite mine in Canada, at Sydenham, Ontario, is owned and operated by the General Electric Co., of Schenectady, New York. The same company owns several other properties in the vicinity and has a large mica-manufacturing plant at Ottawa, Ontario. Other smaller interests are held chiefly in Canada.

Davol & Co., of New York, is the only foreign firm known to be actively working mica mines in Brazil at present.

Mica mines in the United States are, as far as is known, all owned by Americans.

POSITION OF LEADING NATIONS

The _United States_ was before the war the world’s largest consumer of mica of all kinds. The development of her electrical industry is dependent upon her supply of mica, a large part of which is imported from India and Canada. Production in this country has been considerable, but has not proved nearly adequate to supply the demands, particularly the demand for the higher grades of mica for use in magnetos, radio condensers and spark plugs, and for mica splittings used in making built-up mica.

The large demands and high prices created by the war did not increase domestic production enough to warrant the belief that this country can become independent as regards sheet mica under any but the most artificial conditions. The producer is protected by a 25 per cent. import duty on unmanufactured (rough or knife-trimmed) mica and a 30 per cent. duty on cut mica, splittings and other manufactured forms. Production in this country is largely in the hands of individuals and small companies, who are financially incapable of increasing output spasmodically even under very favorable conditions. Moreover, many consumers have a decided prejudice in favor of imported mica.

If the United States is to take a dominant position in the electrical industry of the world an adequate supply of mica must be assured. The most promising field for the development of such a supply is undoubtedly South America. This is particularly true because the important development of the electrical industry in England during the war places that country in the dominating position formerly occupied by Germany and Austria. This development will require a large part of the Indian mica formerly available for export trade.

The _British Empire_ possesses in India the most important source of muscovite mica in the world and in Canada the only important supply of amber mica. In spite of England’s tremendous advantage with regard to raw material, Germany, through her important position in the electrical industry and the large measure of control acquired in the Indian mines, threatened to dominate the mica market of the world at the outbreak of the war. Since the outbreak of the war England has secured her position not only as the controlling center for raw mica but as the chief producing nation of electrical equipment. London is the distributing center of the world for Indian mica and London prices regulate the market. During the war Indian mica was controlled by the British Ministry of Munitions, and allotments were made to the associated nations at fixed prices.

In the development of the South American fields lies the best possibility of the world lessening its dependence upon England for this most essential raw material.

_France_ is entirely dependent upon outside sources for her supply of mica. Before the war her demands were not large and were filled by Indian mica.

Although _Germany_ before the war was entirely dependent on outside sources for mica, her leading position in the electrical industry enabled her to gain control of much of the Indian production. Every advantage was taken of this opportunity, and in 1914, according to the British Secretary of Munitions, the mica market of the world was at the point of being transferred from London to Hamburg. The deposits of German East Africa were being actively exploited and German commercial interests were being extended to South America.

Although probably possessing very large stocks in 1914, Germany felt very acutely the shortage of mica during the war. High prices were paid to Norway for the output of that country, but this source is probably entirely incapable of meeting the normal demands of Germany.

SUMMARY

Sheet mica is essential to the manufacture of a vast variety of electrical equipment, and must, therefore, be classed as one of the important raw materials of the world. The magneto, such a vital factor in modern transport on land, air and sea, depends upon mica for its construction, and mica condensers are indispensable in modern radio equipment.

India, Canada, and the United States are at present the most important mica-producing countries. Deposits of future importance from which production has thus far not been great are known in German East Africa, Brazil, Argentina and the Transvaal; from several other parts of the world more sporadic production is reported.

Unlike the development of many other raw materials the production of mica has not been universally undertaken on a large scale, nor have scientific methods been applied to its extraction. This has been largely due to the irregular nature of the deposits and their scattered position within a district, making considerable investment of capital in mining a particularly hazardous venture. Trading interests have, therefore, played an important part in controlling production and markets.

The British Empire, having within its boundaries a large proportion of the important mica-producing districts, at present dominates the situation politically. British commercial control, threatened by Germany’s leading position in the electrical industry and wide interests in Indian mines at the outbreak of war, is now firmly re-established, and Great Britain has taken the place in the electrical industry formerly held by Germany and Austria.

The United States, the largest consumer of mica among the nations of the world, has relied upon imports to supply a considerable part of the consumption and will probably continue to do so in the future, although steady development of domestic sources of supply may be expected. Brazil offers the most promising foreign field for the development of an independent source of supply for American markets.