PART III

EARTHY MINERALS

CHAPTER XII
THE MICAS, ASBESTOS, AND GEMS

THE non-metallic minerals include many species of value
from special physical and optical properties.

Tur Micas

DistriBuTioN AND Usks—The micas are anomalous in
distribution ; they are ubiquitous in minute flakes, being
essential constituents of many abundant rocks, such as
granite; gneiss, mica-schist, minette, and kersantite, being
common in micaceous sandstones, forming the bulk of some
clays, and giving the blue colour to the Swiss lakes. Nevertheless
 mica of industrial service is exceptionally local; it is
found only in deep-seated pre-Paleozoic rocks; 70 per cent.
of the world’s supplies comes from one district in India, and
most of the rest is from two or three localities.
The chief uses of mica are for windows and lampshades,
where heat or vibration are too great for glass, and as an
Insulator in electrical machinery. The most useful micas
are the white mica, muscovite, and a brown species, phlogopite.
 Muscovite is usually found with pegmatite, as in
Bihar and Nellore in India, in the Transvaal, Tanganyika
Territory, Kenya Colony, Brazil, Quebec, the Eastern United
States, and Russia.
PNEUMATOLYTIC Or1GIN—The mica of Bihar is found with
sheets, “blows ” or lenticles, and irregular masses of pegmatite
 in schist and gneissose granite. Pegmatite is sometimes
 injected as a molten intrusion; but some occurrences
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