The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

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. Neverthe- less 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, phlogo- pite. 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 peg- matite in schist and gneissose granite. Pegmatite is some- times injected as a molten intrusion; but some occurrences 157

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