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

166 ECONOMIC GEOLOGY may be worth £300 a carat; itis mostly mined from a meta- morphic pre-Paleozoic limestone in Upper Burma. Some varieties are named after gems of the same colour ; thus the green is the ‘‘ oriental emerald,” the yellow the ““ oriental topaz,” and the violet the “ oriental amethyst.” The star-ruby and star-sapphire have six rayed series of inclusions. The impure massive form of corundum, emery, is of great value as an abrasive. Large crystals of opaque corundum are found in metamorphic contact zones, and are used for the bearings in instruments, etc. The ruby and sapphire are now manufactured by the fusion of alumina. Emerald—The emerald—the precious variety of beryl {(Be3Al,(SiOy)g), a beryllium aluminium silicate coloured by chromium—is emerald green. It crystallizes in hexagonal prisms, has the hardness of 7-5 to 8, and sp. gr. of 2:63 to 2:8. It usually occurs in pegmatites, mica-schists, and altered limestones. Some beryls in pegmatite are among the largest known crystals and weigh a ton each. Emeralds in ancient times were found mostly in Upper Egypt. In recent years the supply has come from slate in Colombia, from mica-schist in Siberia, and from pegmatite in North Carolina and New South Wales. Topaz — Topaz (a fluosilicate of alumina, (AlF),Si0, rhombic; hardness, 8; sp. gr., 3-4 to 3:6) crystallizes in long rhombic prisms, which are traversed by basal joints. When pure it is colourless, but it is typically yellow ; it has been formed where gneiss, schist, and granite have been acted upon pneumatolytically by fluoric acid. It is generally associated with tin ores. The gems mostly come from the Ural Mountains and Brazil. Zircon (ZrO,, SiO,, silicate of zirconia ; hardness, 7%; 3p. gr., 4-2 t0 4-8) is second in brilliancy only to the diamond, for which it has often been mistaken. If has been called jargon owing to its variable colour, and varieties are known as hyacinth and jacinth. Zircon is a common constituent of granite and syenite, and as it is extremely resistant to weather- ing it is one of the commonest minerals found in the heavy residues of sands and sandstones, Fade, a mineral of value from its translucency and coolness to the touch, includes two species. Nephrite, the green jade of New Zealand and Siberia, is a variety of amphibole. of

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