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

138 ECONOMIC GEOLOGY pp. 84-5), the tricalcic silicate and aluminate of alite are decomposed by water and form an especially active lime hydrate (CaH,0,), which crystallizes and acts as cement, The tricalcic aluminate decomposes first and its calcic hydrate sets first. The tricalcic silicate decomposes more slowly, and its silica is deposited in the setting mass and hardens it, The celite is almost inert, so that it usually takes no part in the reaction but is enclosed in the set alite. Price—The manufacture of Portland cement on a great scale began in England and it was not until about 18g3 that it was largely made in the United States, which has since become the greatest producer with about 26 million tons per annum; Germany is second with 7 million tons; the British Empire makes 5 million tons; France and Belgium 2 million tons apiece. The average price from 1870 to 1880 was 12s. a barrel (380 lb. before 1921, later 376 1b.) ; it had fallen to 8s. a barrel by 1893; American competition and improved methods of manufacture lowered the price to 3s. 6d. in 1908; it rose at the end of the War to 8s., and in [026 to 10s. a barrel, Gypsum CEMENTS—PLASTER OF Paris—Plaster of Paris is a cement made from gypsum (CaSO, 2H,0), which when pure consists of 79-1 per cent. of sulphate of lime and 20-9 per cent. of water. Alabaster is the white fine-grained variety used for statuary : selenite is the water-clear crvstal- line variety. Primary gypsum, due to the evaporation of sea-water, often occurs as beds or lenticles interbedded in red shales or marls. Gypsum, formed by the hydration of anhydrite (CaSO,), has disturbed and slicken-sided margins owing to its expansion. Gypsum occurs in veins, in irregular pockets traversing calcareous rocks, as a limestone-gypsum breccia, and in masses in contact with corroded surfaces of limestones, which, to use J. V. Harrison's phrase, look worm-eaten. Such secondary gypsum is due to the decomposition of pyrites having produced sulphuric acid, which has invaded limestone and altered the carbonate into sulphate. The process happens on a small scale in clay and forms selenite in crystals, nodules, and casts of shells. This explanation was adopted by Dana (1871, Manual of Geology, P. 248) for gypsum deposits in

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