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

CLAY 16¢ that heat melts some of the constituents, the fluxes, which, on re-solidification, cement the rest into a compact material. Clay may be thus burnt into brick, earthenware, or porcelain. In ordinary brick clay the fluxes are usually alkali from a felspar, carbonate of lime, and oxide of iron; they fuse at a moderate temperature. Common brick is therefore made with a low expenditure of fuel, but will not withstand a high temperature. A good brick clay contains about 45 per cent. of silicate of alumina, 35 per cent. of silica, 3 to 6 per cent. of iron oxide, 3 to 8 per cent. of carbonate of lime, 1 to 4 per cent. of magnesia, 3 to 6 per cent. of potash and soda, and 4 to 6 per cent. of water. Bricks made with more silicate of alumina shrink too much in burning, and those with more silica are too brittle. The red or yellow colour depends on the iron oxide present. Clay with a low proportion of fluxes or a high proportion of infusible material serves as fireclay; it requires greater heat in burning, but its products withstand temperatures as high as 3400° F. The temperature in the kilns is determined by the use of *“ Seger cones ”’ of various clay mixtures, which bend and melt at different temperatures. The refractory nature of some fireclay, such as the under- clay of a coal seam, is due to the alkalis having been with- drawn by the growth of plants; in others it is due to the clay particles having been washed during their deposition, and their soluble constituents thus leached out; or it may be aided, as in the Glenboig fireclay {silica 62-5, alumina 34 per cent., Fe,Oy 2-7 per cent., alkalis and loss -8 per cent.), by the large size of the quartz grains, which are only slightly fretted at a high temperature. Pottery clays and the better clays used in earthenware usually consist of weathered felspathic material which is white owing to the absence of iron, and highly refractory owing to the poverty in lime and alkalis. Cuina-CLay—PnEumMaTOLYTIC ORIGIN—The weathering of felspar produces various amorphous silicates of alumina, such as halloysite, and some products of which the particles are so minute that they cannot be determined under the micro- scope, and are grouped as * clay substance.” The action of hot acids on felspar produces a crystalline hydrous silicate of alumina—kaolinite (Al,04, 28i0,, 2H,0),

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