Fig. 1. Graphic representation of volume change in weathering of a Georgia granite.[ToList]
Mineral products formed by weathering from common igneous rocks include soils, clay, bauxite, and certain iron, chromite, and nickel ores. Again the commercial importance of this group is not large, as compared with products formed in other ways described below.
The same weathering processes described above for igneous rocks cause considerable changes of economic significance in deposits formed as igneous after-effects. In some cases they result in removing the less valuable minerals, thus concentrating the more valuable ones, as well as in softening the rock and making it easier to work; and in other cases they tend to remove the valuable constituents, which may then be redeposited directly below or may be carried completely out of the vicinity. The oxide zones of many ore bodies are formed by these processes.
Sedimentary Processes
Sedimentary rocks are formed by the removal and deposition of the weathered products of a land surface. Air, water, and ice, moving under the influence of gravity and other forces, all aid in this transfer. The broken or altered rock materials may be merely moved down slopes a little way and redeposited on the surface, forming one type of terrestrial or subaërial deposits, or they may be transferred and sorted by streams. When deposited in streams or near their mouths, they are known as river, alluvial, or delta deposits. When carried to lakes and deposited they form lake deposits. Ultimately the greater part of them are likely to be carried to the ocean and deposited as marine sediments.
Part of the weathered substances are carried mechanically as clay and sand, which go to make up the shale and sandstone sediments. Part are carried in solution, as for example lime carbonate and magnesium carbonate, which go to make up limestone and dolomite. Some of the dissolved substances are never redeposited, but remain in solution as salts in the sea, the most abundant of which is sodium chloride. Some of the dissolved substances of weathering, such as calcite, quartz, and iron oxide, are carried down and deposited in openings of the rocks, where they act as cements.
The sediments as a whole consist of three main types,—shales (kaolin, quartz, etc.), sandstones (quartz, feldspar, etc.), and limestones or dolomites (carbonates of lime and magnesia). Of these, the shale group is by far the most abundant. There are of course many sediments with composition intermediate between these types. There are also sediments made up of large undecomposed fragments of the original rocks, cemented to form conglomerates, or made up of small fragments of the original rocks cemented to form arkoses and graywackes. These, however, may be regarded as simply stages in the alteration, which in repeated cycles of weathering must ultimately result in producing the three main groups,—shales, sandstones, and limestones.
Mineral products formed by sedimentary processes include sandstones, limestones, and shales, used as building stone and road materials; certain sedimentary deposits of iron, like the Clinton ores of the southeastern United States and the Brazilian ores; important phosphate deposits; most deposits of salt, gypsum, potash, nitrates, etc.; comparatively few and unimportant copper deposits; and important placer deposits of gold, tin, and other metals, and precious stones. With the aid of organic agencies, sedimentary processes also account for the primary deposition of coal and oil.