Many persons upon learning that average shale, and even “clay dirt,” may contain 15% alumina, Al₂O₃ (equivalent to almost 8% metallic aluminum), become thoughtlessly and erroneously enthusiastic about aluminum ore possibilities on their farms or properties. The aluminum is there all right, but it is so securely combined with silica and other elements that the cost of extraction is now greater than the price of aluminum obtained from less abundant ores. Until chemists find a method of extraction of the metal from ordinary clay or shale that can be carried out at considerably less expense than is now possible, the vast quantities of clay and shale on the earth’s surface must be considered a distant reserve of a prohibitively high cost aluminum.
Missouri possesses a little bauxitic clay in the southeastern part of the state but unfortunately does not contain deposits of high grade bauxite, the chief ore of aluminum, and so does not contribute to the aluminum production of the United States (see the discussion under [DIASPORE CLAY]). Arkansas is a leading producer of bauxite, but the geological conditions present in that bauxite locality are so different from Missouri geology that little hope is held for finding bauxite in Missouri, except possibly in the extreme southeastern part.
Fire Clay
Fire clay resembles shale in that it is also a clayey rock and becomes muddy upon wetting and rubbing. It differs from shale at sight in that it (fire clay) is not laminated like shale, but occurs instead in a massive structure which is relatively uniform throughout. Fire clay fractures naturally into blocky or irregular fragments ranging in size from boulders to rough flakes, whereas shale weathers into layered, platy chips.
Shales are commonly buff, yellow, reddish, greenish, or brown in addition to gray in color, whereas good useable fire clay predominates in white, cream, and gray to almost black (if much organic matter is contained in it). Shale is ordinarily gritty with hard sand particles, but most good Missouri fire clay contains only a small amount of sand. Of course, fire clay may grade into sandstone through a sandy clay phase, but this part would not be confused with a layered, gritty shale.
The really determining characteristic of fire clay is its resistance to melting under high temperature. The most positive test for this property is to heat the fire clay to a white heat in comparison with standard preparations (Pyrometric Test Cones) whose fusion temperatures are known. Most of Missouri fire clay will withstand a clean oxidizing heat of over 3000° Fahrenheit without melting.
Clay minerals originate, in general, from the weathering of previously existing silicate rocks and have therefore been called, on occasions, “rotted rocks.” The writer has long insisted that clays, particularly fire clays, should be thought of instead as purified or refined rocks. The original silicate rocks and minerals, which were rich in constituents melting at low temperatures, have been soaked, leached, and washed by chemically active ground water and rain water until many of the undesirable elements have been carried away, leaving a refined material which we use and know as fire clay. Missouri possesses one of the largest reserves of finest quality fire clay in the world. Special bulletins on Missouri fire clay are published by the State Geologist, Rolla, Missouri, and may be obtained from his office.
Plastic fire clay. Shows typical break and naturally polished slicken-sided surfaces. From Mexico, Missouri.
