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Subject: Clays in Soils

Where does clay come from? How can I identify clay in soil? How does one purify it for use?

and then also asked this followup question

Are you aware of any books that might be helpful to beginners. I would like to take a group of young children looking for clay. Then I would like to clean it and use it. Is this possible?

    Clay minerals are produced through hydrolysis weathering reactions which, in words is the reaction between hydrogen ions and a aluminosilicate mineral (such as feldspar) to form soluble cations plus silicic acid plus a clay mineral. Most igneous rocks at the Earth's surface are rich in feldspars and thus weather away to form clays. Clay is also a size term for any sedimentary particle smaller than 0.004mm. Specific clay minerals can be identified in soil by using X-ray diffraction techniques. Clays can also be sorted out of soils using sieving techniques. Certain sedimentary rocks, called claystones or shales are very rich in clay and are of commercial value. The value is dependent on the clay's mineralogical and chemical composition and purity. Common impurities include quartz, mica, calcium carbonates, iron oxides, sulfides and feldspar. Processing clay involves open pit mining and pulverizing the claystone so that it will pass through a sieve with a particular mesh size, such as 90% being retained on a 200 mesh screen (0.074mm openings) with the largest particle being retained on a 30-mesh screen (0.59mm openings). In other cases, the material may have to be pulverized to 99.9% finer than the 325 mesh (0.044mm). All commercial clays are produced in this method. Clays are thus not purified from soils for use.

    References on clays:

  1. R.E. Grim, Clay Mineralogy, 1953; McGraw-Hill Book Company, Inc., New York, 596pp (A classic) The standard work used by geologists and many others. 14 chapters.
  2. R.E. Grim, Applied Clay Mineralogy, 1962, McGraw-Hill Book Company, Inc., New York, 422pp (A classic)
  3. C.I. Rich and G.W. Kunze, Soil clay mineralogy, 1964
  4. A.B. Searle, and R.W. Grimshaw, Chemistry and Physics of Clays and Other Ceramic Materials, 3ed. edition, 1959.
  5. Chamley, H., 1989, Clay Sedimentology, Springer-Verlag, Berlin, 623 pp.
  6. Velde, B, 1985, Clay Minerals,Elsevier, Amsterdam, 427pp.
  7. Weaver, C.E., 1989, Clays, Muds and Shales, Elsevier, Amsterdam, 890pp.

    I don't know where you are nor the geology of your location, so I can not say if it is possible or not to take the studnets on such an exercise. Perhaps you can contact a local geologist at your nearest college or university?

Dr. Craig Glenn, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

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