A B S T R ACT: Many problems--from soil research to ceramics--require a reliable characterization of the clay minerals involved. This can be done using four clay-organic reactions: (i) staining tests and dye adsorption; (ii) glycerol and glycol adsorption; (iii) intercalation; (iv) alkylammonium ion exchange. Dye adsorption (staining tests) and glycerol adsorption allow a preliminary identification of the clay mineral groups. Intercalation reactions indicate minute differences between kaolins which cannot be detected by XRD and DTA. Alkylammonium ion exchange provides the best method for characterizing smectites and is sensitive to changes in the layer charge.About 1840, staining techniques were introduced in biological investigations. The availability of suitable staining tests was the decisive requirement for the detection of virus and bacillus infections. In 1882 Robert Koch detected the tubercle bacillus by staining with methylene blue. The importance of staining methods in biology and medicine induced some scientists to apply the tests in a quite different field of investigation, i.e. for identifying clay minerals. The first results were reported by Behrend in 1881; methylene blue rapidly became one of the most important dyes and has remained so until the present day.The staining tests developed in the 19th century may be considered the basis for the identification of clays by organic reactions. Today, three additional reactions allow a very detailed characterization of clay minerals. These are: the adsorption of neutral molecules, commonly ethylene glycol and glycerol (Bradley, 1945; McEwan, 1948), the intercalation of neutral molecules in kaolins (Wada, 1961;Weiss, 1961) and the exchange of interlayer cations by alkylammonium ions in three-layer clay minerals (Weiss & Kantner, 1960).
FOUR REACTIONS FOR CHARACTERIZING CLAY MINERALS
Adsorption of dyes and staining tests