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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

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Hydroxy double salts (HDS's) comprise a class of layered materials which are similar to layered double hydroxides (LDH's) and show a comparable intracrystalline reactivity. All samples described in this paper were prepared by reacting a solid oxide MeO with a solution of a nitrate M(N03)2: ZnO with solutions of Ni(N03)2-6H20, Co-(N03)2-6H20, and Cu(N03)2-3H20; NiO with Cu(N03)2-3H20; and CuO with Ni(N03)2-6H20, Co(N03)2-6H20, Zn(N03)2-6H20, andCu(N03)2-3H20. Typical compositions of HDS's are [(Me,M)2(0H,N03)4] and [(Me,M)5-(OH)8](N03)2. The hydroxy double salts easily exchange anionic surfactants for the interlayer nitrate anions. Toward short-chain acid anions the HDS's are less reactive than the LDH's. The anion exchange capacity is due to the exchange of anions incorporated in the hydroxide layer (as in the basic copper nitrate [Cu2(0H)3N03]) or of anions bound as gegenions of the positively charged hydroxide layer. The positive charge originates from an excess of the divalent metal ions as in basic zinc salts [Zn5(OH)8] (N03)2-2H20 or [Zn5(0H)8]Cl2-H20. The arrangement of long-chain alkyl sulfate ions in the interlayer space is deduced from basal spacing measurements. In many HDS's the surfactant anions are largely tilted to the hydroxide layers and are aggregated in bilayers. Technical secondary alkanesulfonates which are mixtures of isomers are arranged in the bimolecular film in a way that a constant interlamellar separation is attained. The organic derivatives of the HDS's adsorb neutral molecules between the layers. Typical is the interlamellar adsorption of primary 1-alcanols such as hexanol, octanol, and decanol by the alkyl sulfate derivatives.