Smectites identification

Fibrous saponite, with the analyzed chemical formula (Mgs g, AIq i, F o.i)(Si6.76> All o4 Fe 0.2) 02o(OH)4 IOH2O, also had Cao.i and Mgo4 as exchangeable ions (Midgely and Grass, 1956). Saponite is an example of a trioctahedral smectite. The variable chemical compositions of the smectites adds to the difficulties of accurate identification of these minerals. 

Some glauconites have been identified in hydrothermally altered basalts together with celadonites (Alt et al., 1992 Clayton and Pearce, 2000). This material appears to form a mixed-layer mica-ferric smectite series. The formation of glauconite mixed-layer minerals is therefore not restricted to peloids under shallow-ocean-bottom conditions. However, the identification of glauconite as distinct from ferric Ulite is difficult. Perhaps these mineral occurrences should be given another name. 

In these formulas Na and Ca are the exchangeable ions. Smectites normally occur in extremely small crystals with diameters of less than 1 fim. This makes them too small to identify with the optical microscope. Identification is instead usually accomplished by X-ray diffraction techniques. 

FIGURE 8.25 Simplified scheme of the procedure for XRD identification of clay minerals, showing the XRD patterns resulting for a hypothetical mixture of smectite (S), chlorite (C), vermiculite (V), illite (I), and kaolinite (K). (Reprinted from Amonette, J.E., Soil Mineralogy with Environmental Applications, Soil Science Society of America, Madison, WI, 2002, with permission). 

Srodon, J. (1980) Precise identification of illite/smectites interstratifications by x-ray powder diffraction. Clay Clay Miner., 28, 401-411. 

Intracrystallihe swelling was not discovered until the advent of the X-ray diffraction method of analysis (Hofmann et al. [1933], Hofmann and Bilke [1936]). X-rays are, therefore, very important in the study of smectites. Their role is not limited to the analysis of the structure and the identification of the different types of smectites. Instead, X-ray diffraction is the principal method used in the qualitative and quantitative description of smectite properties. 

The following remarks are a resume of the role that crystalline arrangement can play in the identification of smectites. 

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