Smectites and Vermiculites: The Distinction between Di- and Tri-Octahedral Minerals and Grain Size Determination
The minerals of the montmorillonite group (smectites) can be distinguished from the vermiculites, as they are very similar to them in their thermal behaviour, as regards the temperatures of the exothermic peak. This peak appears in DTA curves of silicates rich in Al (= kaolinites, Alchlorites, montmorillonites) at >900° C. Smectites are mostly di-octahedral (Al-rich montmorillonites); the tri-octahedral types hectorite, saponite and sauconite occur very rarely, whereas vermiculites are generally tri-octahedral. They can, of course, become partly di-octahedral by substitution of the main octahedral cation Mg++ by Al3+ or Fe3+. The temperature of the exothermic peak is a suitable measure for the dioctahedral portion of a vermiculite, and further a hint at the nature of the substituent (Fe3+ or Al3+): the exothermic peak temperature of a “normal” tri-octahedral Mg-vermiculite lies between 830° and 850° C (Grim and Rowland; Barshad). With increasing substitution of Mg++ this temperature will be lowered in case of Fe3+ being the substituent, or it will be raised in the case of Al3+ being the substituent. A (hypothetic!) di-octahedral vermiculite that has occupied half of the central positions of the octahedral layers by Fe3+ and half of them by Al3+, however, would not differ in its exothermic peak temperature from that of a pure Mg-vermiculite.
KeywordsClay Mineral Differential Thermal Analysis Exothermic Peak Half Height Decomposition Peak
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