Allophane, Opal, and Organic Matter of Soils and Sediments

Abstract

Allophanes are hydrated, X-ray amorphous alumosilicates of variable composition occurring in soils. Their thermal behaviour is similar to that of halloysites (White), in which allophanes will be quickly transformed in nature by ageing and ordering (Chukrov et al.). The endothermic dehydration peak between 130 and 185° C shifts a little towards higher temperatures with decreasing particle size (Campbell et al.): the larger surface of finer particles is able to adsorb more water. This relation between peak temperature and surface of fine particles can be taken to estimate the grain size of fine-grained clay minerals (see III-5). Analogous to the DTA curves of halloysite and kaolinite, the curve of allophanes shows between 930 and 1000° C the exothermic peak reflecting the formation of spinel. The temperature of this exothermic peak decreases with increasing Fe-contents of allophanes (ΔT decreases, too) until the peak disappears at Fe/Al relations >0.3, in DTA curves of hisingerites, after Ramdohr and Strunz allophanes containing iron. Allophanes are not found frequently in natural samples because they are quickly transformed to halloysite or fireclay.