Halloysite-kaolinite transformation at room temperature

Abstraet--Halloysite (metahalloysite) of various particle sizes has been altered with oxalic and EDTA acids, at room temperature and during different periods of time (5-90 days). The oxalic acid attack at first achieved only a recrystallization f halloysite. The recrystallization is much more significant the smaller the size of the treated halloysite particles. Later the material is destroyed. The EDTA treatment also has provoked uring the first days a recrystallization f the halloysitic material which is destroyed again after about 20-25 days. Later kaolinite is formed. The kinetic curve of kaolinite formation is symmetrical with respect o that corresponding to the diminution of amorphous material in the sample. The influence of the halloysite particle size and the complexing effect of the acids in relation to the resulting products are discussed. INTRODU CTION The transformation f halloysite to kaolinite at room temperature is theoretically possible, since halloysite is metastable with respect to kaolinite. The free ener-gies of formation of these minerals (Hem et al., 1973) are, respectively: mG ~ H/>- 898-4 kcal/ml AG ~> K =-902.9 kcal/ml. The difference between them logically favors the transformation f haUoysite to kaolinite. In Nature, such a transformation was proved by Tamura and Jackson (1953), who established that in certain kaoliniferous oils formed from feldspathic rocks, there existed a successive sequence of transfor-mations from feldspar to kaolinite, passing through amorphous tages (allophanes), hydrated halloysite and metahalloysite. Subsequently, Kinoshita an