Iron in the kaolinites from kaolin and bauxite weathering crusts after granites: EPR data

Abstract

Kaolinite is an economically important clay mineral. It is proposed to use electron paramagnetic resonance (EPR) in order to estimate the quality of kaolin concentrate while studying deposits [1]. This is related to the fact that most EPR signals of kaolinite are ascribed to Fe 3+ ions. Some of them occur as an isomorphic impurity in the mineral structure, while others are incorporated in various micro- and nanoinclusions. The iron admixture significantly affects the physical, chemical, and, of particular importance, technological properties of kaolins. One of the main kaolin sources is weathering crusts, whose formation is accompanied by structural, morphological, and chemical transformations of kaolinite [2, 3]. However, the reasons for its transformation in the weathering profiles of different mineralogical-genetic types and ages remain insufficiently studied. In this work, we present the results of EPR study of kaolinites from the Zhuravlinyi Log (South Urals) and Chaimat (South Vietnam) deposits, which are confined to the weathering crusts after compositionally similar leucocratic granites. The weathering crust of the Zhuravlinyi Log deposit is made up of one kaolin zone up to 40 m thick with a subzone of 10 m and more of alkali kaolins in its lower part. The eluvial kaolin is overlain by the Miocene clays of the Svetloe Formation. The weathering crust at the Chaimat deposit consists of two zones: the lower 10- to 16-m-thick kaolin zone and laterite bauxites in the upper 1‐4 m. The ages of the weathering crusts are respectively, Cretaceous and Middle Miocene. Samples for study were taken from the lower, middle, and upper horizons of the kaolin zones of the considered objects, as well as from laterite bauxites of the Chaimat deposit. Samples were collected through intervals along borehole 2005 at the Zhuravlinyi Log deposit and the section of the Chaimat deposit (Fig. 1). EPR spectra of clay fractions (<63 µ m) of the studied kaolinite samples were recorded on a compact PS-100 radiospectrometer in the X-range at room temperature.