Microstructural Properties of Natural Allophane/Gibbsite from a White Bauxite Deposit in Montenegro

Abstract

Microstructural properties of white bauxite from Montenegro were investigated. XRD analysis showed the presence of allophane (Al2O3 · 2SiO2 · 3H2O) and gibbsite (γ-Al(OH)3). The allophane phase showed very broadened diffraction lines, thus indicating poor crystallinity as well as very fine nanoparticles in this phase. Upon heating of natural allophane/gibbsite at 1000 °C mullite (Al6Si2O13) was formed, but with a lower degree of crystallinity. FT-IR spectra of allophane, gibbsite and mullite were interpreted. FE-SEM images of natural allophane/gibbsite showed a dominantly amorphous-like morphology. Upon heating of natural allophane/gibbsite two-dimensional (2D) microstructure was visible. EDS analysis showed traces of calcium, potassium, magnesium and iron. The 57 Fe Mössbauer spectrum of iron traces showed a small relative intensity magnetic splitting component due to the presence of hematite and a central quadrupole doublet due to the presence of Fe 3+ ions substituted for Al 3+ in octahedral positions and/or a superparamagnetic fraction of hematite particles. The possible geochemical mechanism of white bauxite formation in Montenegro is discussed.