Nano-mineralogy and -geochemistry of high-grade diasporic karst-type bauxite from Parnassos-Ghiona mines, Greece

Abstract

In the present work, a combination of various techniques is utilized for the study of nano-mineralogy and -geochemistry of high-grade karst-type bauxite (Al-rich and Fe-depleted samples; Al2O3 ca. 80wt.%) from the Parnassos-Ghiona mines located in Greece. Initial characterization using PXRD and electron microscopy in microscale and mesoscale (SEM-EDS including STEM mode), proved the presence of “Fe-Cr-Ti-containing diaspore”, anatase and minor rutile. The study by means of 57Fe Mössbauer spectroscopy, in correlation with magnetic susceptibility measurements and, complemented, with Synchrotron-based spectroscopies at the microscale (SR micro-XRF and micro-XANES/-EXAFS), indicated that Fe3+, in contrast to [6]Cr3+, is not exclusively a component of the diaspore structure. While Cr3+ substitutes Al3+ in octahedral sites of diaspore ([6]Cr3+↔[6]Al3+), the electron microscopy in nanoscale (TEM-EDS & EELS) revealed that Fe exists in the form of peculiar Fe3+-bearing nanominerals (most likely maghemite-type phases) between 25 and 45nm in size, in addition to the Fe3+ ions substituting Al3+ in the diaspore structure. Moreover, it was proven that TiO2 polymorph mineral nanoparticles, particularly rounded anatase mesocrystals and nanocrystals and individual needle-shaped rutiles, are dispersed into the diaspore matrix. Thus, diaspore in the studied bauxite concerns -in fact- a distinct Fe3+-Cr3+-AlOOH low-T authigenic phase, demonstrated for the first time in literature. On the other hand, the observed TiO2 mineral nanoparticles (formed, together with diaspore, during diagenesis) and Fe nanominerals (formed during epigenesis) were hitherto unknown not only for the allochthonous karst-type bauxite deposits of Greece, but also for the overall bauxite deposits, worldwide.