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

© 2016 Elsevier B.V. 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; Al 2 O 3 ca. 80 wt.%) 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 57 Fe 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 Fe 3+ , in contrast to [6] Cr 3+ , is not exclusively a component of the diaspore structure. While Cr 3 + substitutes Al 3 + in octahedral sites of diaspore ( [6] Cr 3+ ↔ [6] Al 3+ ), the electron microscopy in nanoscale (TEM-EDS & EELS) revealed that Fe exists in the form of peculiar Fe 3+ -bearing nanominerals (most likely maghemite-type phases) between 25 and 45 nm in size, in addition to the Fe 3+ ions substituting Al 3+ in the diaspore structure. Moreover, it was proven that TiO 2 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 Fe 3+ -Cr 3+ -AlOOH low-T authigenic phase, demonstrated for the first time in literature. On the other hand, the observed TiO 2 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.