QEMSCAN® analysis can be applied to bauxite ores, allowing a rapid quantification of mineralogy (including trace detrital phases) and assessment of the individual textural characteristics of the bauxite lithotypes, showing a detailed image of the distribution of economic and non-economic minerals and their intergrowths. The published data on this
subject are currently relatively scarce, and are unhelpful for understanding the basic principles and advantages of applying QEMSCAN® analysis to this kind of ore. In this study the QEMSCAN® development work has been applied to the South Italian karst bauxites, which consist of small deposits that are currently uneconomic, but that can be considered as a model analogue for truly economic karst bauxite ores. The
representative samples considered for this study come from two different localities: the Matese Mountains and the Caserta district. The mineralogical composition of bauxite has been quantified by combined QEMSCAN® and XRD Rietveld Quantitative Phase Analysis (QPA), as well as with SEM-EDS techniques. Our results show that the QEMSCAN® data allow a detailed textural characterization and add significant information on the major and trace mineral distribution. This methodology can augment or replace other time-consuming quantitative phase analyses for mineralogical studies of bauxites, provided that the SIP (species identification protocol) database has been carefully validated by preliminary use of XRD and SEM-EDS. However, the XRD (QPA) and QEMSCAN® analytical techniques can be complementary for bauxite ore evaluation, and a very powerful tool for exploitation and mineral processing, as shown in several examples. The results obtained with each technique are compared, and the advantages extensively discussed
Analcime (Na x Al x Si 3-x O 6 ⅐ [(3-x)/2]H 2 O, where x varies from ϳ ϳ0.78 to ϳ ϳ1.06) is one of the most common rock-forming zeolites. It forms in a wide range of geologic environments that span a range of temperature and pressure from ambient to magmatic conditions. Cluster variation method analysis of 29 Si magic angle spinning nuclear magnetic resonance spectra indicates 1) the presence of at least two distinct states of short range Si/Al disorder [low (less disordered) and high (more disordered) analcime], and 2) that configurational entropy associated short-range Si-Al disorder within each of these states increases regularly with increasing Si content. Hydrofluoric acid (HF) solution calorimetry at 50°C was used to determine the enthalpy of formation of five pure analcime samples of varying composition (range of x approximately 0.95 to 1.05) and Si-Al disorder. Enthalpies of formation from the elements at 25°C (⌬H f ) for these samples fall on a linear trend, except for one sample of high analcime for which ⌬H f was about 6.1 ؎ 3.0 kJ/mol less stable than a low analcime of the same composition. Comparison with the results of previous calorimetric studies indicates negligible excess enthalpies of mixing in both low and high analcime solid solutions (that is, the solid solutions are athermal). The configurational entropies derived from cluster variation analysis were in turn used to derive activity-composition relationships for low analcime solid solutions whose compositions are bounded by an aluminous endmember (Na 1.05 Al 1.05 Si 1.95 O 6 ⅐ 0.975H 2 O) and a siliceous endmember (Na 0.75 Al 0.75 Si 2.25 O 6 ⅐ 1.125H 2 O). These relationships were used to retrieve thermodynamic properties for the endmembers from experimental observations of equilibria between analcime, albite, and aqueous solutions. Retrieved values of ⌬H f are in excellent agreement with the calorimetric results of this study. Comparative analysis of equilibrium observations in the literature indicate that one sample of analcime from the Mont St. Hilaire alkaline intrusive complex used for analcime solubility measurements is high analcime. The Gibbs energy of disordering at 298.15 K, 1 bar consistent with the retrieval calculations is ϳ ϳ 6 kJ/mol. The thermodynamic properties of disordering for analcime indicate that hydrated low analcime is stable with respect to hydrated high analcime everywhere in Earth's crust. Phase relations between low analcime, quartz, albite, and aqueous solutions calculated from the retrieved thermodynamic data indicate that at quartz equilibrium, low analcime should become more Si-rich with increasing temperature and pressure and that the composition of analcime is a sensitive function of the chemical potential of SiO 2 . Stable equilibrium between analcime, albite, quartz and H 2 O occurs at much lower temperatures than suggested by earlier phase equilibrium experiments. The breakdown of analcime plus quartz to form albite in geologic systems probably reflects metastable equilibrium in which the composition of...
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