Crystal chemistry of melanites from Mt. Vulture (Southern Italy)

Melanites from Mt. Vulture, an extinct volcano located in the South of Ital y (Potenza, Basilicata), have been investigated via electron probe microana lysis (EPMA), single crystal X-ray diffraction (XRD) analysis, Mossbauer sp ectroscopy (MS) and infrared spectroscopy (IR). The investigations were per formed on six samples from two different Mt. Vulture volcanic deposits. The combination of the results from these techniques led to the following cati ons site distribution: Ca, Mg, Mn, Fe2+ at X; Al, Fe2+, 3(+), Ti3+ 4(+) at Y; Si, Fe3+, Ti4+ at Z. The oxidation states of the transition elements were determined directly fo r iron and indirectly for titanium via Mossbauer spectroscopy. The latter i nvestigation yielded the following iron partitioning: Fe3+(Y) > Fe2+f (X) > Fe2+ (y) = Fe3+ (Z). To clarify the ambiguity about the location of the Fe 2+ (X) absorption doublet, two fitting models were compared. The best model was used both to estimate correctly iron occupancies and to infer Ti distr ibution and oxidation state. Good agreement was found between direct (XPS, previous study) and indirect (Mossbauer, this study) determination of Ti oc cupancy. In particular it was confirmed that Ti may exist as Ti3+ and Ti4at Y and as Ti4+ at Z. IR spectroscopy confirmed the presence of hydroxyl groups in the studied ga rnets, which was also indicated by the chemical analyses. Considering the c ation sites and valence states, the main substitution mechanisms affecting the Mt. Vulture melanites were found to be: 1) Ti4+(Y) + Fe3+(Z) <-> Si4+(Z ) + M3+(Y) (M = Al, Fe, Ti); 2) (O4H4)(4-) <-> (SiO4)(4-); 3) Fe2+(Y) + Ti4 +(Z)+ OH- <->, Fe3+(Y)+ Si4+(Z) + O2-. Finally, the observed substitution mechanisms and their effects on the geom etric details of the structure of the garnets have been supported by crysta l chemical arguments. They correspond to the best configuration, in terms o f bond strength theory, whether hydrogen is incorporated into the melanite structure as in 2) and 3) substitutions, or not, as in 1)substitution.