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

Melanites from Mt. Vulture, an extinct volcano located in the South of Italy (Potenza, Basilicata), have been investigated via electron probe microanalysis (EPMA), single crystal X-ray diffraction (XRD) analysis, Mossbauer spectroscopy (MS) and infrared spectroscopy (IR). The investigations were performed on six samples from two different Mt. Vulture volcanic deposits. The combination of the results from these techniques led to the following cations 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 for iron and indirectly for titanium via Mossbauer spectroscopy. The latter investigation yielded the following iron partitioning: Fe3+(Y) > Fe2+f (X) > Fe2+ (y) = Fe3+ (Z). To clarify the ambiguity about the location of the Fe2+ (X) absorption doublet, two fitting models were compared. The best model was used both to estimate correctly iron occupancies and to infer Ti distribution and oxidation state. Good agreement was found between direct (XPS, previous study) and indirect (Mossbauer, this study) determination of Ti occupancy. In particular it was confirmed that Ti may exist as Ti3+ and Ti4+ at Y and as Ti4+ at Z. IR spectroscopy confirmed the presence of hydroxyl groups in the studied garnets, which was also indicated by the chemical analyses. Considering the cation 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 geometric details of the structure of the garnets have been supported by crystal chemical arguments. They correspond to the best configuration, in terms of bond strength theory, whether hydrogen is incorporated into the melanite structure as in 2) and 3) substitutions, or not, as in 1)substitution.