High-temperature vesuvianite: crystal chemistry and surface considerations

A multi-methodical approach has been applied for characterizing the bulk and surface crystal chemical features of a high-temperature vesuvianite crystal from skarns of Mount Somma-Vesuvius Volcano (Naples, Italy). Vesuvianite belongs to the space group P4/nnc with unit cell parameters a = 15.633(1)angstrom, c = 11.834(1)angstrom and chemical formula (Ca-18.858 Na-0.028 Ba-0.004 K-0.006 Sr-0.005 a-(0.098))(19.000) (Al-8.813 Ti-0.037 Mg-2.954 Mn-0.008 Fe (0.114) (2+) Fe (1.375) (3+) Cr-0.008 B-0.202)(13.511) Si-18.000(O-0.261 F-0.940 OH7.799)(9.000). Structure refinement, which converges at R = 0.0328, demonstrates a strong positional disorder down the fourfold axes, indicating that the Y1 site is split into two positions (Y1A and Y1B) alternatively occupied. However, because of X4 proximity to Y1B and Y1A, X4 cannot be occupied if Y1B or Y1A are. Overall Y1 occupancy (Y1A + Y1B) reaches approximately 0.5, as common in vesuvianite and occupancy of Y1B site is extremely limited. Moreover, T1 position, limitedly occupied, accommodates the excess of cations generally related to Y position. A small quantity (0.202 apfu) of boron is sited at the T2 site that, like T1, is poorly occupied. The determination of the amount of each element on the (100) vesuvianite surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si-2p, Al-2p, Mg-1s, and Ca-2p core levels, evidences that a greater amount of aluminum and a smaller amount of calcium characterize the surface with respect to the bulk. Although both of these features require further investigation, we may consider the Al increase can be related to preferential orientation of Al-rich sites on the (100) plane. Furthermore, the surface structure of vesuvianite suggests that Al, Ca, and Mg cations maintain coordination features at the surface similar to the bulk. Silica, however, while presenting fourfold coordination, shows also a [1]-fold small coordinated component at binding energy 99.85 eV, due to broken Si-O bonds at the surface. The presence of eight- and nine-coordinated Ca cations is suggested by a large XPS feature resulting from the merging of Ca-2p3/2 and Ca-2p1/2 peaks at 348.45 and 352.05 eV, respectively.