The relationships of bulk structure, surface structure, chemistry, and physical properties of mineral phases with six- and eight-membered silicate rings

The goal of this research is to better understand the relationship between chemical composition and atomic structure (both bulk and surface structure) of minerals having six-membered silicate rings, such as osumilite and eight-membered silicate rings, such as vesuvianite. Osumilite under investigation occurs in thin fissures within a rhyolite from the volcanic massif of Mt. Arci, Sardinia (Italy). Vesuvianite sample is from skarns of Somma-Vesuvius volcano (Italy) instead.The osumilite and vesuvianite bulk structures were refined via single crystal X-ray diffraction. Osumilite sample is hexagonal, with symmetry P6/mcc and unit cell parameters a = 10.1550(6) c = 14.306(1) (Å). The structural formula is (K0.729)C (Na0.029)B’ (Si10.498 Al1.502)T1 (Al2.706 Fe2+0.219 Mg0.075)T2 (Mg0.660 Mn0.091 Fe2+1.258)AO30. When compared to literature data [1], osumilite sample from Mt. Arci is characterized by a significantly high Fe2+ content (1.478 apfu).X-ray photoelectron spectroscopy (XPS) enabled the crystal chemical characterization of osumilite surface, where iron was still observed in its divalent state, thus confirming a similarity in osumilite surface and bulk chemical composition. A further evidence of Fe oxidation state at osumilite surface is provided from X-ray absorption spectroscopy (XAS) at Fe-L2,3 edge.Vesuvianite is a complex tetragonal silicate. Vesuvianite ideal formula is Ca19(Al,Mg)13(B, Si)18O68(OH, O, F)10 and crystallizes, generally, in the space group P4/nnc. Vesuvianite presents in nature two different structural type: a high temperature type (at about 1000 °C) and a low temperature type (at about 900 °C). To recognize which type of vesuvianite, our sample belongs to, we studied its thermal decomposition via X-ray powder diffraction method by using a Philips X’Pert PRO diffractometer equipped with X’Celerator area detector. Finally we could assure that our sample is a high temperature vesuvianite with high symmetry P4/nnc.Unlike osumilite, vesuvianite surface is very complex to study because of its roughness on given faces of the crystal. However we could obtain preliminary information on the chemical composition of the surface via XPS and subsequently compare these data to bulk chemical composition. Moreover, unlike osumilite sample, vesuvianite presents Fe in octahedral site both in divalent and trivalent state, as confirmed by XAS experiments performed at the L2,3 edge of iron.