Mixed-Valence Minerals of Iron and Titanium: Correlations of Structural, Mössbauer and Electronic Spectral Data

Abstract

A large cariety of mixed-valence silicate and oxide minerals occur in rocks from the Earth, Moon and Meteorites, They frequently contain Fe2+ -Fe3+, Ti3+ -Ti4+ and Fe2+ -Ti4+ assemblages and often display anomalous color or opacity, high thermal and electrical conductives, and unusual spectral variations with temperature or pressure. Such properties are not attributable to transitions in single cations and indicate the presence of intervalence transitions. The intervalence transitions are observed either as charge transfer (CT) bands in the visible-near infrared region and frequently cause blue colors in FeFe2+ -Fe3+ and Fe2+ -Ti4+ bearing minerals (e.g. viviante, sapphire, dordierite, kyanite, bearing minerals (e.g. vivianite, sapphire, cordierite, kyanite glaucophane, aquamarine), or as electron decolalozation (ED) often causing opacity and semiconductor behavior (e.g. magnetite, ilvaite, deerite, riebeckite). Many of the physical and spectral properties of mixed-valence silicate and oxide minerals may be correlated with crystal structure features and site Occupancies of the metals. Structural variables which affect properties of mixed-valence oxide and silicate minerals or distorted octahedra, tetrahedra, dodecahedra), type of polyhedral linkage (e.g. face-shared, edge-shared, corner-shared) metal-oxygen (M-O) distances, metal-metal (M-M) distances, and M-O-M bond angles; and orientation and extend of linked polyhedra (e.g. isolated pairs, chains, bands, sheets; blocking by Mg2+ and AI3+ in solid solution series). These features may be used to interpret the temperature and pressure dependencies of CT bands in minerals with isolated Fe2+-Fe3+ and Fe2+-Ti4+ clustes (e.g. vivianite, sapphire, glaucophane, kyanite, cordierite, omphacite, pyroxenes, etc.), and the occurance of Fe in intermediate electronic environments in minerals with infinite chains of edge-shared Fe2+-Fe3+ polyhedra (e.g. magnetite, ilvaite, deerite, schorlomite garnets, riebeckite, and potentially aenigmatite, howieite, babingtonite).