Structural adjustments induced by heat treatment in ilvaite

Ilvaite is a mixed-valence Fe-bearing sorosilicate, typically occurring as a late forming mineral in Ca-Fe-Si skarn deposits (Burt 1971). Its chemical composition does not differ significantly from the ideal end-member, Ca Fe2Fe (Si2O7)O(OH), except for a considerable amount of Mn, mainly substituting for Fe and, to a lesser extent, Ca (Carrozzini 1994); minor replacement of Fe and Fe by Mg and Al respectively were also observed (Naslund et al. 1983; Finger and Hazen 1987; Carrozzini 1994). Ilvaite was considered to be orthorhombic (Belov and Mokeeva 1954; Beran and Bittner 1974; Haga and Takéuchi 1976), until Bartholomé et al. (1968) pointed out the existence of a monoclinic phase (space group P21/a). In the orthorhombic phase, Fe occupies two distinct octahedral positions: (1) the eightfold Wyckoff position 8d (labeled M1) and (2) the fourfold Wyckoff position 4c (labeled M2). The M1 site is randomly occupied by Fe and Fe, whereas M2 is completely filled by Fe and Mn, if any is present (Haga and Takéuchi 1976). At room temperature, ordering of Fe and Fe cations occurs, thus causing a small deviation from orthorhombic Pnam symmetry: as the mirror plane normal to the c axis is lost, the eightfold M1 site splits up into two independent crystallographic sites, M11 (4e) and M12 (4e), which mainly accommodate Fe and Fe respectively. This crystallographic phase transition occurs within the range 60–70 °C (Ghose et al. 1984a; Ghose et al. 1985; Robie et al. 1988). In the chemically pure synthetic ilvaite it occurs at 117 °C (Ghazi-Bayat et al. 1992). Complete ordering between Fe and Fe should result in a β angle value of 90.45°, whereas complete disordering leads to β = 90.00° (Takéuchi et al. 1983). Ilvaites with β angle values ranging from 90.00° to ca. 90.35° are observed in nature (Takéuchi et al. 1993) and the degree of disorder is considered to vary accordingly. However, as Takéuchi et al. (1983) pointed out, different factors can combine to simulate disorder greater ABSTRACT