Mineralogy and Petrology (2001) 72: 249±257 Re¯ectance variations in heat-treated ilvaite P. Bonazzi 1 , L. Bindi 1 , and F. Olmi 2 1 Dipartimento di Scienze della Terra, Universita Á di Firenze, Firenze, Italy 2 CNR ± Centro di Studio per la Minerogenesi e la Geochimica Applicata, Firenze, Italy With 4 Figures Received June 5, 2000; revised version accepted November 14, 2000 Summary Ilvaite, a black mixed-valence iron silicate, undergoes oxidation-dehydrogenation reaction when heated in air. Variations in the optical properties were investigated as function of the structural adjustments accompanying the process. Re¯ectance measurements were carried out in air on crystallographically oriented crystals heated at increasing temperatures (selected wavelengths of 471.1, 548.3, 586.6, and 652.3nm). As expected on the basis of the strong structural anisotropy, sections parallel to the c axis (i.e. the chain axis) exhibit maximum bire¯ectance. The most relevant variation induced by heating affects the R e value which appears to increase as consequence of the shortening of the M1-M1 contact along the c axis. Introduction Ilvaite, CaFe 2 2 Fe 3 Si 2 O 7 OOH, is a black mixed-valence iron sorosilicate, typically occurring as a late forming mineral in Ca-Fe-Si skarn deposits (Burt, 1971). The structure of ilvaite is based on iron octahedral ribbons, running parallel to the c axis. Seven-coordinated Ca 2 ions and Si 2 O 7 groups interconnect the octahedral chains. At room temperature, ilvaite is monoclinic with space group P2 1 =a (Bartholome  et al., 1968) with Fe ions ordered on three crystallographically independent sites. One half of the Fe 2 ions occupies the M2 site, the other half the M11 site and the Fe 3 ions the M12 site. By increasing temperature, the structure undergoes a crystallographic phase transition to the orthorhombic symmetry Pnam, due to the complete disordering of Fe 2 and Fe 3 ions within the double-chain of M1-octahedra. This transition occurs within the range 60±70  C(Ghose et al., 1984a, 1985; Robie et al., 1988). In the chemically pure synthetic ilvaite the transition occurs at 117  C(Ghazi-Bayat et al., 1992). A number of monoclinic structure re®nements have been performed using both X-ray single crystal intensities (Finger et al., 1982; Take Âuchi et al., 1983, 1993,