Temperature dependence of reflectance spectra and color values of hematite by in situ, high-temperature visible micro-spectroscopy

We measured visible reflectance spectra and colors of hematite (alpha-Fe2O3) reagent powders and a natural feldspar grain containing dispersed hematite microcrystals from room temperature LIP to 800 degrees C using an in situ, high-temperature visible micro-spectrometer with dark field optics. The spectrum of room-temperature hematite powder is characterized by a nearly constant reflectivity in the range 400-550 nm, a shoulder near 620 nm, and a reflectivity maximum near 750 nm. The reflectance spectrum is similar to the diffuse reflectance spectra Measured by a spectrophotometer and a conventional spectrometer with an integrating sphere. This result indicates that the dark field objective is Suitable for measuring visible reflectance spectra of hematite powders with the visible micro-spectrometer. The reflectance of hematite powders in the longer wavelength region (>550 nm) decreases gradually With increasing temperature. The shoulder centered around 620 nm and the reflectance maximum near 750 nm also become indistinct at high temperatures. The calculated L* (dark-light), a* (red-green). and b* (blue-yellow) color values decrease with increasing temperature. This means that the red color of hematite becomes black with temperature increase. The calculated absorption intensities (Kubelka-Munk functions) Suggest that this temperature dependence of the hematite powder-reflectance spectra can be mainly explained by a change in hand gap absorption edges for a semiconductor (Urbach rule). The visible spectra and colors of a natural feldspar grain containing dispersed hematite microcrystals show a similar change with temperature, indicating that the temperature dependence can be observed under natural conditions at high temperatures, Such as in volcanic eruptions.