The visible diffuse reflectance spectrum in relation to the color and crystal properties of hematite

Hematite (alpha-Fe2O3) possesses distinct spectral properties which facilitate its identification in mineral mixtures. This paper reports the relationships between the visible diffuse reflectance (DR) spectrum and the color and crystal properties of a group of 81 natural and synthetic hematites. The visible DR spectra for powdered hematite samples diluted to 4 wt.% with BaSO4 white standard were recorded and used to calculate the corresponding Munsell colors. The second derivative of the Kubelka-Munk function of the DR was used to estimate the position and intensity of the main absorption bands, which occurred at similar to435, similar to485 and similar to545 nm. The Munsell hue ranged from 9.5P to 5.3YR and was negatively correlated with the position of the similar to545 nm band, so it became yellower as the band shifted to shorter wavelengths (higher energies). The Munsell value, which ranged from 4.9 to 8.6, and chroma, which ranged from 1.4 to 8.3, were negatively and positively correlated, respectively, with the intensity of the similar to545 nm band, the position of which exhibited a weak negative correlation with the degree of Al substitution (x). The position and intensity of this band also exhibited a weak negative correlation with the specific surface area (SSA); both, however, were uncorrelated with domain shape as measured by the ratio between the X-ray mean coherence lengths (MCLs) in the [110] and [104] directions. The properties of the similar to435 and similar to485 nm bands were unrelated to x, SSA or MCL110/MCL104. The negative relationship between the position of the similar to545 nm band and x does not support the assignment of this band to the 2(6A(1)) --> 2 (T-4(1g)((4)G)) electron pair transition (EPT), which relates to the to Fe3+-Fe3+ magnetic coupling between face-sharing octahedra. The similar to485 nm band might reflect a 'goethitic' structural component in Fe-defective hematites as it appears at the same wavelength as the hypothetical EPT related to Fe3+-Fe3+ magnetic coupling between edge-sharing octahedra in goethite (alpha-FeOOH).