VISIBLE SPECTRA AND COLOR OF SYNTHETIC ALUMINUM-SUBSTITUTED GOETHITES ANDHEMATITES

Various iron oxide minerals were synthesized under different conditions and studied by diffuse reflectance spectroscopy in the visible region using a Cary 17DX spectrophotometer. The visible spectrum (absorbance versus wavelength) has broad and weak bands of absorption or reflectance, but the second derivative of the curve, calculated and plotted with a computer, has narrower and sharper peaks. Derivative patterns of iron oxides are qualitatively different from each other and characteristic of the mineral. The most distinctive features of the second derivative curves are: Goethite, minimum at 423 nm and maximum at 447 nm; hematite, minimum at 547-575 nm; maghemite, minimum at 655 nm; and lepidocrocite, minimum at 485 nm. Second derivative spectroscopy can be used for quantitative estimation of iron oxides in soils. The method is very sensitive at low concentrations of the minerals (lower than 5%). Samples with higher concentrations can be diluted before analysis. Another derivative maximum, around 565 to 605 nm, is related to the color of goethite and hematite and mixtures of the minerals, with the yellowest minerals having a peak around 565 nm and the reddest around 605 nm. The minimum around 475 to 495 nm is related to Al substitution in goethite with most Al resulting in peaks around 475 nm and the least, around 495 nm. The color of the iron oxides was calculated from spectral measurements and converted to the Munsell notation. Hematite showed a wide range of color, from about 7.5R 3/3 to 2.5YR 4/11. Munsell value increased with increasing Al substitution and decreasing unit cell dimensions. No relation was found between color parameters and crystallite size measured by x-ray diffraction or laser beam techniques. Goethites showed a narrower range of color, from 7.5YR 5/10 to 10YR 7/10. The Munsell value increased with increasing crystallite dimension, while the redness increased with decreasing crystallite size measured by x-ray diffraction and laser beam techniques. No relation was found between color and unit cell dimensions or Al substitution. Poorly crystallized goethites derived from green rust acted differently from goethites derived from derrihydrite. Infrared absorption bands of OH stretching or bending modes of vibrations were related to Al substitution in goethites. The bands shifted to higher wavenumbers with increasing Al substitutions