Relationship among Derivative Spectroscopy, Color, Crystallite Dimensions, and Al Substitution of Synthetic Goethites and Hematites1

C. S. Kosmas2, D. P. Franzmeier and D. G. Schulze
Agronomy Department, Purdue University West Lafayette, Indiana 47907
1 Journal Paper No. 10,504.
2 Present address: Athens Faculty of Agriculture, Laboratory of Soils and Agricultural Chemistry, Botanicos 118 55, Athens, Greece.

Abstract: Nine hematites and 22 goethites were synthesized by a variety of methods to obtain monomineralic samples having a range of Al substitutions and particle sizes. The second derivative of absorbance and Munsell color designations were calculated from visible reflectance spectra obtained from the dry powders. Unit-cell dimensions, Al substitution, infrared band positions, mean crystallite dimensions (MCD) from X-ray powder diffraction, and particle size from fiber-optic Doppler anemometry (FODA) were determined. Previously reported correlations between Al substitution, goethite unit-cell dimensions, and OH-stretching and -bending band positions were confirmed. For hematite, the position of the second derivative peak at ∼600 nm was negatively correlated with Al substitution (r = -.86). Munsell value and chroma were positively correlated with Al substitution (r = .94 for both), but hue was not related to Al substitution. Hue appeared to become redder, however, as particle size measured either by FODA or MCD increased. For goethite, the position of the second derivative minimum at ∼485 nm was negatively correlated with Al substitution (r = -.99). Munsell hue appeared to be related to both Al substitution and MCD perpendicular to (110), MCD110, with hues becoming redder with increasing Al substitution and yellower with increasing MCD110. Correlations between Munsell value and chroma and parameters such as Al substitution, particle size, and OH-stretching and -bending band positions were poor, but goethites synthesized by oxidation of Fe2+ solutions at room temperature had higher chromas than goethites synthesized hydrothermally from an Fe3+ system. Visually determined colors agreed well with calculated ones. Second-derivative spectra and color designations calculated from visible spectra appear to be potentially useful for quickly estimating other properties of goethite and hematite, such as Al substitution and particle size.

Key Words: Aluminum • Color • Fiber-optic Doppler anemometry • Goethite • Hematite • Visible spectroscopy

Clays and Clay Minerals; December 1986 v. 34; no. 6; p. 625-634; DOI: 10.1346/CCMN.1986.0340602
© 1986, The Clay Minerals Society
Clay Minerals Society (www.clays.org)