Color Variations Associated with Rapid Formation of Goethite from Proto-Ferrihydrite at pH 13 and 40°C

T. Nagano1, S. Nakashima2, S. Nakayama1, K. Osada1 and M. Senoo1
1 Environmental Geochemistry Laboratory Japan Atomic Energy Research Institute Tokai, Naka, Ibaraki 319-11, Japan
2 Research Institute of Natural Resources, Akita University Tegata Gakuenchol-1, Akita 010, Japan

Abstract: Color variations from brown to yellow of synthesized goethite have been studied colorimetrically and spectroscopically. Goethite with various colors was synthesized at pH 13 and 40°C by varying the incubation time. Colorimetry revealed that the b* value (yellowish chroma) in L*a*b* color space was a quantitative indicator of color variations of the diluted samples. From UV-VIS-NIR spectra, the increase in the b* value was found to be caused by the increase in crystal field absorptions due to goethite formation around 500 nm. The b* value was a good indicator of the relative proportion of goethite in the precipitates including ferrihydrite. X-ray diffraction patterns and infrared spectra revealed that crystallization of goethite was associated with loss of water from the proto-ferrihydrite.

Key Words: Crystallization • Colorimetry • Goethite • Infrared spectroscopy • L*a*b* color space • UV-VIS-NIR spectroscopy • X-ray diffractometry

Clays and Clay Minerals; October 1992 v. 40; no. 5; p. 600-607; DOI: 10.1346/CCMN.1992.0400515
© 1992, The Clay Minerals Society
Clay Minerals Society (www.clays.org)