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Torrent et al. (1980), for example, dem- onstrated the usefulness of a "redness index" based on the. Munsell notation to predict the hematite content in some soils.
Clays and Clay Minerals, Vol. 32, No. 2, 157-158, 1984.

INFLUENCE OF ALUMINUM

SUBSTITUTION ON THE COLOR OF

SYNTHETIC HEMATITES Key Words--Aluminum, Color, Hematite, Iron oxides, Soil.

The red color of many soils and sediments has usually been attributed to hematite, a mineral having a high pigmenting power when present in a finely dispersed form. Soil scientists, in particular, have been frequently interested in obtaining significant correlations between soil color and the content of hematite (and other Fe-oxides), as shown in a recent review by Taylor (1982). Torrent et al. (1980), for example, demonstrated the usefulness of a "redness index" based on the Munsell notation to predict the hematite content in some soils of river terraces from Spain. More recently, Torrent et aL (1983) found quantitative relationships between redness indexes and hematite content for a large number of soils from different geographic areas. The use of color to predict hematite content can be of more than academic interest because the usual procedures to determine hematite need costly equipment and are time-consuming, especially when hematite is present in low concentration (Schwertmann and Taylor, 1977). The color of hematite itself, however, depends on several factors. In this report we present data on the effect of A1 substitution on the color of synthetic hematites as a logical step toward a better understanding of the chromatic properties of hematite-bearing soils and sediments.

ment. The reflectance measurements were converted to tristimulus values (X, Y, Z) by the selected ordinate method and, finally, to Munsell hue, value and chroma as described by Wyszecki and Styles (1967).

EXPERIMENTAL Several series of hematites were prepared by coprecipitating 10 mmole of Fe + A1 (as nitrates) with 2 M KOH to a final volume of 200 ml and storing the suspension under various conditions. The mole % AI varied in each series from 0 to 1315%. For series 1, precipitation was stopped at pH 8, and the precipitates were stored at 100-150~ for 3 days. For series 51, 52, and 53, Na-citrate at concentrations of l0 -5 M, 2 X 10 -~ M, and 10 -4 M, respectively, was added before precipitation. Final pH, storing time, and temperature were, respectively, pH 9, 13 days, and 70-75"C for series 51; pH 9, 8 days, and 100-105~ for series 52, and pH 10, 8 days, and 100-105~ for series 53. For series 61 and 62, precipitation was accomplished in 10-4 ML-tartaric acid, and final pH, storing time, and temperature were, respectively, pH 9, 8 days, and 70-75* C for series 61 and pH 10, 8 days, and 100-105~ for series 62. In some complementary experiments (series 3 and 4) precipitation was carried out in solutions having oxalic acid at various concentrations (from 2.5 X 10-a M to 10-2 M) and at a final pH from 6 to 8.5. Finally, the precipitates were stored at 100-i05~ for periods of 5 to 38 days. After storage, all products were washed free of salts, and the residual noncrystalline material was removed with pH 3 ammonium-oxalate. The crystalline products were then dried at 60~ crushed, and examined by X-ray, powder diffraction (XRD). The widths at half height (WHH) of the hematite 104 and 110 peaks were measured from the diffractograms and corrected for instrumental broadening by reference to wellcrystallized Pb(NO3)2. After each synthesis experiment AI substitution was estimated from the position of the 104 and 110 hematite peaks using the 200 and 311 peaks of admixed Pb(NO3)2 as an internal standard (Schwertmann et al., 1979). Finally, for a more accurate determination of A1 substitution, hematites were dissolved in 6 M HCI and Fe and A1 analyzed in the extracts by atomic absorption spectroscopy. These latter values were used in all subsequent work. The color of powdered hematites was measured in a Bausch & Lomb Spectronic 20 equipped with a reflectance attachCopyright 9 1984, The Clay Minerals Society

157

RESULTS AND DISCUSSION For the range of sizes of our hematites (plate width: 1503000 A), hue ranged from 9.9R to 3.8YR; value from 2.7 to 5.0, and chroma from 5.3 to 8.8. Color depended on the percentage of A1 substitution. Within each series Munsell hue remained relatively constant or became redder with increasing A1 substitution; value increased and chroma increased or remained nearly constant. When all series were considered together a good linear correlation was found between Munsell value, and mole % A1 (Figure 1) whereas the correlation of either hue or chroma with the mole % A1 was not significant. A1 substitution in hematite synthesized at low temperature usually results in a reduced growth of the crystal in the z-direction as shown by Schwertmann et al. (1979). Therefore, this particular morphology rather than A1 substitution per se could be the cause for the color properties of our samples. The Al-substituted hematite crystals from series 1, 51, 52, 53, 61, and 62 showed thinning in the z-direction as evidenced by differential line broadening in XRD (the ratio WHH(104)/ WHH(110) increased with mole % A1) and by electron mi-

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Figure 1. Correlation between Munsell value (V) and percentage of A1 substitution for powdered hematites.

158

Clays and Clap"Minerals

Barron and Torrent

croscopy. It is not surprising, therefore, that a linear correlation existed between Munsell value (V) and the WHH(104)/ WHH(110) ratio (V = 3.04 + 0.426WHH(104)/WHH(110); r 2 = 0.536; 40 samples). Proof that morphology was not a significant factor involved in color came from hematites synthesized in the presence of oxalate (series 3 and 4). All of these hematites were essentially equidimensional as shown by XRD (WHH(104)/WHH(110) ~ 1) and electron micrographs, but AI substitution still explained satisfactorily the variation in value [V = 2.85 + (0.119 • mole % AI); r 2 = 0.903; 28 samples]. In addition, for all of the nonoxalate samples, the linear regression of V on mole % A1 was [V = 3.23 + (0.111 • mole % A1); r 2 = 0.822; 40 samples] and not signficantly (1% level) different from that of oxalate samples. Color did not depend on particle size because none of the color parameters was found to be related to WHH(110), a parameter which can be taken as an indirect measure of plate width. Therefore, the effect of both particle size and particle morphology seemed to be unimportant compared to that of A1 substitution. AI substitution seems to influence the chromatic characteristics of hematite by making them closer to those of corundum. This relationship is suggested by the following fact: If luminance (Y) is taken as a better measure of the lightness of hematite, instead of Munsell value (V), a good linear relation between Y and mole % AI is obtained [Y = 6.54 + (0.732 X mole % A1); r 2 = 0.843; n = 68]. By extrapolating this line to mole % A1 = 100 (i.e., corundum composition), luminance has a value of 80 which is close to the luminance measured for powdered corundum (~90). It is evident from the above results that soils having similar amounts of hematite can show differences in color due, among other factors, to the degree of A1 substitution in this mineral. The observation that Al-substituted hematites are lighter than nonsubstituted ones is in agreement with certain observation of the color of red Mediterraneean soils. For example, in acid soils, which probably have Al-substituted hematites (because more A1 is available in solution), red color is usually lighter than in neutral soils (e.g., Terra Rossa) having similar amounts of hematite. Many more data, however, are needed before we

achieve a good comprehension of the factors involved in the color of hematite-containing soils and sediments. ACKNOWLEDGMENTS This research was supported by funds from the Comisifn Asesora de Investigaci6n Cientifica y Tfcnica Project no. 0253/ 81. The authors also thank Dr. J. Morales (Departamento de Quimica Inorg~mica, Universidad de C6rdoba) for taking the electron micrographs.

Departamento de Edafologfa Escuela T~cnica Superior de Ingenieros AgrOnomos Apartado 3048 COrdoba, Spain

V. BARRON J. TORREYT

REFERENCES Schwertmann, U., Fitzpatrick, R., Taylor, R. M., and Lewis, D . G . (1979) The influence of aluminum on iron oxides. Part II. Preparation and properties of Al-substituted hematites: Clays & Clay Minerals 27, 105-112. Schwertmann, U. and Taylor, R, M. (1977) Iron oxides: in Minerals in SoilEnvironments, J. B. Dixon and S. B. Weed, eds., Soil Sci. Soc. Amer. Madison, Wisconsin, 145-180. Taylor, R. M. (1982) Colour in soils and sediments--Review: in Proc. International Clay Conference, Bologna and Pavia, 1981, H. van Olphen and F, Veniale, eds., Elsevier, Amsterdam, 705-761. Torrent, J., Schwertmann, U., Fechter, H., and Alffrez, F. (1983) Quantitative relationships between soil color and hematite content: Soil Sci. (in press). Torrent, J., Schwertmann, U., and Schulze, D. G. (1980) Iron oxide mineralogy of some soils of two river terrace sequences in Spain: Geoderma 23, 191-208. Wyszecki, G. and Styles, W. S. (1967) ColorScience--Concepts and Methods, Quantitative Data and Formulas: Wiley, New York, 478--487.

(Received 12 March 1982; accepted 1 June 1983)