MUNSELL COLORS OF SOILS SIMULATED BY MIXTURES OF GOETHITE AND HEMATITE WITH KAOLINITE

The colors and color patterns of soils are widely used to infer other soil properties. It is desirable to understand, at least semiquantitatively, the effect various pigments have on soil color. The purpose of this study was to determine how hematite and goethite pigments influence soil color. We prepared simulated soil mixtures consisting of kaolinite and 2.5, 5, 10, 20, 40, 80, 160, 320, 640, and 1000 g kg-1 of pigment (either a synthetic goethite, one of two synthetic hematites, or a mixture of goethite and hematite). We determined the reflectance spectra of the samples, and calculated their colors in the Munsell system. For hematite and goethite pigments mixed with kaolinite, more than half of the change in hue occurred as the Hm/(Hm+Gt) ratio increased from 0 to 0.25, and hue changed relatively little as the Hm/(Hm+Gt) ratio increased from 0.50 to 1.00. Most of the change in color occurred as pigment content increased from 2.5 to 160 g kg-1. Doubling of pigment content from 160 to 320 g kg-1 had very little impact on the colors of the mixtures, while further increases in pigment content between 320 and 1000 g kg-1 caused some changes in color, but less than for the 2.5 to 160 g kg-1 range. A one unit increase in chroma corresponded approximately to a doubling of pigment content in the series 2.5, 5, 10, 20, 40, 80. 160 g kg-1 for the goethite as well as the hematite pigments studied. For one hematite, hues became progressively redder and changed by more than 5 units of hue as pigment content increased from 0.25 to 160 g kg-1.