Color Identification of Iron Oxides and Hydroxysulfates

Abstract

Colors are widely used to describe soil profiles but seldom to identify minerals. This study was conducted to determine whether color measurements with a colorimeter could be employed to identify Fe oxides and hydroxysulfates, either as monomineralic samples or in soils. We measured the color of 277 Fe oxide and hydroxysulfate mineral samples, as well as 309 soils and soil fractions. Discriminant analysis was employed to separate the minerals in three different color systems: CIE‐Yxy, CIE‐L*a*b*, and Munsell. Using the color of monomineralic samples, 100% of maghemites and jarosites could be correctly classified, 95% of goethites, 90% of feroxyhites, 84% of lepidocrocites, 83% of hematites, 56% of ferrihydrites, 50% of akagan e ´ ites, and 44% of schwertmannites. In soil samples, goethite (90% correct classifications) could be reliably distinguished from hematite–goethite mixtures (82% correct classifications), and from lepidocrocite–goethite mixtures (89%). The identification of ferrihydrites, akagan e ´ ites, and schwertmannites largely failed because of similar average colors and because of high color variability. Because the color variability is a mineral‐intrinsic property rather than produced by errors of measurement, color is in principle unsuited for identifying ferrihydrites, akagan e ´ ites, and schwertmannites. However, color may be used to identify goethite, hematite, lepidocrocite, jarosite, maghemite, and feroxyhite with a relatively high reliability. The choice of color system is not crucial for this type of analysis.