Composition of Typical Soil Minerals and Quantitative Analysis for Influence of Iron and Manganese Forms on Purple Soil Color in Northeastern Sichuan, China

Eight soil samples from the Jurassic Shaximiao Formation (J2s) in the Sichuan Basin, China, were taken as the research object to explore the correlation and quantitative relationship between soil color and mineral composition, iron, and manganese (free state Fed/Mnd, active state Feo/Mno, and complex state Fep/Mnp). The results show that the kaolinite and muscovite contents in purple clay are higher than those in soil of other colors, and most albite content is lower than that in soil of other colors. The iron and manganese contents in purple soil are higher than those in other soils. Silicon and sodium elements make the soil brighter; magnesium reduces the brightness; aluminum, iron, potassium, titanium, and manganese make the soil redder; and manganese elements will make the soil more yellow. The soil particle size is significantly positively correlated with L* (brightness), a* (red–green degree), and b* (yellow–blue degree), and organic matter is significantly negatively correlated with L* while positive correlated with a*. L* is affected by Feo, Fep, Mno, and Mnp, of which Mnp contributes the most to the brightness of L*; a* is affected by Feo, Fed, and Mno, of which Mno has the greatest contribution to a*; and b* is only affected by Fep. In summary, the particle size and the types and contents of clay minerals and elements will affect the color of the soil, and the various forms of iron and manganese have a significant effect on the soil color. The results provide an important reference for the study of purple soil system classification.