Soil-color changes by sulfuricization induced from a pyritic surface sediment

• A soil-color chronosequence reflected the course of a sulfuricization process.
• Spectral and parametric colors can identify acid sulfate soil materials.
• Sulfidic materials, achromatic, increased their Munsell value during oxidation.
• Schwertmannitic materials, from 8.5YR to 0.2Y, had an absorption band at 480 nm.
• Jarositic materials, from 1.3Y to 5.3Y, had an absorption band at 440 nm.

Colors are widely used to describe hydroxysulfate minerals and acid sulfate soils but seldom to study an active sulfuricization process. Our research was designed to measure the spectrophotometric colors of a soil sulfuricized by pyritic sediment over 15 years (8 profiles, 75 samples) and to determine whether color could be employed to identify the new soil materials. The Munsell value of gray sulfidic materials deposited on the soil surface changed over time from 3.0 to 5.6 (R2 = 0.97) because of Fe leaching and the formation of whitish sulfates. The underlying native soil was first pigmented yellowish brown by illuvial precipitates appearing in SEM as “bubble wrap” coating soil particles and having the EDX peaks of Fe, S and O, and the XRD peaks at 0.255 and 0.166 nm. Between pH values 3 and 4 its spectral character in the second-derivative Kulbelka–Munk function registered a maximum at 440 nm and a minimum at 480 nm, both drastically attenuated after oxalate Fe-extraction. Therefore, we attributed most of precipitates to schwertmannite phases. Their subsequent progressive transformation to jarosite in parallel to a pH decline was colorimetrically detected by a displacement of the spectral maximum to 450 nm, its intensity reduction with increasing jarosite content, and yellowing of aggregates and ground-soil samples from 8.5YR to 0.2Y. Finally, the dominance of jarosite below pH 2.4 resulted in soil materials with a spectral minimum at 440 nm and Munsell hue between 1.3Y and 5.3Y. Because sulfidic, schwertmannitic, and jarositic materials, as well as their compositional changes, were unambiguously identified, soil sulfuricization could be determined by colorimetry.