Serpentine Soil Redness, Differences among Peridotite and Serpentinite Materials, Klamath Mountains, California

Peridotite soils were perceived to be redder than serpentinite soils. These redness differences were confirmed by relating soil redness to bedrock specific gravities and to heavy mineral concentrations in fine sand fractions of soils. The redness differences are explained by mineralogical differences between peridotite and serpentinite. Soil redness in well-drained soils of the Klamath Mountains is closely related to free iron in oxides and oxyhydroxides. Most of the iron in peridotite is in olivine, which is readily weathered to release iron that is then oxidized in well-drained soils, whereas most of the iron in serpentinite is in magnetite and serpentine. Magnetite is very resistant to weathering and serpentine weathers more slowly than olivine and pyroxene. This relationship predicts untested redness differences in soils derived from serpentinized dunite (redder soils) compared to those from more silicic serpentinized peridotite such as lherzolite.