Mineral Transformations in Carolina Blue Ridge‐Piedmont Soils Weathered from Ultramafic Rocks

Abstract

Abstract Soils developed from ultramafic rocks in the Blue Ridge province of North Carolina are morphologically different from soils developed from chemically similar rocks in the Piedmont province of South Carolina. Three pedons were studied by petrographic microscopy, x‐ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) to determine mineralogical properties and weathering sequences that contribute to soil differences and to observe micromorphological features of coarse silt primary minerals that may suggest pathways for secondary mineral formation. Amphiboles and chlorite dominate the sand fractions of two pedons, whereas pyroxenes and hornblende dominate the sand fraction of the third pedon. Talc is a major component of the clay fraction (<2.0 µm) with estimates as high as 600 g kg ‐1 . Kaolinite composes from 113 to 330 g kg ‐1 of the coarse clay (2.0–0.2 µm) and as much as 561 g kg ‐1 of the fine clay (<0.2 µm). Dehydroxylation and oxidation of the hydroxide sheet of Fe‐rich chlorite during metamorphic processes and weathering of ferromagnesians under well‐drained conditions account for higher free Fe contents in two of the pedons. Smectite composes up to 550 g kg ‐1 of the clay fractions in one soil, impedes drainage, and possibly aids additional smectite formation. Primary chlorite alters to a vermiculite‐type mineral, randomly interstratified chlorite‐vermiculite, kaolinite, and 2:1 intergrade. Amphiboles and pyroxenes weather to kaolinite, 2:1 intergrade, and smectite. Talc weathers to smectite. Goethite is a weathering product of the ferromagnesians. Microscopic surface features on amphiboles and pyroxenes suggest secondary clay mineral formation via both solid‐state transformation and neogenesis. Faults and fractures in the parent rock, water flow, mineral stability, and the physicochemical regime created by initial weathering products appear responsible for the differences among these soils.