Mineralogical differences between seam and prism material in loess-derived fragipans in western Kentucky

Fragipan horizons are common diagnostic subsurface features typically identified by their dense, brittle nature under field conditions. Fragipan horizons are often hypothesized to contain various silicate bonding agents, including phyllosilicates, amorphous aluminosilicates/silica, and silica sorbed to iron oxyhydroxides. However, the association of silica (Si) with other minerals in these horizons remains uncertain. To fill this knowledge gap, we sampled the prism and seam material of four loess-derived fragipan horizons in western Kentucky. We characterized and compared the clay mineral composition of the prism and seam material using X-ray diffraction (XRD), selective extractions, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Fragipan prism and seam material exhibited a mixed clay mineralogy comprised of vermiculite, mixed layer minerals (hydroxyl-interlayered vermiculite), illite, kaolinite, and quartz. We identified goethite in fragipan prisms in much greater quantities when compared to seam material, confirmed by both XRD and DRIFTS and corroborated by higher dithionite-extractable iron in prisms. DRIFT spectra showed a peak near 3180 cm⁻¹ assigned to OH-stretching mode of goethite, which was prominent in prism materials. Goethite 110 XRD peak positions were negatively correlated to dithionite-extractable aluminum (Al) and Si content of the prism material, which may suggest greater Si sorbed to the surfaces of Al-substituted goethite. These results demonstrate the potential importance of Al-substituted goethite in fragipan prisms and provide insight about possible associations with Si.