Pedon Distribution of Minerals in Coastal Plain Paleudults

Abstract

AbstractMineral distributions in three Florida Paleudults were investigated with respect to particle size, horizon, depth, and location within the soil matrix. Mineralogy of sand, silt, and clay was determined using optical microscopy, x‐ray diffraction, and thermal analysis. Also, mineralogy of sand‐grain coatings was compared with intergrain matrix material. Elemental‐oxide composition of siltsized phyllosilicate grains was determined by electron microprobe analysis. Sand fractions consisted mainly of quartz, with lesser amounts of feldspar and mica, which were more prevalent in lower horizons. Coarse‐ and medium‐silt fractions were also dominated by quartz, but contained appreciable feldspar and mica in B horizons. Kaolinite and hydroxy‐interlayered vermiculite (HIV) were most prevalent in fine‐silt and clay fractions. Abrupt changes in silt and clay mineralogy across the E‐ and B‐horizon boundary (E‐B) included a decrease in quartz and HIV and an increase in kaolinite. Relative amounts of HIV did not markedly change with depth in arenic epipedons, but decreased steadily below the E‐B boundary for all pedons. Sand‐grain coatings from Bt horizons contained more mica than did the intergrain matrix, whereas coatings in A and E horizons contained appreciably more HIV. Distribution of phyllosilicates in these pedons could be explained by a hypothetical sequence of processes which includes (i) illuviation of kaolinite formed from weathering or dispersed from clay lenses in the sediments, (ii) comminution of sand‐ and silt‐sized mica, (iii) incorporation of finegrained mica in sand‐grain coatings, and (iv) transformation of finegrained mica to HIV in highly permeable eluvial horizons.