Nonexchangeable Potassium Associated with Hydroxy‐Interlayered Vermiculite from Coastal Plain Soils

Abstract

AbstractHydroxy‐interlayered vermiculite (HIV) from a Quartzipsamment, a Haplaquod, and a Paleudult from Florida was studied to determine levels of nonexchangeable K associated with this mineral. Mineralogy was characterized by x‐ray diffractometry (XRD) and thermal analysis. A scanning electron microscope equipped with an energy‐dispersive x‐ray system was used to examine morphology of silt‐sized phyllosilicate grains and to qualitatively determine elements present. An electron microprobe equipped with a wavelength‐dispersive x‐ray system was used to obtain K x‐ray images and determine elemental oxide compositions of 82 randomly selected grains. Total K was determined for bulk samples using Na2CO3 fusion. Extractable K by double acid, HNO3, and H2SO4 was determined for clay fractions. Results confirmed that most phyllosilicate grains from quartz‐ and HIV‐dominated silt fractions of all samples contained sufficient K to be detected in K x‐ray dot images, though little or no mica was detectable by XRD. Mean K2O contents for mediumsilt grains from the Quartzipsamment, Haplaquod, and Paleudult were 2.3, 4.1, and 5.6%, respectively. Fine‐silt grains averaged 1.3, 1.3, and 2.3%, respectively, despite lack of infinite thickness. High K2O values, mica‐like grain morphology, and relatively large size of these grains suggest that they are transformation products of mica. Acid extracts from the clay also contained detectable levels of K. Nonexchangeable K may reside within grains as small occluded mica zones with insufficient periodicity for XRD detection, or could be retained in the 1.4‐nm structure. Results suggest a more direct transformation of mica to HIV rather than one involving a significant vermiculite intermediate.