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

Hydroxy‐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 Na 2 CO 3 fusion. Extractable K by double acid, HNO 3 , and H 2 SO 4 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 K 2 O 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 K 2 O 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.