Occluded Mica in Hydroxy-Interlayered Vermiculite Grains from a Highly-Weathered Soil

Abstract

Hydroxy-interlayered vermiculite (HIV) is a ubiquitous phyllosilicate in the <0.05-mm fraction of sandy soils on the U.S. southeastern coastal plain. Extensive areas of soils with abundant HIV (i.e., peninsular Florida) have no detectable mica; yet the coarseness, platy habit, and nonexchangeable K associated with HIV grains suggest a mica precursor. The objectives of this study were: (1) to probe for mica zones (1.0-nm) within HIV grains, using high-resolution transmission electron microscopy (HRTEM), and (2) to determine intragrain elemental distributions via electron microprobe analysis (EMA). HIV grains from a Quartzipsamment medium-silt fraction, which contained no detectable mica by X-ray diffraction (XRD), were concentrated via high-density liquid separation. EMA transects and X-ray dot maps showed zonation or trends of K depletion near edges of some grains, with K2O contents ranging from trace levels to >40 g kg-1. Elemental oxide data indicated a dioctahedral phyllosilicate structure, with some octahedral substitution of Fe and Mg for Al. Intermittent 1.0-nm lattice-fringe images obtained by HRTEM supported the presence of mica zones within grains. There were no detectable 1.4-nm fringes, despite the dominance of a 1.4-nm XRD peak, indicating the instability of the HIV specimen under the electron beam. Results support a transformational link between mica and HIV in these soils. Rapid incursion and polymerization of Al following loss of K from mica may limit the extent of the vermiculite intermediate. The latter idea is consistent with the paucity of vermiculite in Florida soils. Traces of occluded mica may be the last remnants of the precursor grain. A sand-sized mica precursor would likely have weathered in place during the period when colloidal components such as kaolinite illuviated to deeper zones. Thus, the transformation product (HIV) would comprise a significant proportion of the <0.05-mm fraction persisting in sandy eluvial horizons.

Highly weathered mineralogy of select soils from Southeastern U.S. Coastal Plain and Piedmont landscapes

“…Hydroxy-interlayered vermiculite content is stable, changing in concentration by kaolinite depletion (surface) and accumulation (subsurface). Data from Harris et al (1989Harris et al ( , 1992 for some Florida Coastal Plain soils support a similar mechanism.…”

“…In fact, relatively abrupt changes of these mineral quantities across E/Bt horizon boundaries have been suggested as evidence of an eluvial pathway (Harris et al, , 1992. Workers attribute the coarser size of the hydroxyinterlayered mineral in some Coastal Plain soils to a mica precursor (Harris et al, 1988;Harris et al, 1992). Jenny's state factor equation indicates soils are a function of parent material, climate, relief, and organisms, all acting over time (Jenny, 1941).…”

The distribution coefficient for cesium in different clay fractions in soils developed from granite and Paleozoic shales in Japan

“…As Thompson and Ukrainczyk (2002) pointed out, XRD analysis may not be suitable for the quantification of illitic minerals in soil clays. Particularly, illitic minerals associated with hydroxy-interlayered vermiculite are known to be poorly detected by XRD analysis (Harris et al 1992). Although, the relative intensity of the 1.0 nm peak in the XRD analysis may be effective for a rough comparison of the 137 Cs adsorption potential between soils (Ishikawa et al 2007), the absence of the 1.0 nm peak does not necessarily mean the absence of illitic minerals in the soil clays.…”

Radiocesium interception potential (RIP) of smectite and kaolin reference minerals containing illite (micaceous mineral) as impurity

“…Laird et al (1991) estimated the proportion of the illitic phase as 18-36% in smectitic fine clays which showed no XRD peak for illite, using a nonlinear regression model method calculated with total elemental composition. Harris et al (1992) detected a conspicuous amount of K in the aluminosilicate structure by using electron-optical analyses (electron microprobe analysis (EMA) and high-resolution transmission electron microscopy (HRTEM)), although the same sample showed no XRD peak for illite.…”