³⁹K Solid-State NMR Studies of Potassium Tecto- and Phyllosilicates: The In Situ Detection of Hydratable K⁺ in Smectites

Abstract

Abstract--We report the first application of 39K solid-state NMR to the study of tecto-and phyllosilicates. Under high field (11.7 Tesla) and with the application of a spin-echo sequence, informative 39K spectra can be obtained for several compounds of interest to the geologist and the agronomist. Tectosilicates and phyllosilicates can be distinguished from the uncorrected frequency (Sc6) of the observed NMR peak. A series of montmorillonites submitted to increasing numbers of wetting and drying cycles was studied in order to discriminate between mobile and "fixed" forms of K+: when the spectra are run on hydrated samples, two different signals are observed corresponding to K § in different hydration states, and NMR data can be correlated with the amount of exchangeable K + measured by ion exchange. Thus, it appears that NMR can provide useful information on K fixation complementary to classical chemical methods.

New Opportunities for High-Resolution Solid-State NMR Spectroscopy of Oxide Materials at 21.1- and 18.8-T Fields

“…A previous 39 K study of a similar potassium feldspar (orthoclase) and several sheet silicates including muscovite used partially oriented single crystals at 11.7 T, and noted surprisingly broad peaks [31]. Peak positions obtained in that study cannot be directly compared with the new data at 18.8 T because powder averages were not obtained.…”

State and Location of Water Adsorbed on Clay Minerals: Consequences of the Hydration and Swelling-Shrinkage Phenomena

“…The 2 peaks observed for fully dehydrated samples were assigned to 2 different interlayer sites. Lambert et al (1992) report data obtained with a Kmontmorillonite submitted to wetting and drying (W-D) cycles. Figure 7 gives 39K solid-state NMR spectra of potassium in montmorillonite pastes prepared with samples submitted to 0, 11 and 100 W-D cycles and after a replacement of exchangeable K by Sr in the 100 W-D sample.…”

Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite

“…For example static 39 K NMR spectra from a range of model inorganic potassium salts showed relatively small dipolar coupling, resulting in sharp powder pattern features of the observed central transition allowing accurate extraction of the quadrupole parameters as chemical shift anisotropy appeared to make only a very small contribution [34]. In potassium-containing aluminosilicates it has been shown that from the position of the centre-of-gravity (cg) of the static 39 K resonances, the tectosilicate orthoclase (~205 ppm) could be distinguished from phyllosilicates in which the cg position is below 40 ppm [35]. Potassium NMR could also distinguish structural and exchangeable potassium in the clay mineral montmorillonite as it undergoes wetting and drying [35].…”

“…In potassium-containing aluminosilicates it has been shown that from the position of the centre-of-gravity (cg) of the static 39 K resonances, the tectosilicate orthoclase (~205 ppm) could be distinguished from phyllosilicates in which the cg position is below 40 ppm [35]. Potassium NMR could also distinguish structural and exchangeable potassium in the clay mineral montmorillonite as it undergoes wetting and drying [35]. For potassium aluminosilicate geopolymers the cg positions varied from −47 ppm (more typical of a hydrated potassium phase) in the unheated geopolymer, to −120 ppm in the material heated at 1300 o C, consistent with the position of an unheated feldspar [11].…”