Structural studies of imogolite and allophanes by aluminum-27 and silicon-29 nuclear magnetic resonance spectroscopy

27Al Mas NMR and Aluminum X-Ray Absorption Near Edge Structure Study of Imogolite and Allophanes

“…In allophanes, the AFV/Ap ~ ratio determined from NMR increases with decreasing A1/Si ratio ( Figure 2). It is highest for the Okamoto allophane (0.21) in which the A1/Si ratio is close to unity, as shown previously by Goodman et al (1985). The AFV-~p values found in this study (58.6-59.8 ppm) are in the range reported for framework silicates (55-65 ppm: Fyfe et al, 1982;Kirkpatrick et al, 1985) and A1TM in peralkaline glasses (58-62 ppm: Oestrike and Kirkpatrick, 1988).…”

“…Bs and Kip allophanes (with A1/Si ratios 2.3 and 1.78, respectively) yield spectra similar to those of imogolite, as previously observed by Goodman et al (1985). The 6p of the main resonances are 5.8 and 6.0 ppm, respectively.…”

“…The 6p value of the KIG imogolite is more positive than that of the same imogolite in another study (-2 ppm: MacKenzie et aL, 1989) due to the higher H0 magnetic field we used (Kinsey et al, 1985). It is similar to, though slightly higher than the 6p value measured in another natural imogolite, 5.8 ppm (Goodman et al, 1985). The 27A1 NMR spectra of Goodman et al (1985) for one synthetic and one natural imogolite show, in addition to the peak for A1 vI, the presence of a second small peak at 63 +_ 1 ppm due to AI TM that was interpreted as due to the presence of an alumino silicate gel or an allophane.…”

“…Sirich allophanes (AI/Si ~ 1) are thought to have a defect kaolin-or halloysite-structure, a tetrahedral sheet being the framework of the structure . The presence of both 4-and 6-coordinated A1 in natural allophanes has been recognized by various authors (Goodman et al, 1985;Childs et al, 1990). However, the change in the relative concentration of A1 with different coordination numbers (CN) as a function of allophane chemistry is unknown.…”

“…Spectroscopic methods like XANES (e.g., Brown et al, 1988) and nuclear magnetic resonance (NMR) are direct probes of local Al-environments. 27A1 NMR is an important structural tool for determining AI coordination number in minerals (Miiller et al, 1981 ), particularly in clay minerals (Sanz and Serratoza, 1984;Kinsey et al, 1985;Woessner, 1989) and in low temperature, XRD-amorphous aluminosilicate allophanes from soils and stream deposits (Wilson et al, 1984;Goodman et at., 1985;Childs et al, 1990). XANES has provided useful information about Al-coordination in crystalline and glassy silicates (McKeown et al, 1985;McKeown, 1989).…”