O ATOM SITES IN NATURAL KAOLINITE AND MUSCOVITE: 17O MAS AND 3QMAS NMR STUDY

Abstract

The layer silicates are among the most common minerals in the Earth’ s surface environment, play important roles in many geological processes, and have diverse technological applications. While it has been suggested that O isotope exchange and dissolution kinetics in aqueous solutions are controlled by chemical bonding and local atomic structures, the effect of atomic environment around O atom sites in clay minerals on their site-specific reactivities with H2O are not well known, mainly because direct experimental evidence is lacking. Here, we present for the first time detailed high-resolution 17O NMR data for 17O-exchanged natural kaolinite [Al2Si2O5(OH)4] and muscovite [KAl2(AlSi3)O10(OH)2] using 17O triple quantum magic angle spinning (3QMAS) and MAS NMR at high fields. At least two basal O atom sites in kaolinite are resolved: O4, and (O3 + O5). Apical O atoms ( [4] Si-O-2 [6] Al) and hydroxyl groups are also shown in these spectra. The 17O 3QMAS spectrum for muscovite shows improved resolution over the 17O MAS NMR spectrum, allowing us to resolve several basal O atoms, including ( [4] Si-O-[ 4] Al), as well as hydroxyl groups. The fraction of each O atom appears to deviate somewhat from the stoichiometric value, suggesting that each crystallographically distinct site may have a different rate of exchange with the O atom in H2O.