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.

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