THE EXTENT OF DISORDER AND KINETIC STABILITY OF OXYGEN SITES IN PHYLLOSILICATES : OXYGEN-17 NMR STUDY

The phyllosilicates are one of the most dominant constituents of the Earth’s surfaces and have diverse applications as an industrial catalysis and a backfill material for nuclear waste storage. While it has been speculated that oxygen isotope exchange and dissolution kinetics are controlled by chemical compositions and topology, the atomic environment around oxygen in clay minerals, a key factor in kinetic stability, has not been explored.

Here, we present, for the first time, the detailed oxygen atomic configurations in several model phyllosilicates including pyrophyllite, kaolinite and muscovite using oxygen-17 multiple quantum magic angle spinning (MQMAS) and MAS NMR where several types of crystallographically distinct, basal and apical oxygens as well as hydroxyl groups are clearly resolved. We also show that these oxygen sites have different kinetic stability in aqueous solutions. Three resolved basal oxygen sites (Si(4)-O-Si(4) (I,II & III) in synthetic pyrophyllite due to varying distance with hydroxyls in octahedral layer are resolved. On the other hand, only two basal oxygen sites (Si(4)-O-Si(4) (I,II)) are resolved in synthetic kaolinite and natural kaolinite reacted with 17O enriched water at hydrothermal conditions. No Al(4)-O-Al(4) was found in O-17 enriched natural muscovite with similar fraction of Al(4)-O-Si(4)/ Si(4)-O-Si(4), confirming a homogeneous distribution of Al in tetrahedral layer. The relative site population of oxygen in phyllosilicates reacted with O-17 enriched water can provide crucial information on relative kinetic stability of each site and the extent of disorder among tetrahedral cations. The fraction of Si(4)-O-Si(4)(II)/ Si(4)-O-Si(4)(I) in hydrothermally altered natural kaolinite is less than 1 (=0.85), demonstrating that these two sites have different kinetic stability and the nearby hydroxyl affects the dissolution and exchanges kinetics by hampering the protonation of basal oxygen. The apical oxygens in O-17 enriched natural phyllosilicates have significant intensity, implying diffusion of water perpendicular to octahedral and tetrahedral layers as well as that parallel to the interlayer.

Above results can shed light on more complete, atomic-level understanding on the geochemical processes, such as weathering and oxygen isotope fractionation.