Dense hydrated Mg-silicates in diamond: Implications for transport of H2O into the mantle.

Water in Earth's upper mantle is a minor and yet critically important component that dictates mantle properties such as strength and melting behavior. Minerals with stoichiometric water, such as those of the humite group, are important yet poorly characterized potential reservoirs for volatiles in the upper mantle. Here, we report observation of hydroxyl members of the humite group as inclusions in mantle-derived diamond. Hydroxylchondrodite and hydroxylclinohumite were found coexisting with olivine, magnesiochromite, Mg-bearing calcite, dolomite, quartz, mica, and a djerfisherite-group mineral in a diamond from Brazil. The olivine is highly forsteritic (Mg# 97), with non-mantle-like oxygen isotope composition (δ18O +6.2‰), and is associated with fluid inclusions and hydrous minerals-features that could be inherited from a serpentinite protolith. Our results constitute direct evidence for the presence of deserpentinized peridotitic protoliths in subcratonic mantle keels, placing important constraints on the stability of hydrous phases in the mantle and the origin of diamond-forming fluids.

Significant Variability in the Photocatalytic Activity of Natural Titanium-Containing Minerals: Implications for Understanding and Predicting Atmospheric Mineral Dust Photochemistry.

The billions of tons of mineral dust released into the atmosphere each year provide an important surface for reaction with gas-phase pollutants. These reactions, which are often enhanced in the presence of light, can change both the gas-phase composition of the atmosphere and the composition and properties of the dust itself. Because dust contains titanium-rich grains, studies of dust photochemistry have largely employed commercial titanium dioxide as a proxy for its photochemically active fraction; to date, however, the validity of this model system has not been empirically determined. Here, for the first time, we directly investigate the photochemistry of the complement of natural titanium-containing minerals most relevant to mineral dust, including anatase, rutile, ilmenite, titanite, and several titanium-bearing species. Using ozone as a model gas-phase pollutant, we show that titanium-containing minerals other than titanium dioxide can also photocatalyze trace gas uptake, that samples of the same mineral phase can display very different reactivity, and that prediction of dust photoreactivity based on elemental/mineralogical analysis and/or light-absorbing properties is challenging. Together, these results show that the photochemistry of atmospheric dust is both richer and more complex than previously considered, and imply that a full understanding of the scope and impact of dust-mediated processes will require the community to engage with this complexity via the study of ambient mineral dust samples from diverse source regions.

An unprecedented uranyl phosphate framework in the structure of [(UO2)3(PO4)O(OH)(H2O)2](H2O).

The new uranyl phosphate [(UO2)3(PO4)O(OH)(H2O)2](H2O) (1) with an unprecedented framework structure has been synthesized at 150 and 185 degrees C. The structure (tetragonal, P4(2)/mbc, a = 14.015(1) A, c = 13.083(2) A, V = 2575.6(4) A(3), Z = 8) contains uranyl phosphate chains composed of uranyl pentagonal and hexagonal bipyramids and phosphate tetrahedra linked by sharing of polyhedral edges. The uranyl phosphate chains are aligned both along [100] and [010] and are linked into a novel framework structure involving channels along [001]. Topologically identical chains occur linked into sheets in more than a dozen uranyl phosphate minerals, but these chains have never been observed in opposing orientations and linked into a framework as in 1.