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Abstract: Experimentally determined phase relations in the system MgO–AlO1.5–SiO2 at pressures relevant to the upper part of the lower mantle indicate that Mg–silicate perovskite incorporates aluminum into its structure almost exclusively by a charge-coupled reaction. MgSiO3-rich bulk compositions along the joins MgSiO3–MgAlO2.5 and MgSiO3–MgAl2O4 crystallize assemblages of perovskite coexisting with periclase. MgO-saturated perovskites along these joins have ambient unit cell volumes consistent with those measured and calculated for aluminous perovskite along the charge-coupled substitution join, MgSiO3–AlO1.5. The compressibility of aluminous perovskite along the MgO-saturated joins is not anomalously low as predicted for oxygen-defect perovskites. The bulk moduli, however, are consistent with previous measurements made for aluminous perovskites along the charge-coupled substitution join. These results agree with first-principles calculations showing very limited stability of O-defects in Mg-perovskite at pressures and temperatures corresponding to lower mantle conditions, but are inconsistent with earlier experimental results showing unusually compressive aluminous perovskite. The maximum solubility of alumina in perovskite is ∼25 mol% along the MgSiO3–AlO1.5 join within the ternary MAS-system (i.e. pyrope composition), and the join is apparently binary. Although primitive mantle peridotite compositions are MgO-saturated and fall nearly on the oxygen vacancy join, alumina substitution into perovskite is expected to occur primarily by charge-coupled substitution throughout the lower mantle. The compressibility of aluminous perovskite in primitive mantle is expected to be only a few percent lower than for end member MgSiO3 perovskite. [Copyright &y& Elsevier]

Systematic correlation in alkaline-earth carbonate compounds between the deviation of the CO3 units from the perfect D3h symmetry and their (13)C nuclear magnetic resonance (NMR) chemical shift anisotropy (CSA) parameters is established. The (13)C NMR CSA parameters of amorphous calcium carbonate (ACC) are measured using two-dimensional (13)C phase adjusted spinning sidebands (PASS) NMR spectroscopy and are analyzed on the basis of this correlation. The results indicate a distortion of the CO3 units in ACC in the form of an in-plane displacement of the C atom away from the centroid of the O3 triangle, resulting from hydrogen bonding with the surrounding H2O molecules, without significant out-of-plane displacement. Similar distortion for all C atoms in the structure of ACC suggests a uniform spatial disposition of H2O molecules around the CO3 units forming a hydrogen-bonded amorphous network. This amorphous network is stabilized against crystallization by steric frustration, while additives such as Mg presumably provide further stabilization by increasing the energy of dehydration.