Magnesium silicate perovskite

Frost D. J. and Langenhorst F. (2002) The effect of AI2O3 on Fe—Mg partitioning between magnesiowiistite and magnesium silicate perovskite. Earth Planet. Sci. Lett. 199, 227-241. 

J. S. Sweeney and D. L. Heinz, Melting of iron-magnesium-silicate perovskite, Geophys. Res. Lett. 1993, 20, 855-858. 

Simultaneous measurements were made of the self-diffusion of oxygen and silicon in magnesium silicate perovskite. The oxygen diffusivity was some 2 orders of magnitude higher than the Si self-diffusivity and Fe-Mg interdiffusion in perovskite ... 

Kellogg et al. (1999), however, have suggested, on the basis of a transition in seismic heterogeneity observed at —1,600 km depth, the possibility of a very deep layer extending hundreds of kilometers above the core-mantle boundary. One possibility is that a relic layer of dense, primordial crystalline differentiates (e.g., magnesium- and calcium-silicate perovskite) may have remained buried in the deep lower mantle until the present. Such a layer is a potential storehouse for trace elements, including radioactive heat-producing elements, and potentially could provide an important reservoir for bulk silicate Earth chemical mass balance... 

The effects of covalency on the quartz to stishovite phase transition in silica and the spinel to perovskite phase transition in magnesium silicates can be compared. Both of these phase transitions are from a phase based on tetra-coordinated silicon ions to a phase based on hexa-coordinated silicon ions. For the phase transition in silica, the inclusion of covalent effects reduces the transition pressure by over 15 GPa stishovite is stabilized relative to quartz because it has more bonds, and the bonds are significantly covalent in both structures. In contrast, for the magnesium silicate phase transition, the inclusion of covalent effects increases the transition pressure by 20 GPa although there are more bonds in perovskite than in spinel, spinel is stabilized more because the bonds are more covalent in spinel than in perovskite. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

The refractory condensate model has fallen out of favor, including with Lewis (1988). Nevertheless, it is a useful end-member case. Goettel (1988) calculated the composition of the silicate portion of an ultrarefractory Mercury (Table 2, column 2). This model composition contains no FeO or volatiles, and has large concentrations of the refractory elements—aluminum, calcium, and magnesium. We calculated the thorium and uranium contents of such refractory condensates by assuming chondritic Al/Th and Al/U ratios. A surface of this composition will contain many of the phases in calcium-aluminum-rich inclusions (CAls), such as forsterite, anorthite, spinel, perovskite, hibonite, and melilite. 

The lower mantle is made predominantly of magnesium perovskite, and thus a solid understanding of element partitioning between perovskite and silicate melt is relevant to the early history of the Earth. Much of the early work with perovskite was done on aluminum-free or low-aluminum materials. However, magnesium perovskite can accommodate AI2O3 into its structure (Wood, 2000 Stebbins et ai, 2001),... 

Magnesium sihcate has several potymorphs (enstatite, majorite, perovskite) of which the enstatite t5rpe is the most common and most stable form. It is therefore likely that it is this form of magne-siinn silicate which has been described for use as a paint extender. 

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