Silicate perovskite crystal structure

Surrounding the core, the mantle has a thickness of about 2900 km. Its mass is estimated at 4 x 1024 kg. It is composed mainly of high-density silicates of Mg and Fe. It is divided into three layers lower (2000 km), transition (500 km), and upper mantle (360 km). The lower mantle is predominantly formed by Mg-perovskite, Mg-wurstite, and Ca-perovskite, which contain water in their crystal structures. Incredibly as it may seem, because of this water content the lower mantle is believed to contain more water than the oceans. 

Following the widespread acceptance of the view that silicate perovskites may be major components of the lower mantle (see Jeanloz and Thompson, 1983), there have been a number of attempts to calculate the structure, elastic properties, and equations of state of these materials (Wolf and Jeanloz, 1985 Wolf and Bukowinski, 1985, 1987 Matsui et al., 1987 Hemley et al., 1987). A great deal of interest has also been generated in the crystal chemistry of perovskite-structure phases because of their high-temperature superconducting properties. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

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