Molecular dynamics simulation of aluminous perovskite

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Mg-rich perovskite, the most abundant mineral of the lower mantle, is thought to be the host phase of alumina Al203 in the region. We have applied molecular dynamics (MD) method to aluminous perovskite (Mg, Al)(AL, Si)O3 in order to predict the effect of alumina on the physical properties. The solid solution was made by the following two methods :1. Random substitution of Mg(2+) and Si(4+) ions in MgSi03 perovskite with Al(3+) ions.2. Substitution of adjacent pairs of {Mg(2+) ion + Si(4+) ion} in MgSi03 perovskite with Al(3+) ions, so as to satisfy the local charge balance.Although the latter solid solution model gives a little lower values of enthalpy, both of the models give almost the same results, including the following characteristics :1. At P=0 GPa and T= 298 K, the unit-cell parameter c and the cell volume increase remarkably with the increase of alumina content, which is consistent with the powder X-ray diffraction data obtained under atmospheric pressure and temperature (e.g., Irifune et al., 1996 ; Kubo, 1999).2. Under the higher pressures, the increase in cell parameter c and cell volume with alumina gets smaller. At around 40-50 GPa, the volume is almost insensitive to the content of alumina.3. The Al203-enriched perovskite is more compressible than aluminum-free perovskite, which is consistent with the results of high-pressure in situ X-ray studies below 15 GPa (Kubo et al., 1999 ; Zhang and Weidner, 1999). The value of the bulk modulus at zero pressure for the perovskite with pyrope composition (Mg3Al2Si3012) is more than 20 %, smaller than that for MgSi03 perovskite. However, the difference of the value of the compressibility gets smaller with the increase of pressure.4. Positive excess enthalpy for MgSi03-Al203 binary system is observed.