Molecular dynamics simulation of majorite solid solution

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The molecular dynamics (MD) method was applied to majorite solid solution in the system Mg3A12Si3O12 (pyrope)-Mg4Si4012 (pyroxene component), in order to predict the effects of composition and cation distribution on the cell parameters, volume, bulk modulus, and molar enthalpy. The solid solution crystals Mg3(Mg[x]Si[x]Al[2-2x])Si3O12 (0 【less than or equal】 x 【less than or equal】1) with the following two types of cation distribution were prepared :1. Tetragonal solid solution with the ordering of Mg and Si on the two nonequivalent octahedral sites.2. Cubic solid solution with the random distribution of Mg and Si in the octahedral site.The MD calculation was performed under constant pressure and temperature, by using two-body interatomic potential model CMAS94 (Matsui, 1994). The characteristic results are as follows :1. The cell parameters of tetragonal solid solution shows the relation a > c. The difference in the parameters becomes smaller with the decrease in the Mg4Si4O12 content x. At the pyrope composition (x = 0), the crystal system becomes cubic.2. In Al-enriched composition (x 〜 0), the two solid solution models give almost the same values of molar volume, bulk modulus, and molar enthalpy. With the increase in the Mg4Si4O12 content x, the cubic solid solution becomes to show larger molar volume, smaller bulk modulus, and higher enthalpy, than the orthorhombic solid solution.The bulk modulus of the majorite solid solution determined by high pressure X-ray diffraction studies decreases with x in the compositional range of 0 【less than or equal】 x 【less than or equal】 0.8, and increase rapidly in the range 0.8 【less than or equal】 x 【less than or equal】 1. This is explained by the progressive ordering of Mg and Si on the octahedral sites around the composition of x = 0.8.