The structure analyses of pyrope (Mg₃Al₂Si₃O₁₂) and grossular (Ca₃Al₂Si₃O₁₂) glasses by X-ray diffraction method

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The structures of Mg₃Al₂Si₃O₁₂ and Ca₃Al₂Si₃O₁₂ glasses were investigated by means of the radial distribution function (RDF) analysis based on X-ray diffraction data. The Al and Si atoms in these glasses are essentially in four-fold coordination. The T–O (T=A1 and Si) distance of Mg₃Al₂Si₃O₁₂ glass (1.68Å) estimated from the peak position of radial distribution function curve is almost equal to that of Ca₃Al₂Si₃O₁₂ glass (1.70Å). These T–O distances are consistent with the common average T–O distance in AlO₄ and SiO₄ tetrahedra. Mg–O distance in Mg₃Al₂Si₃O₁₂ glass is 2.1Å and coordination number of Mg atom is 4.8. These values are smaller than those in pyroxene and pyrope crystals. Ca–O distances in Ca₃Al₂Si₃O₁₂ glass is 2.4Å and coordination number of Ca atom is 7.1–7.4. These values are in conformity with those obtained in grossular and diopside crystals. The short range structures of Mg₃Al₂Si₃O₁₂ and Ca₃Al₂Si₃O₁₂ glasses, the oxygen coordination around T, Mg, and Ca atoms resemble those of MgSiO₃ and CaSiO₃ glasses, respectively. Especially, the G(r) curve of Mg₃Al₂Si₃O₁₂ glass is very similar to that of MgSiO₃ glass.