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

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.