COMPOSITION MICROHETEROGENEITY OF UVAROVITE GROSSULARITE GARNETS

A solid-solution series with up to 50 mol% uvarovite (Ca3Cr2Si3O12) an d grossularite (Ca3Al2Si3O12) was synthesized by the sol-gel method at 1543 K. Colloidal silica was used as the starting reagent, together w ith chloride salts of the remaining components. X-ray diffraction anal ysis of the evolution of the systems showed the formation of uvarovite in a nominal composition of 100 mol% uvarovite; a garnet-phase solid solution; and gehlenite (Ca2Al2SiO7), pseudowollastonite (alpha-CaSiO3 ), and alpha-cristobalite in a nominal composition of 100 mol% grossul arite. In some samples, small amounts of impurity phases appeared alon g with the major phase. The general microstructure was observed by sca nning electron and transmission electron microscopy, and an agglomerat ion of crystals with 2-mum average size was revealed. The crystals wer e similar and rounded in shape for all compositions. Additional observ ations carried out by transmission electron microscopy (TEM) (using th e powder replica method) showed that the end series consists of cubic crystals displaying some deformation. Quantitative and semiquantitativ e scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) and t ransmission electron microscopy/scanning electron microscopy/energy-di spersive X-ray (TEM/STEM/EDX) microanalysis (by the Cliff-Lorimer meth od, working at 200 kV) were also conducted in all compositional ranges of the solid solutions synthesized in the present study.