Mullite-beta-spodumene composites from aluminosilicates

Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al2O3 and SiO2 in the presence of Li2CO3 and LiCl as fluxing agents. Various compositions of the above were prepared, an d conventional sintering studies were conducted at temperatures of 900 degr ees -1450 degreesC with soaking periods of 1-3 h. Kaolin, metakaolin, and a morphized kaolin in the presence of Li2CO3 showed nucleation centers of bet a -spodumene as pink specks, whereas synthetic mixtures of Al2O3 and SiO2 f ailed to behave in the same manner. To determine whether the pink specks fo rmed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence proper ties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impuritie s present in the natural kaolin were the cause of this phenomenon. Moreover , the LiCl-based samples did not result in pink specks, even though the kao linitic samples contained iron as an impurity. Therefore, although beta -sp odumene was formed in aluminosilicates in the presence of Li2CO3 and LiCl, the pink variety of beta -spodumene (kunzite) formation occurred only in th e presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained bas ed on XRD, DTA, and SEM studies.