Characterization of silica polymorphs in kaolins by X-ray diffraction before and after phosphoric acid digestion and thermal treatment

To characterize silica polymorphs (silicas) in some kaolins, orthophosphoric acid digestion (240 °C, 15 min), and thermal treatment (1050 °C, 24 h) were applied to eight samples. The original, digested, and heated samples were examined by X-ray diffraction (XRD). Crystalline silica quartz (SiO2) was identified from the standard XRD patterns of the original kaolins; all contained quartz. On the other hand, hydrated partially crystalline silicas (SiO2·nH2O), such as well-ordered opal-C and opal-CT, were not readily distinguished from high-temperature crystalline silica, α-cristobalite, using standard XRD patterns of the original kaolins because, sharp and intense characteristic XRD peaks (h k l = 1 0 1) centered near 0.4 nm for opal-C, opal-CT, and α-cristobalite coincided. In order to distinguish these silicas the XRD patterns of the digested and heated samples were evaluated. It was observed that the 1 0 1 peaks disappear and sharpen in the course of digestion and heating, respectively. Because, the crystallinity of α-cristobalite does not change by these treatments, it was concluded that the kaolins contain opal-C and opal-CT or their mixtures in amorphous opal-A (SiO2·nH2O), but not α-cristobalite, which is probably human carcinogen. Because, the crystallinity increases in order opal-CT and opal-C, the narrowing in width at half-maximum peak height (FWHM) 1 0 1 must be more for opal-CT than opal-C by heating. Therefore, to distinguish opal-CT and opal-C from each other, the FWHM values before and after the heating process, were examined. Based on the results, it was estimated that six kaolins contain opal-CT in opal-A matrix, one kaolin contains only opal-A in a trace amount, and one kaolin contains non-opals.