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 degrees C, 15 min), and thermal treatment (1050 degrees 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 center dot nH(2)O), such as well-ordered opal-C and opal-CT, were not readily distinguished from high-temperature crystalline silica, alpha-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 alpha-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 alpha-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 center dot nH(2)O), but not alpha-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. (c) 2005 Elsevier B.V. All rights reserved.