Differentiation of α-cristobalite from opals in bentonites from Turkey

Because of the similarity of the XRD-patterns, well-ordered opal-C and opal-CT have been misidentified as α-cristobalite in many publications. This is important, because flying α-cristobalite is probably a human carcinogen and the opals are not. The evidence of α-cristobalite in bentonites is hazardous for the human health and reduces their marketing. To distinguish the opal-C and opal-CT from α-cristobalite, the XRD-patterns of bentonites before and after H3PO4-digestion (240 °C, 15 min) and heat treatment (1050 °C, 24 h) were evaluated in details. It was seen that the most characteristic XRD-reflection (hkl:101) centered near 0.40 nm for crystalline α-cristobalite and paracrystalline opals disappeared after the digestion and sharpened after the heat treatment. Since the crystallinity of α-cristobalite was not affected from the digestion and the heat treatment, it was concluded that the bentonites contain opal-CT or opal-C in amorphous opal-A matrix, but do not contain α-cristobalite. Since the paracrystallinity increases in order opal-CT and opal-C, the narrowing in full width at half-maximum reflection height (FWHM) 101 must be more for opal-CT than opal-C by heat treatment. Therefore, these opals were distinguished approximately from each other depending on the narrowing in the FWHM of 101 XRD-reflection by heating. Based on this result, it would not be decided that whether bentonites or other clays contain α-cristobalite or not, without performing the H3PO4-digestion and heat treatment.