Analysis of kaolinite/chrysotile mixtures by ashing and x-ray diffraction

Abstract

A simple ashing procedure for a mixture containing kaolinite and chrysotile is described that converts kaolinite to amorphous metakaolinite while retaining the diffraction intensity of chrysotile. This ashing procedure removes the X-ray diffraction (XRD) pattern overlap between kaolinite and chrysotile that can interfere with the analysis of even high concentrations of chrysotile. Samples are ashed at 460 °C in a muffle furnace for 40 h to completely convert kaolinite to metakaolinite. The complete conversion of 1 g of kaolinite under these conditions was determined for two standard kaolinite samples from Georgia, KGa-1 and KGa-2. Two of the most common types of commercial chrysotile, long-fiber Canadian and short-fiber Californian chrysotile, are demonstrated to retain diffraction intensity after ashing at 460 °C. Both chrysotile samples have the same integrated intensity for the (002) reflection prior to ashing, although the peak breadths for the two samples are quite different. Ashing at 480 and 500 °C reduces the diffraction intensities of both chrysotile samples by 15%, and broadens the peaks by approximately 3%. Using the prescribed ashing procedure and x-ray diffraction with an internal corundum standard, two kaolinite-bearing building materials containing chrysotile near 0.01 mass fraction were analyzed. The ashing procedure has additional advantages in reducing some samples to powders and removing volatile components, thereby eliminating some sample preparation procedures and concentrating any chrysotile present in the sample. The removal of volatile components improves the sensitivity of XRD analysis to concentrations below 0.01 mass fraction chrysotile.

A new method for the detection of low levels of free fibres of chrysotile in contaminated soils by X-ray powder diffraction

“…This method not only removes the organic compounds, but also enhances the chrysotile peak through a gravimetric reduction of the matrix components. The peaks of kaolinite disappear because the structure is collapsed as a consequence of the dehydroxylation reaction: 24,25 The exothermic peak at 825 uC corresponds to the recrystallization of the remaining anhydrous material to forsterite, in agreement with literature data. 27,28 Additional small and broad endothermic areas are due to the dehydroxylation of brucite at approximately 400 uC.…”