Abstract
NIOSH Method 7601 gives incorrect high values by colorimetry for crystalline silica associated with sodium montmorillonite in Wyoming bentonite deposits because the digestion of amorphous silica and silicate minerals is incomplete in phosphoric acid at 240°C. However, the partial digestion of silicates and amorphous silicas concentrates crystalline silica for x-ray diffraction and other analytical methods to meet the 0.1% threshold for quantitative analysis required by federal and local government agencies. When the NIOSH Method 7500 x-ray procedure is applied to phosphoric acid digestion residues, use of the absorption correction factor to correct for thicker than ideal particles or films gives incorrect results and should not be used. This phosphoric acid digestion also provides a chemical method of distinguishing soluble opal (SiO 2· xH 2O) from inert cristobalite of high temperature origin. Based on the greater chemical reactivity in phosphoric acid of the silica phase associated with a number of Wyoming bentonites, and the x-ray diffraction patterns for these phases, the bentonite deposits are best described to contain opal-CT and not cristobalite.
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