Physicochemical studies on dusts. V. Examination of finely ground quartz by differential thermal analysis and other physical methods

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Abstract
Physical evidence consistent with the concept of a disturbed surface‐layer, containing non‐quartz silica, on finely ground quartz, is presented: Differential thermal analysis of quartz dusts is markedly affected by the presence of this layer on fine particles. Below 5–10 μ, the measured quartz content decreases sharply, values X‐ray‐diffraction method is similarly influenced by the disturbed layer on fine particles, line‐intensity being markedly increased by etching the finely ground dusts. The mean thickness of the disturbed layer is estimated at 0.11–0.15 μ by calculations based jointly on differential thermal analysis and the change in density shown by quartz on grinding. Calculations based on density changes alone, and on previous solubility studies, give lower estimates (0.03–0.05 μ and 0.02–0.03 μ respectively). The refractive index and mean density of the disturbed layer lie nearer to the values for quartz than to those for vitreous silica. Empirical correction‐factors previously used to compensate for quartz losses in a chemical method of analysis are now given a rational interpretation on the basis stated above, and possibly provide an independent estimate of layer thickness (0.04–0.07 μ).

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