A test has been made of the use of quartz as a standardizing substance for very accurate lattice-spacing determinations. With CuK α radiation it gives a very good photograph with many sharp K α doublets, which may be measured with accuracy. A redetermination of the axial ratio from an X-ray powder photograph gave c/a =1.10002 ± 0.00004, and this value was confirmed by X-ray measurements on several other crystals. Assuming Bergqvist's value for d 100 (4246.02 X. units) we obtained a =4902.9 X., c =5393.3 X. Various specimens of quartz from different sources were compared by taking powder photographs in the same camera, one of the clear, colourless specimens being used to calibrate the camera, on the basis of the above values. The values of d 100 at 18° C. all lay between 4245.95 and 4246.30. The clear colourless crystals gave 4246.0 ± 0.05.
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Bradley et al. (1933) studied this question.