Data for three hydrobiotites from different sources give mean basal spacings of 24.51 Aa measured to 0.0.31 for individual flakes and 0.0.22 for oriented powder layers coefficient of variation (CV) values for these samples are 1.25, 0.35, and 0.98 when equal weight is given to all reflections. When the first-order diffractions are omitted, the CV values are 0.67, 0.29, and 0.20. The three samples studied have 45%, 53%, and 49% biotite layers; calculations also show that if the percentage of biotite layers falls outside the range 45-55% biotite, variable basal spacings with CV >0.75 will be obtained.--Modified journal abstract.

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