Microabsorption results in biased phase fraction estimates where weakly-absorbing phases exhibit greater intensities than expected, while strongly-absorbing phases exhibit lower intensities. Calculation of the linear absorption coefficients for the cement phases (Table 1) and that of a mixture, RM 8486, show that ferrite, periclase, and free lime (when present) may be expected to have the greatest estimate errors unless compensated for. This effect is not problematic in studies utilizing standardization mixtures, as the error was inherent in the standardization curve; however, in Rietveld analyses this effect may be significant.
Fine-grinding reduces microabsorption effects, but trials in our laboratory using binary mixtures of ferrite and periclase show it may still be problematic. Use of the Brindley absorption correction changes the scale factors relative to the differences between the phase and mixture linear absorption coefficients, with an adjustment made for particle size [13].