Fig. 3. Part of powder diffraction patterns obtained for alite using a specimen with minimum preferred orientation (black) and intentionally induced preferred orientation (red). For the peaks 32.1° and 32.7, the intensity ratio should be about 1.2, whereas the the red pattern has a peak intensity ratio of 1.5.
can test for the effect of coarseness by obtaining patterns on the same powder sample that has been removed and repacked in the sample holder; each repacking should produce a different ratio of peak intensities if the bias is caused by coarseness.
Some preferred orientation is difficult to avoid for powders with anisometric grains, such as platelets or needles, because they tend to naturally pack with the particles aligned. Minerals with strong cleavage planes, such as alite, calcite, and gypsum all are susceptible to preferred orientation. Corrections for preferred orientation are available in most XRD analysis software, but they seem to be most effective after steps have been taken to minimize orientation effects by careful specimen preparation, as described in Sec. 2.3.
A third potential source of error in XRD stems from the limited interaction depth of the X-ray beam with the specimen. The interaction depth varies with the X-ray absorptivity of the powder specimen while, for a fixed divergence slit setting, the interaction volume remains constant. A typical interaction depth for cement powder, assuming 2θ = 40°, is about 15 μm. For multiphase powders like cement and ground