X-ray diffraction techniques are commonly used in the
identification and quantification of fine-grained materials. A
relationship exists between the size of crystals in a sample and
the degree to which they diffract X-rays. The Scherrer equation
relates the broadness of diffraction peaks to the size of crystals
responsible for diffraction (Klug and Alexander, 1974).
Relatively large crystals of the order of I - 10 m strongly
diffract X-rays; the diffractogram of such crystals exhibit narrow
intense peaks. As the size of the crystals in a sample is reduced,
diffraction peaks become broader and less intense. Crystals less
than 10 nm generally exhibit broad, diffuse and low intensity
peaks. Some material is so fine-grained and poorly-diffracting
that X-ray patterns show very little information in a normal
one-hour scan thereby necessitating very long acquisition times
of up to 12 hours. The term poorly-diffracting material (PDM)
was coined for such material found within the bauxite deposit of
Weipa, northern Queensland (Tilley and Eggleton, 1994; 1996).
Common regolith minerals such as hematite, goethite and layer
silicate clays are quite commonly fine enough to possess a
poorly-diffracting character. Noncrystalline and poorly crystalline
Fe-Si-Al-oxyhydroxides such as allophane, opal, ferrihydrite and
hisingerite, have either a nondiffracting (amorphous) or
poorly-diffracting nature (Eggleton, 1987).