The Application of Micro-Beam Methods to Iron Minerals in Soils

Abstract

The term “micro-diffraction” may be used to describe diffraction in which the incident beam is very narrow, or it may refer to diffraction from very tiny crystals. Iron-bearing minerals in soils are generally very fine-grained, commonly less than one micrometer — a size considerably smaller than the collimated beam of most available X-ray cameras. Hence, as it applies to X-ray diffraction, “micro” in this Chapter will refer to the crystal size. Most routine X-ray diffraction work is done on large crystals (1–5 μm diameter and thus more than 1000 unit cells across). Most of the theory and practice, Bragg’s law and the Laue equations for example, assume many coherently diffracting unit cells and simplify the results accordingly. The simplified theory is inadequate when the crystal size is reduced below about 100 unit ceils, and serious error in the measurement of d-spacing using Bragg’s law may be introduced below 20 unit cells. Many Fe minerals in soils are this small, and the full diffraction theory is needed to avoid errors in interpretation of X-ray diffraction results by routine methods.