ABSTRACT
Crystalline materials exhibit long-range order in their atomic structure. Structural features repeat over scales of at least micrometers in a single crystal or over several particles in a powder. Such minerals display narrow x-ray di¦raction peaks and their crystal habit can be directly observed at the scale of an optical microscope or by the naked eye in the case of large crystals. Amorphous materials, on the other hand, exhibit no order, even in the local
environment of the atoms. ¤e atoms are arranged in a variety of states and structures where bond lengths, coordination, and geometry vary from site to site. Poorly crystalline materials fall in between these two extremes, showing short-range, mediumrange, or limited long-range order. Short-range order is observable by techniques that probe the local environment of each atom, such as nuclear magnetic resonance (NMR) or x-ray photoelectron spectroscopy. X-ray di¦raction peaks are indicative of, at least, medium-range order, whereas long-range order is detectable by electron microscopy. Poorly crystalline materials lack medium-range repetition of structural units in at least one of the three spatial dimensions; that is, well dened electron or x-ray diffraction patterns (XRD) are not produced. Allophane and imogolite practically represent the range from amorphous to crystalline materials (Fyfe et al., 1987). In the interest of brevity, allophane, imogolite, and poorly crystalline iron hydrous oxides will be referred to collectively as short-range ordered (SRO) materials.
