Allophanes diffraction

The index of refraction of allophane ranges from below 1.470 to over 1.510, with a modal value about 1.485. The lack of characteristic lines given by crystals in x-ray diffraction patterns and the gradual loss of water during heating confirm the amorphous character of allophane. Allophane has been found most abundantly in soils and altered volcanic ash (101,164,165). It usually occurs in spherical form but has also been observed in fibers. 

X-ray diffraction pattern for allophane shows the characteristics of non-crystalline material (Wada, 1985) imogolite patterns consist of a... 

Secondary silicates form as clay minerals in soils after weathering of the primary silicates in igneous minerals. The secondary silicates include amorphous silica (opal) at high soluble silica concentrations and the very important aluminosilicate clay minerals kaolinite, smectite (montmorillonite), vermiculite, hydrous mica (il-lite), and others. Kaolinite tends to form at the low silicate concentrations of humid soils, whereas smectite forms at the higher silicate and Ca concentrations of arid and semiarid soils. The clay fraction of soils usually contains a mixture of these day minerals, plus considerable amorphous silicate material, such as allophane and imogolite, which may not be identifiable by x-ray diffraction. 

The reporting of mica in soil clays depends somewhat on the method of detection. Jackson and Mackenzie [1964] state that some soil clays, which show no indication of mica based on X-ray diffraction, may contain from 5 to 20 % or more of micas based on chemical analysis and on the basis of 10% K2O in mica. According to Schuffelen and van der Marel [1955], soils high in allophane fix very considerable quantitities of potassium. Thus, potassium does not necessarily reside altogether in micas and feldspars in soils. Some of it may be in amorphous material. However, some of the potassium may be in micalike zones of particles, which are largely montmorillonite or vermiculite and have weathered from micas. Such zones may be too small to be detected by X-ray diffraction (Knibbe and Thomas [1972]). 

If much well-ordered kaolinite is present, the assymmetric peaks are not prominent in the patterns from random samples, and the basal reflections are sharper and much enhanced in intensities in patterns from oriented samples. If much disordered kaolinite is present, the assymmetric peaks are prominent in the first patterns, and the basal reflections are much enhanced in the second. Chemical pretreatments prior to X-ray diffraction, such as those proposed by Wada [1965] and Alexiades and Jackson [1965], are sometimes useful in determining amounts of kaolinite and halloysite. Where the halloysite is tubular, it is easily detected in electron micrographs, although the amount can seldom be determined. Amounts of hydrated halloysite can be determined if allophane is not present in differential thermal analysis by calibrating and measuring the low-temperature endothermic peak. 

The advent of X-ray methods of study in the twentieth century showed that materials appearing amorphous through the miscroscope could be subdivided into submicroscopic crystalline materials and materials that appeared amorphous to X-rays. With the technique of X-ray powder diffraction, a large proportion of specimens classed as allophane from museums, collections, or regions described in the literature were shown in fact to include finely crystalline material, frequently halloysite. 

The availability of such instrumental techniques as X-ray powder diffraction, differential thermal analysis, electron micrography, and infrared absorption spectrophotometry, together with refinements in measurements of surface properties, particularly since 1950, has improved methods of defining allophane. It is, therefore, necessary to consider the nature of the evidence of these and other techniques with respect to forms of allophane occurring in soils, in order to appreciate the range of soil properties conferred by this material. 

Fieldes [1968] has studied subsoils of suites of brown loams and red loams, soils of increasing maturity derived from basalt. Relative proportions of amorphous (allophanic) constituents, kaolin, and gibsite were assessed by X-ray diffraction and differential thermal analysis. Results are listed in Table 7. The following points are observed ... 

The following section reviews evidence concerning allophane provided by the techniques of X-ray powder diffraction, electron diffraction differential thermal analysis, infrared absorption, and electron micrographic methods of analysis. 

Samples of allophane are, by definition, amorphous to X-rays, and their X-ray powder diffraction patterns show no pronounced X-ray powder diffraction maxima. The general scatter of X-rays by allophane samples mounted, for instance, on glass plates is, however, greater than that of the glass itself, particularly at low angles. This scatter differs for different samples, often as a result of differences in particle size or differing amounts of constituents other than allophane such as compounds of iron. Diffuse bands near 10 A, sometimes reported, are possibly due to hydrated halloysite. 

X-ray diffraction patterns of ordered structure in allophanic soil clays are evidence of the presence of crystalline impurities. It is, however, possible that finely divided particles showing some evidence of crystallinity by X-ray or electron diffraction patterns may have allophanic surface properties, and such materials (e.g., hydrous feldspars) may be conveniently regarded as special cases of allophane. 

The presence of absorption bands at particular wavelengths can be related to the presence of particular interatomic bonds. Thus infrared absorption spectrophotometry provides evidence of the structure of materials such as allophane that are amorphous to X-ray diffraction. 

IiMURA [1969] has studied, by a variety of methods, allophane extracted from weathered pumice and volcanic ash soil, including DTA and TGA, X-ray diffraction, and CEC estimation with barium hydroxide. He describes the average formula of allophane as... 

Sanchez-Calvo, M. C., 1961. Allophane and other colloids in the brown loam clays and their changes in the Western Canary Islands. I. Chemical analysis, DTA (differential thermal analysis), and X-ray diffraction. Anales. Edafol. Agrobiol. (Madrid) 20 189. 

---