Estimation of the Total Amorphous Content of Hawai'i Soils by the Rietveld Method

Abstract

The crystalline and amorphous phases of the clay fraction of 10 agriculturally important Hawaiian surface soils were analyzed by x‐ray diffraction (XRD) and the Rietveld method. The clay fraction of these soils was also analyzed by x‐ray fluorescence (XRF). A balance sheet method was used to assign the oxide content of the clays to the crystalline phases. The remaining oxides after the crystalline phases were accounted for served as an estimate of the chemical composition of the amorphous fraction. Amorphous phase mass fractions ranged from 0.28 to 0.77. Loss of weight on ignition (LOI) was found to be an important constituent of the amorphous fraction. Acid ammonium oxalate extractions were compared with the results of the Rietveld method. Because of the differences between the results of the two methods, it was apparent that they were determining different fractions of the amorphous material. The Rietveld method determines the total amorphous content of a sample, whereas oxalate extraction determines what we refer to as the chemically active portion of the amorphous fraction. The influence of the amorphous fraction on a soil's chemical and physical properties has remained speculative because the determination of the actual percentage of the total noncrystalline component was not possible by past analytical procedures. Furthermore, because the amorphous component is not considered in the Rietveld computation of the mass fraction breakout of the crystalline phases, estimates of the actual crystalline phases' mass fractions cannot be made without a reliable estimate of the total amorphous content.