CHARACTERISTICS OF THE MINERAL CONSTITUENTS OF SOME ALBIC AND SPODIC HORIZONS AS RELATED TO THEIR CHARGE PROPERTIES

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Abstract
Some physical, chemical and mineralogical properties of five albic and six spodic horizons were studied in an attempt to relate their composition to their charge properties. The samples investigated contained less than 3% clay and had specific surface areas less than 30 m2/g. pH values measured in M NaF were lower than 9.5 for the A2 horizons but higher than 10.5 for all the Bir horizons studied. Mineralogy of the clay fraction was studied by using X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis along with selective chemical dissolution analyses. Chloritized vermiculite was the dominant phyllosilicate in the Bir samples, while montmorillonite dominated in the clay fraction of most of the A2 samples. Amorphous constituents, as determined by acid ammonium oxalate and 0.5 N NaOH extractions, represented up to 40% of the clay-sized fraction of some of the samples. Primary minerals such as quartz, feldspars and to a lesser extent hornblende and magnetite were the major constituents of the coarser fractions. The change in chemical and mineralogical compositions of the A2 and Bir horizons sharply reflected the variation that existed in their overall electrochemical behavior as shown by the potentiometric titration curves. A2 horizons usually behaved like a constant surface charge system, while the Bir samples reacted like a constant surface potential system with PZC values varying between pH 4.1 and 4.9.