Some Physico-Chemical Properties of Amorphous Mineral Colloids

Abstract

The amorphous mineral colloids were separated by the particle size method from four Hawaiian soils and a Japanese soil known to have high contents of these colloids. These five soils represent weathering conditions ranging from moderately to strongly weathered, with varying silica-alumina ratios. The physico-chemical properties of amorphous mineral colloids were studied by several methods: X-ray diffraction analysis, differential thermal analysis, elemental analysis, electron microscopy, infrared absorption spectroscopy, surface area determination, and cation exchange capacity determination. The fine clay fraction (less than O.1 μ) was x-ray amorphous with various amounts of weakly formed 2: 1 minerals. The coarse clay fraction consisted mainly of the crystalline minerals mica, kaolin, and gibbsite. From OTA patterns it was found that the first endothermic reaction was closely related to the degree of crystallinity estimated from the x-ray data. The silica-alumina molar ratios were within the range of 0. 927 to 1. 940, and the loss on ignition was found to be inversely proportional to the silica-alumina ratio. The micromorphology of the fine fraction as examined under the electron microscope showed fibrous and sponge-like features. The fibrous material decreases with an increasing degree of crystallinity. The morphology of the coarse fraction, however, changes as the degree of crystallinity increased, from irregularly-shaped flakes to pseudohexagonal flakes, accompanied by an increase in the size of the flakes and a decrease in the amounts of fibrous and sponge-like material. The main infrared absorption band at 10 μ shifted from 9.75 to 10.3 μ as the Al203/ (Al203+Si02) weight ratio increased from 50% to 65% reflecting the change in the coordination state. The surface area determinations were inconsistent with observations made of clay specimens with -the aid of the electron micrograph. The CEC increased with increasing pH, increasing more rapidly when alkaline. The A. CEC increased curvilinearly with the Al203/ (Al203+Si02) weight ratio from 45% to 65%. This may be attributed to the strong acid behavior of AlIV.