High-temperature minerals are unstable at the temperatures of the biosphere, and they break down readily to form clays. The precise reactions depend on factors such as temperature and moisture, the nature of exudates from plants that can react chemically with the minerals, and the chemistry of the parent material. Different species of clay minerals may be formed under different conditions of environmental weathering. Figure 8.12 Diagram of a clay crystal. In the soil, clay surfaces are covered with a negative charge, and positively charged ions are adsorbed all over the surface.
Clay minerals are remarkable substances.
A colloid is a finely divided material that is dispersed through another medium. It shares many properties with a solution. For example, colloidal particles introduced into water tend not to settle out, and they can pass through very fine filters. In a soil, the colloidal clay particles can be carried by water percolating through the soil. Much of the leaching in topsoil involves the movement of clay colloids.