The environmental aspects of clay minerals include relations of clay minerals to two environments that may differ radically from each other: the environment of origin (where formed) and the environment of deposition (where found).

Environmental aspects are viewed as parts of geologic history and change. Three components constitute geologic history and change: (1) earth materials that undergo the changes and simultaneously serve as substantive records of the geologic changes, (2) the energies (climate, weathering, sedimentation, etc.) that effect the changes on the materials, and (3) the flow of time through which the energies act on the materials.

Parent materials of clay minerals are argillaceous and nonargillaceous parent rocks, plus ions added minus ions removed during genesis. Energies effecting argillation include activities of the H⁺ (protons), other cations (notably K, Mg, Ca, Na, Fe) and anions (carbonate and silica complexes), oxidation potential (electrons, e^-), and organisms. Geological terms for these agents are fresh rain water, alkaline ocean water and brines, and hydrothermal solutions, but they can be quantified with the equilibrium diagrams and thermodynamic calculations of the geochemist. Time in geology correlates with kinetics of geochemistry.

In the environment of deposition the already formed clay minerals may respond to (1) mechanical energy of sorting, (2) colloidal effects of flocculation or dispersion with consequent modification of surface areas and surface chemistry, ion exchange, dialysis, and the probably important but poorly understood interactions with organic materials, and (3) reaction with the ions of the aqueous medium in which deposition occurs.

Following deposition, the clay minerals respond to the energies of mechanical compaction and mechanical dehydration, further thermal dehydration that is significantly important near the temperature of boiling water, activities of ions of concentrated brines accelerated in reactivity by high temperatures, and decrease in oxidation potential of surroundings.

A cumulative record of events is imprinted on clay minerals insofar as changes are produced in the minerals. A specimen of clay minerals extracted from a core of shale from a 3-mi depth may have an exceedingly complicated genealogy, not completely interpretable. Its interpretation is a function of both the fidelity of the mineral record, and the competence of the interpreter.

End_of_Article - Last_Page 535------------