ABSTRACT

Clay minerals are reactive responses of geologic materials to energies characterizing certain environments during time. They may therefore be indicators of the reacting environments. Conversely, after a clay mineral has formed, it becomes a reactive and characteristic part of the environment--whereupon the environment acts also as an indicator of the clay mineral in it.

A mineral used as an indicator is not only geologic, but also is psychologic in import because it must be recognized, and likewise is conceptual because its use includes interpretation.

Environments are described in geologic jargon from a geologic viewpoint, and alternatively in geochemical parameters which permit higher precision in description in relation to stabilities of specific minerals. It is suggested that greatest utility of indicator minerals will be achieved if they are defined first by geochemical parameters which are then translated into geologic environments. Environments may be micro-, macro-, and mega- in scale; they may radically differ among themselves in geochemical characteristics although one may constitute a scale part of another.

Clay minerals found here may not have formed here. Therefore, maximum caution must be used when environmental interpretations are drawn from clay mineral occurrences. Primary-stage clay minerals are those formed as clay minerals for the first time; they are formed here. Other clay minerals that are altered or deposited in an environment different from their primary genesis are termed N + 1 stage clay minerals.

Examples are cited in the paper of primary-stage clay minerals deposited from solution, by replacement, by crystallization from a colloidal gel, and alteration of non-layer silicates. The geochemical environments in which these geneses occurred are taken to be described or defined by stability-diagram models such as those which have been prepared for systems of H₂O-SiO₂-Al₂O₃ and Na₂O or K₂O. It is suggested that the solubility of Al and its complexes is less well understood than of the other components of clay minerals, and probably is a more important variable in clay mineral genesis than has been documented.

In clay-mineral forming systems, kaolin tends to be produced when [H₊] is high relative to Metal ions, and

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Al concentration is high. The 3-layer clay minerals tend to be formed where concentrations of Ca²⁺, Mg²⁺ Fe²⁺, and SiO₂ are high in relation to others. Illite requires [K⁺] high relative to other Metal ions.

N + 1 stage clay minerals transformed during deposition in marine environments include chlorite. and illite from antecedent "open illite." Maximum diagenetic illitization and probably chloritization occur, however, upon deep burial and/or contact of clay minerals with relatively high concentrations of K⁺ and Mg²⁺.

Designation of an indicator mineral of marine environment must be accompanied or preceded by a clear cut, geochemical definition of what is meant by "marineness" in a marine environment, a definition that is apparently still lacking. A survey of clay mineral distribution shows that any major-group clay mineral may occur (be found though not necessarily formed) in any major sedimentational environment.

Hydrothermal clay minerals are best recognized from their geologic occurrences.

Metamorphic and igneous clay minerals, i.e.. clay mineral formed in metamorphic and igneous environments, have been reported as saponite in limestones (Dalradian) in sillimanite grade zones, and smectite in andesite.