The analytical fraction termed H2O+, or LOI, of clay minerals and shales contains, in addition to OH, numerous chemical elements and compounds, such as F, Cl, B, Br, NO2, NO3, SO3, SO4, NH4, and C. The pH of liquids condensed from such volatiles ranges from approximately 1 to 10.

The sources of these ions and compounds, driven off at temperatures above the critical temperature of water, may include inheritances from the parent rock and from processes of argillation, diagenesis, and also re-exposure of the clay to surface agents that include the phytosphere and its products. Clay minerals, because of their complex structures, ion exchange on interlayer and surface sites, and varied bonding properties are prime candidates (as zeolites also should be) for holding accessories to OH as important parts of their chemical compositions.

Alternatively, it is possible that some of the volatile elements in the specimen may be non-clay mineral impurities in minor quantity; if they were significantly abundant the specimen would be termed a “clay rock.” These diverse volatile ions and compounds potentially may be used as fingerprints to help interpret the geologic history of the clay minerals and the mud rocks in which they are dominant.

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