Exhalite

Exhalite is a term used in geology to describe mineral-rich crusts or veneers that form by the exhalation of fluids or gases from subsurface sources. The concept emphasizes deposition that occurs as fluids migrate toward the near-surface and precipitate minerals when conditions change, rather than precipitation from open seawater or surface evaporation alone.

Formation and textures

Exhalites develop when mineral-bearing fluids move through rock along faults or fractures and reach zones where pressure, temperature, or chemistry induce precipitation. The resulting deposits can coat rock surfaces, fill small cavities, or produce laminated crusts. Textures are diverse, including crustose sheets, botryoidal rims, and thin laminae, with mineral assemblages reflecting the chemical composition of the exhaled fluids.

Common mineralogy

Because exhalation fluids vary widely, exhalites display a broad spectrum of minerals. Expected constituents may include carbonates (such as calcite and dolomite), sulfates (gypsum and anhydrite), silica minerals, clays, and, in trace amounts, sulfides. The exact assemblage records fluid chemistry, temperature, pH, and redox conditions at the time of precipitation.

Settings and significance

Exhalites are described in settings where hydrothermal, fumarolic, serpentinization-related, or methane-rich fluids ascend toward the surface, including submarine vent zones, volcanic or hydrothermal fields, and certain cave environments. They are useful indicators of past fluid flow and crustal processes and can sometimes host economically important ore minerals, though they are not primarily evaporite-related deposits.

See also

Hydrothermal mineral deposits, evaporites, serpentinization, methane seeps.