When clays are heated, a mass loss occurs due to the evolution of volatiles. Most of these are water vapour, but in addition
minor amounts of a complex assemblage of other species are liberated. The corresponding condensates are colloidal suspensions.
The composition of the volatiles and condensates and the release patterns of the gases are reviewed. The nature of the assemblages
varies from one clay to another and depends on the thermal regime.
Both volatiles and condensates are very reactive. Their reactivity persists even after prolonged storage. They act as acid
catalysts in reactions with organic matter and decompose calcite and other carbonates. Condensates were found to etch the
surfaces of quartz crystals and to dealuminate and partly destroy an Al-rich faujasite (zeolite).
Possible implications of reactions of clay volatiles and condensates for natural processes are discussed.