volatile

volatile
1. Applied to a substance with a high vapour pressure, which passes readily into a gaseous phase.

2. (a) A dissolved element in a silicate magma which would be gaseous at that temperature except for the confining pressure and solvent nature of the magma (e.g. Cl, F, and S) and that therefore becomes gaseous when the magma reaches the Earth's surface or a zone of reduced pressure. Common volatiles include water vapour, carbon dioxide, sulphur dioxide, hydrochloric acid, and there are many more. The melting temperatures of late, volatile, saturated melts may be about 600°C. They form pegmatites which have large crystals and contain some metals, e.g. lithium, molybdenum, uranium, and tin. (b) In *coal, a mixture of combustible gases (hydrogen, carbon monoxide, and methane) with other substances, which is given off when coal is heated without air being present. Peat contains more than 50% volatiles, lignites about 45%, anthracite 10%, and graphite less than 5%.