  Despite its importance in geological sciences, our understanding of
interactions between gas and condensed phases (comprising solids and liquids)
remains clouded by the fact that, often, only indirect evidence remains for
their occurrence. This arises from the tendency for the vapour phase to escape
from the condensed phase with which it interacts, owing to its much lower
density and thus greater volume. For a gas that is sufficiently tenuous that
interactions do not occur between its constituent molecules, this relationship
is quantified in the ideal gas law (Clapeyron 1834): $PV=nRT$ (1) where $P$ is
the total pressure exerted by the gas, $V$ its volume, $n$ is the number of
moles, $R$ the gas constant 8.3145 $Jmol^{-1}K^{-1}$, Horstmann, 1873) and $T$
the absolute temperature. One mole of an ideal gas at 273.15 $K$ and $10^5$
$Pa$ (standard temperature and pressure for gases) has a molar volume of 22,711
$cm^3/mol$, $10^3$ x greater than typical silicate liquids or minerals. As a
result, vaporisation processes in nature are often informed by chemical and
textural evidence remaining in the condensed phase.
