The ‘simmering’ of an igneous rock in its own ‘juices’. After the last drop of silicate melt has crystallized and the rock is, in effect, solid, residual water-rich vapour, concentrated by the crystallization of a large proportion of non-water-bearing minerals, can permeate along crystal boundaries and through crystal fractures and react with the primary minerals. Reaction of the low-temperature vapour with high-temperature magmatic minerals (see magma) is termed ‘deuteric alteration’.