XCO 2 where X is equal to the mole fraction At high temperature and pressure gases begin to behave non-ideally. Composition of Magmatic Gasses Difficult to determine since gasses in magma chamber can not be sampled directly and gases at a vent are significantly contaminated by the atmosphere. Solubility of Water in Silicate Liquids Water saturated - Liquid has dissolved all the water it can hold at a given temperature and pressure and must coexist with a vapor phase. The solution of water in a magma drastically lowers both the solidus and liquidus. Result is depolymerization breaking the O-O bonds of shared tetrahedral groups.
Net result is a drastic drop in viscosity.
Figure shows the effect of increasing PH 2 O on crystallization temperature of albite and anorthite. The figure below shows the effect of increasing pressure on both an anhydrous and hydrous system. Note that in an anhydrous system the P-T slope is positive which means that increasing pressure increases the melting temperature. The following figure illustrates crystallization paths for liquids of two differing water contents. Both have a P total of 4 kb while liquid A has a P H2O of 3 kb and liquid B is anhydrous.