Describe the sequence in which minerals crystallise in a cooling magma using Bowen's Reaction Series.
As a melt cools in a magma chamber is begins to crystallise and becomes a composite mass of solids, the crystals, and liquid, the residual melt. Bowen's Reaction Series gives the order of which the most common rock forming silicate minerals crystallise in a cooling magma. The series applies more so to viscous magmas, where the early forming crystals cannot settle within the magma chamber and change the composition in the chamber. The series has two branches, the discontinuous branch and continuous branch. The discontinuous branch describes the order the mafic minerals will form, starting with olivine as the temperature drops below 1400oC. As the temperature continues to drop pyroxene crystals will form if there is enough silica (SiO2) left in the residual melt. The already existing olivine crystals become unstable at lower temperatures and react with the SiO2 and are converted into stable pyroxene crystals. As the magma cools this process happens again as long as theres enough SiO2 the left and the pyroxene crystals and converted to amphibole, this happens yet again and the amphibole crystals are converted biotite mica.The continuous branch describes the evolution of plagioclase feldspar. The plagioclase will start to crystallise at the same temperature and alongside pyroxene. Initially anorthite will crystallise, which is the calcium (Ca) rich end-member of the plagioclase series. As the temperature drops, the plagioclase will gradually become more sodium (Na) rich, as long as there's enough Na left in the residual magma. At 800oC is continuous and discontinuous branches merge if the original magma composition was rich in SiO2. The merge branch will have the felsic minerals of orthoclase feldspar, muscovite mica and quartz crystallise in that order as the magma cools. The micas within the series may not always be present as hydrogen and aluminium need to be present and abundant in the original magma composition to form the OH complexes within the crystals.
