crystallisation of magma

• Crystallisation of magma is an exothermic process, it releases heat. 
• Formation of melt is an endothermic process - it takes in heat. 
• The first crystals to form are small and called nuclei. If the nuclei is larger that the critical size atoms will attach more easily than detach and a crystal grows. 
• Different minerals crystalise or melt at different temperatures. For a certain mineral when this temperature is reached crystallisation should be spontaneous. Undercooling may occur before crystallisation begins as an extra input of energy is needed to form crystals. 
• The growth rate line indicates how big the crystals are likely to grow at various temperatures, the higher the line the larger the number of crystals. 
• When undercooling reaches a certain temperature, the critical temperature, the number of nuclei increases rapidly. 
• It is possible to imitate natural processes by remelting existing rocks or by making synthetic ones. This means the cooling history of magma can be predicted. 
• Create a powder of a mixture of the two minerals and heat to various temperatures:

  • Heat to 1500 c 
  • Rapidly cool the melt - produces crystals surrounded by glass 
  • Glass corresponds to the liquid phase and the crystals in the solid phase 
  • Repeat for a range of temperatures and a range of mixtures in different proportions. 
• Solidus - temperature below which the substance is entirely solid 
• Liquidus - temperature above which the substance is entirely liquid 
• Olivines:

  • Most rock forming minerals have variable compositions due to substitution of ions of a similar size but of another element 
  • As the composition varies between the two extremes called end-members. The composition of a particular mineral can be represented in terms of the percentage of its end-members 
  • Forsterite - magnesium rich 
  • Fayalite - iron rich 
• Interchanging of ions:

  • Interchangeability of ions is called isomorphic substitution 
  • No change in the crystal lattice structure through the series 
  • Series is known as a solid solution series 
• Characteristics of olivine:

  • Both types have comparable dimensions 
  • Iron and magnesium are easily interchangeable because they have the same size and charge 
• Valency - combining power of an element

  • Linked to the number of electrons in the outer shell 
  • Magnesium and iron ions have a valency of 2 
• Olivine:

  • Slow cooling - first formed crystals stable only at temperature they formed at. As melt cools the crystals react with melt to reach composition at which they are stable again 
  • Eventually melt is completely crystallised, these are the crystals we see - there is no evidence of previous history 
• Olivine:

  • Fast cooling - earlier crystals don’t have time to fully react back with melt so aren't all reabsorbed 
• Result of crystallisation in olivines:

  • As a result, fresh mineral growth may occur around the margins of such crystals and they appear to be zoned 
  • The composition of these crystals changes from the centre outwards 
  • Centre is rich in forsterite 
  • Edge rich in fayalite
  • Porphyritic - phenocrysts richer in forsterite and groundmass richer in fayalite 
• Olivine:

  • As reaction between crystals and liquid is theoretically continuous 
  • This is a reaction series 
• Plagioclase feldspar:

  • Not one mineral but a solid solution series of minerals 
  • High temperature - anorthite 
  • Low temperatures - albite 
  • Isomorphic substitution occurs 
  • Na and Si replace Ca and Al 
• Plagioclase feldspar:

  • Centre - rich in anorthite 
  • Edge - rich in albite