1. The Structures of Magmas No Phase Diagrams!

2. The Structures of Magmas Melt structure controls: The physical properties of magmas The chemical behaviour of magmas (e.g. the solubility of elements and gases). The variation in the properties of a magma with pressure and temperature. With just a little knowledge of melt structure you can predict a lot about the way a magma will behave!!

3. Magmas are Liquids! More disorder than solids (i.e. higher entropy) Higher total energies (enthalpies). Higher velocities of atoms/molecules (i.e. their structures are constantly rearranging).

4. Atomic Bonding: Ionic Na + Cl - Ionic Bonding Net electrostatic attraction Bond length set by the balance between attraction and repulsion (ionic radii). They are not directional

5. Atomic Bonding: Covalent Electronic structure of an atom X Y Z S Orbital (up to 2 electrons) P Orbital (up to 6 electrons) Electrons live in orbitals Filled orbitals are happy orbitals! (as are empty!)

6. Atomic Bonding: Covalent Unhappy atoms (without filled orbitals) Too few! Too many!

7. Atomic Bonding: Covalent Unhappy atoms (without filled orbitals) Too few! Too many! Covalent bonds share electrons between atoms They are stronger than ionic bonds They are directional!

8. Silicates Si 4+ requires 4 electrons, O 2- has two additional. Si forms covalent bonds with 4 oxygens leaving one additional electron on each oxygen.

9. Silicate Minerals O-O- O0O0 Si 2 O 7 6-

10. Silicate Magmas Disordered, rearranging flexible network.

11. Polymerisation NBO = non-bridging oxygens, T = tetrahedra Building larger molecules by the repeated addition of smaller molecular components

12. Polymerisation SiO 4 4- + SiO 4 4- = Si 2 O 7 6- + O 2- Depolymerisation

13. Physical Properties Viscosity of a liquid is the resistance to shear Viscosity increases with polymerisation

14. Density Increases with degree of polymerisation

15. Temperature Degree of polymerisation decreases with T

16. Effects of Composition Metal cations charge balance the silicate network The total electronic charge must be zero!

17. Effects of Composition Adding metal cations causes depolymerisation.

18. Effects of Composition: Alkali Metals Adding alkali metal cations causes depolymerisation.

19. Effects of Composition: Aluminium Certain cations can substitute for Si in tetrahedra (e.g. Al) These act as network modifiers and/or network formers.

20. Effects of Composition: Water Adding water causes depolymerisation Water decreases the viscosity of a silicate melt.

21. Effects of Composition: CO 2 Adding CO 2 causes polymerisation CO 2 increases the viscosity of a silicate melt.

22. Effects of Composition: Volatiles The solubility of H 2 O is highest in acidic magmas (i.e. silica-rich) since these are highly polymerised and have many bridging oxygens. The solubility of CO 2 is highest in basic magmas since the have low degrees of polymerisation and many bridging oxygens.

23. Effects of Composition: Oxygen Free Oxygen (O 2- ) is highly chemically reactive. It’s abundance is describe by the oxygen fugacity (i.e. the activity of oxygen).

24. Effects of Composition: Oxygen SiO 4 4- + SiO 4 4- = Si 2 O 7 6- + O 2- Polymerisation Rewrite in terms of the different types of oxygen

25. Effects of Composition: Oxygen SiO 4 4- + SiO 4 4- = Si 2 O 7 6- + O 2- Polymerisation Rewrite in terms of the different types of oxygen SiO 4 4- has 4 non-bridging oxygens O - Si 2 O 7 6- has 6 non-bridging oxygens O - and one bridging oxygen O 0

26. Effects of Composition: Oxygen 8O - = 6O - + O 0 + O 2- Polymerisation Rewrite in terms of the different types of oxygen SiO 4 4- + SiO 4 4- = Si 2 O 7 6- + O 2-

27. Effects of Composition: Oxygen 8O - = 6O - + O 0 + O 2- Polymerisation Rewrite in terms of the different types of oxygen SiO 4 4- + SiO 4 4- = Si 2 O 7 6- + O 2- K = Products Reactants A reaction has an equilibrium constant K

28. Effects of Composition: Oxygen K = [O - ] 6 [O 0 ][O 2- ] [O - ] 8 K = [O 0 ][O 2- ] [O - ] 2

29. Effects of Composition: Oxygen K = [O - ] 6 [O 0 ][O 2- ] [O - ] 8 K = [O 0 ][O 2- ] [O - ] 2 Increasing the oxygen fugacity causes depolymerisation