The Structures of Magmas

No Phase Diagrams!

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!!

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).

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

Atomic Bonding: Covalent

Electronic structure of an atom

XY

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!)

Atomic Bonding: Covalent

Unhappy atoms (without filled orbitals)

Too few!

Too many!

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!

Silicates

• Si4+ requires 4 electrons, O2- has two additional.

• Si forms covalent bonds with 4 oxygens leaving one additional electron on each oxygen.

Silicate Minerals

O-

O0

Si2O76-

Silicate Magmas

Disordered, rearranging flexible network.

Polymerisation

NBO = non-bridging oxygens, T = tetrahedra

Building larger molecules by the repeated addition of smaller molecular components

Polymerisation

Polymerisation

SiO44- + SiO4

4- = Si2O76- +

O2-

Depolymerisation

Physical Properties

Viscosity of a liquid is the resistance to shear

Viscosity increases with polymerisation

Density

Increases with degree of polymerisation

Temperature

Degree of polymerisation decreases with T

Effects of Composition

• Metal cations charge balance the silicate network

• The total electronic charge must be zero!

Effects of Composition

Adding metal cations causes depolymerisation.

Effects of Composition: Alkali Metals

Adding alkali metal cations causes depolymerisation.

Effects of Composition: Aluminium

• Certain cations can substitute for Si in tetrahedra (e.g. Al)

• These act as network modifiers and/or network formers.

Effects of Composition: Water

• Adding water causes depolymerisation

• Water decreases the viscosity of a silicate melt.

Effects of Composition: CO2

• Adding CO2 causes polymerisation

• CO2 increases the viscosity of a silicate melt.

Effects of Composition: Volatiles

• The solubility of H2O is highest in acidic magmas (i.e. silica-rich) since these are highly polymerised and have many bridging oxygens.

• The solubility of CO2 is highest in basic magmas since the have low degrees of polymerisation and many bridging oxygens.

Effects of Composition: Oxygen

• Free Oxygen (O2-) is highly chemically reactive.

• It’s abundance is describe by the oxygen fugacity (i.e. the activity of oxygen).

Effects of Composition: Oxygen

SiO44- + SiO4

4- = Si2O76- +

O2-

Polymerisation

Rewrite in terms of the different types of oxygen

Effects of Composition: Oxygen

SiO44- + SiO4

4- = Si2O76- +

O2-

Polymerisation

Rewrite in terms of the different types of oxygen

SiO44- has 4 non-bridging oxygens O-

Si2O76- has 6 non-bridging

oxygens O- and one bridging oxygen O0

Effects of Composition: Oxygen

8O- = 6O- + O0 + O2-

Polymerisation

Rewrite in terms of the different types of oxygen

SiO44- + SiO4

4- = Si2O76- +

O2-

Effects of Composition: Oxygen

8O- = 6O- + O0 + O2-

Polymerisation

Rewrite in terms of the different types of oxygen

SiO44- + SiO4

4- = Si2O76- +

O2-

K = Products

Reactants

A reaction has an equilibrium constant K

Effects of Composition: Oxygen

K = [O-]6[O0][O2-]

[O-]8

K = [O0][O2-]

[O-]2

Effects of Composition: Oxygen

K = [O-]6[O0][O2-]

[O-]8

K = [O0][O2-]

[O-]2

Increasing the oxygen fugacity causes depolymerisation