Igneous Petrology
LECTURE 7
The Structure of Magmas
Understanding the atomic structure of magmas is important since it allows us to predict their physical (rheological)
and chemical behaviour. Most magmas on Earth are silicate melts and these have structures similar to silicate
minerals except they are more disordered and undergo continual rearrangement.
1. Revision of Atomic Bonding
Understanding atomic bonding is important if we are to get to grips with atomic structures. There are two main
types of atomic bond. (1) Ionic bonds in which atoms are held together by their mutual electrostatic attraction. The
negative electrons of one atom attract the positive nucleus of another atom and vice a versa. (2) Covalent bonds in
which atoms share electrons in order to become more stable. Generally covalent bonds are directional and much
stronger than ionic bonds.
2. Silicate Mineral Structures
The structures of silicate minerals are based around the silica
tetrahedra which consists of one Si atom surrounded by 4 oxygens.
The tetrahedra has the shape of a three sided pyramid in which all
the faces are equilateral triangles.
The Si-O bonds in the silica tetrahedra are covalent. These are
strong and directional. The tetrahedra, therefore, behaves as a rigid
unit in silicate structures. Because the silicon atom has a charge of
4+ and each of the 4 oxygens has a charge of 2-, the net electronic charge of the tetrahedra is 4-. This is written as
SiO44-.
The huge variety of silicate minerals arises because the oxygen in the silica tetrahedra are able to form covalent
bonds and link tetrahedra together. The tetrahedra share oxygens to form chains, rings, sheets and 3D networks.
Different atoms in silicate
minerals play different roles
in the structure. Silicon is the
basis of the tetrahedra on
which the network is formed
and is, known as a network
former. Oxygen is present in
two forms in silicate minerals.
Where it links tetrahedra
together it is known as
bridging oxygen. Oxygen
atoms that are not shared
between tetrahedra are
known as non-bridging
oxygens. Electropositive
atoms (positively charged
since they have more positive
protons in their nucleus than
electrons in their electron
shell) hold chains, rings and
sheets together through ionic
bonds. However, they also
influence the shape of the
network structure they are
known as network modifiers.
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Igneous Petrology
LECTURE 7
The electronic charge of oxygen also changes depending on its role in the structure. Free oxygen, which is not
involved in silica tetrahedra, has a charge of 2- (O2-), non-bridging oxygen has a charge of 1- (O-) and bridging
oxygen has a charge of zero (O0). The electronic charge depends on the number of electrons that the oxygen is
sharing in covalent bonds. Non-bridging oxygen shares one of its two additional electrons with one silicon atom.
Bridging oxygen shares both its two extra electrons with two silicon atoms. The net charge of a single isolated
tetrahedra is thus 4- (SiO44-). The net charge of two tetrahedra with a shared (bridging oxygen) is 6- (Si2O76-).
Silicate Melt Structures
The structure of a silicate melt, like that of a silicate mineral,
is formed by a 3D network of linked silica tetrahedra. In
silicate melts, however, the structure is more disordered, is
flexible and can rearrange itself through the random
motions of atoms and molecules. The network in the melt
contains many irregular rings and chains.
Atoms in the structure play the same roles as in silicate
minerals. There are network formers (Si), bridging oxygens
(O0) and non-bridging oxygens (O-) making up the
tetrahedra network, free oxygens (O2-) that are not part of
tetrahedra, and network modifiers that support and change the shape of the silicate network.
2.1. Making the Network: Polymerisation.
To construct the silicate network we must add silica
tetrahedra to the framework, turning non-bridging
oxygens into bridging oxygens. This process of building a
larger molecule by the repeated addition of smaller
molecules is known as polymerisation and the network
itself is a polymer. Most hydrocarbons and complex
organic molecules such as DNA and proteins are also
polymers.
In the figure opposite we can see that increasing the size
of the network by adding tetrahedra leads to molecules
with the formulae SiO44-, Si2O76-, Si3O108-, Si4O1310- - note
that the ratio of oxygen to Si decreases because some
oxygens are shared between the tetrahedra as bridging oxygens. We can use the ratio of the number of nonbridging oxygens (NBOs) to the number of tetrahedra (T) as a measure of the degree of polymerisation. Note that
NBO/T=4 for isolated tetrahedra and it decreases to 0 for a structure where all the oxygens are bridging and shared
between tetrahedra. NBO/T thus decreases with increasing polymerisation.
We can write polymerisation as a reaction:
SiO44- + SiO44-  Si2O76- + O2- (reaction 1)
Notice that polymerisation also produces free oxygen and that, like any reaction at equilibrium it proceeds both
ways at the same rate (i.e. it is balanced). In a liquid this is particularly true since the structure is forever being
constructed and deconstructed even though the total degree of polymerisation stays the same. The processes by
which the framework decreases in size is known as depolymerisation.
3.2. The Physical Properties of Magma and Polymerisation
A more polymerised magma (i.e. lower NBO/T) will have a higher viscosity than a less polymerised magma. This is
because the silicate network, although flexible, consists of strong directional covalent bonds that resist deformation.
Viscosity is simply a measure of the resistance of a liquid to flow, and flow requires the structure to deform and
move. The more polymerised the magma the less easily it can deform and the higher its viscosity.
Density, to a lesser extent, is also dependent on the degree of polymerisation. Magmas with large polymers tend to
be less dense since their silicate network contains open rings. Magmas with a low degree of polymerisation are
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Igneous Petrology
LECTURE 7
more compact and thus denser. The abundance of metal cations is also important in controlling the density of a
magma.
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LECTURE 7
2.2. The Effects of Temperature and Pressure
Increasing temperature results in depolymerisation of the silicate network of a magma because the increased
kinetic energy of atoms makes it easier for bonds to be broken. Since the magma becomes more depolymerised it,
therefore, becomes less viscous (density usually still decreases, however, due to expansion of the melt). Decreasing
temperature causes an increase in polymerisation for the opposite reason and increases the viscosity.
Increasing pressure results in polymerisation of the silicate network of a magma because the structure is
compressed and new bonds between tetrahedra form. Increasing pressure causes increases in viscosity and density
of the magma.
2.3. The Effects of Composition on Polymerisation
By considering the degree of polymerisation we can predict changes in the physical (and chemical) properties of
the magma with changes in composition.
3.3.a Silica Content
Increases in the SiO2 content of a magma produces large increases in the degree of polymerisation since there
are more SiO44- tetrahedra available to construct the network. Silica-rich (acid) magmas are, therefore, much more
viscous and have a lower density than silica-poor (basic) magmas. The addition of any network former will cause an
increase in polymerisation.
3.3.b. Metal abundances
The abundance of metal atoms other than Si changes the degree of polymerisation due to charge balancing. The
diagram below shows that we need to balance the negative net electronic charge of the silicate network with
positively charged metal atoms (cations). The number
of cations per tetrahedra in the network needed to
balance the excess negative charge is shown in the
diagram as M/T for divalent (2+) cations. Note that the
larger the network the fewer cations are needed to
balance the charge. Network modifiers, therefore, not
only help support the network (i.e. keeping rings open)
they also balance the negative electronic charge of the
network. The total electronic charge MUST be zero.
If we added more divalent cations to a magma you can
see that it would have a positive total electronic charge.
This is not allowed and depolymerisation would occur,
making the network more electronegative to charge
balance the cations.
Magmas richer in metals such as Fe2+ and Mg2+ relative to SiO2 are, therefore, less polymerised and have lower
viscosities. Note that monovalent cations (charge 1+), such as Na and
K, have a similar effect, however, more of them are needed to charge
balance the network (as shown opposite). These cations have much
larger ionic radii than divalent cations such as Fe and Mg and tend to
make the network more open (i.e. they support larger rings in the
structure). Because the structure is more open it is cannot be as
highly polymerised. Magmas rich in alkali metals (such as Na and K),
therefore, tend to be less polymerised and less viscous.
3.3.c. Aluminium and other network formers
Any atom that can substitute for silicon in the silica
tetrahedra can act as a network former. Aluminum is
the most important in natural magmas and
complicates the structure since it has a lower
electronic charge than Si of 3+. If aluminium is
present as a network former (i.e. replacing Si) then
the silicate network will have a larger overall
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Igneous Petrology
LECTURE 7
negative charge (i.e. it will be more electronegative). More charge balancing cations (network modifiers) will be
needed to charge balance the aluminium-containing network. These can be trivalent (3+), divalent (2+) or
monovalent (1+) cations. Even Al3+ can be present as a network modifier (i.e. outside the tetrahedra) to perform
charge balancing. If there aren’t enough charge balancing cations the network will depolymerise. If there are then
Al will cause polymerisation (because it helps build the network).
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Igneous Petrology
LECTURE 7
3.3.b. Volatiles
Volatiles are gaseous substances such as CO2 and H2O. They are very important in the behaviour of magmas during
eruption, they change the melting point of magmas and they change their physical properties. Water and CO 2,
however, behave differently when incorporated into magmas.
Water
The diagram opposite shows the
reaction mechanism by which water
dissolves in a silicate magma. The
hydrogen of the H2O molecule reacts
with a bridging oxygen of the silicate
network to form a hydroxyl (OH)
molecule which is hydrogen bonded
to a bridging oxygen. The reaction of
the H2O molecule causes
depolymerisation. Adding water to a
silicate melt will, therefore, decrease
its viscosity. The viscosity of
polymerised melts, such as acidic
melts, decreases more significantly
with water content than less polymerised (basic) melts.
As a reaction this can be written as:
Si2O76- + H2O = 2SiO44- + 2HThe degree of polymerisation also controls the amount of water that can be dissolved in a magma. Polymerised
magmas (acidic) can dissolve large amounts of water since they have more bridging oxygens. The solubility of water
in magmas, therefore, decreases with SiO2 content due to the decrease in polymerisation (i.e. acid magmas can
contain more dissolved water than basic magmas).
With decreasing pressure the solubility of water in a magma decreases because of the decrease in polymerisation.
The explosive volcanism of acid magmas is a result of this behaviour since as they rise through a conduit the
pressure decreases until the water can no longer be dissolved in the magma and exsolves as water vapour. The
large expansion of the vapour causes the explosive nature of the eruption.
Carbon Dioxide
Carbon dioxide (CO2) dissolves in a magma by
reacting with a non-bridging oxygen of the
silicate network to form a carbonate molecule
(CO32-). The removal of the non-bridging oxygen
causes polymerisation of the network as shown
and thus increases in viscosity. Because CO2
solubility is dependent on non-bridging oxygens
it is higher in depolymerised magmas. The CO2
content of basic magmas is, therefore, higher
than acidic magmas.
As a reaction this can be written as:
Si2O76- + CO32- = 2SiO44- + CO2
A complication is in the presence of Al3+ as a network former (i.e. in the tetrahedra) since the carbonate ion formed
by the reaction of CO2 forms a complex with a divalent cation which can no longer balance the charge of the Al in
the network. In the case of Al-rich magmas CO2 can cause depolymerisation.
3.3.c. Oxygen
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Igneous Petrology
LECTURE 7
Oxygen is the most abundant element in igneous rocks and has important effects on the structure of silicate
magmas and the minerals that crystallise from them. Free oxygen is a powerful chemical agent (because of its
bonding capabilities) and its abundance is described by the oxygen fugacity.The higher the abundance of free
oxygen the higher the oxygen fugacity.
To see the effects of oxygen fugacity on the structure of a silicate melt we can write reaction 1 just in terms of
bridging (O0), non-bridging (O-) and free (O2-) oxygen.
4O- + 4O-  O0 + 6O- + O2- (reaction 2)
for any reaction at equilibrium its progress is constant (because its balanced) and can be described by an
equilibrium constant k where [O0] is the abundance of bridging oxygen. k is constant at constant pressure and
temperature.
k
[O 0 ].[O  ]6 .[O 2 ] [O 0 ].[O 2 ]
(equation 1)

[O  ]4 .[O  ]4
[O  ]2
Because k is a constant if we increase the oxygen fugacity (i.e. increase [O 2-]) we must decrease the number of
bridging oxygens relative to non-bridging oxygens (i.e. [O0]/[O-]). Increasing the oxygen fugacity (the abundance of
free oxygen), therefore, causes depolymerisation.
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