52
WHAT HAPPENS IF THE TETRAHEDRAL SHEETS AND OCTAHEDRAL
SHEETS DON’T FIT TOGETHER?
The serpentine group minerals with a general formula Mg3Si2O5(OH)4 has three forms
: lizardite, chrysotile and antigorite. In each of these minerals there is a different way
of coping with the tetrahedral-octahedral mismatch.
In lizardite, the sheets are flat, but the tetrahedra have to rotate so that the apices can
connect to the octahedral sheet, just as in kaolinite.
In chrysotile, the tetrahedra are tilted and the layers are curved to accommodate the
longer repeat length wuthin the octahedral layer. Continued growth of the curved
layer results in a crystal in which the layers are rolled up as in a carpet.
In antigorite, the tilting of the tetrahedra which leads to the curved layers is
accompanied by a periodic switching of the direction of the tetrahedral layer.
Another consequence of the misfit between the sheets is that the crystals are usually
rather small, This is the case in clay minerals.
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CLAY MINERALS
Clay minerals are fine-grained
(<0.002mm) sheet silicate minerals
which form as a result of weathering of
other silicates. The main clay minerals
are:
Kaolinite - Al2Si2O5(OH)4
O
T
7Å
O
T
Illite ~ K0.8Al2 (Al0.8Si3.2)(OH)2
i.e. essentially fine-grained muscovite
T
O
T
K+
10 Å
layer charge is not as high as in illite, so
there are fewer cations in the interlayers,
and also H2O can easily move in and out
of the interlayers, causing the structure
to expand and contract. The interlayer
spacings can change from 10Å (when
the clay is dry) to 15.2Å (when there are
two layers on water molecules between
the TOT layers). Smectites are therefore
sometimes referred to as the swelling
clays.
T
O
T
Smectite ~
Ca0.17(Al,Mg,Fe)2(Si,Al)4O10(OH)2 .
nH2O
Vermiculite ~
(Mg,Ca)0.3(Mg,Fe2+,Fe3+,Al)3(Si,Al)4O1
0(OH)2
Another group of swelling clays
minerals is called vermiculite. It usually
formed by the alteration of biotite by the
oxidation of some Fe2= to Fe3+, thus
lowering the negative charge in the
biotite TOT layer. It has a higher layer
charge than smectite.
Chlorite:
(Mg,Fe,Al)3(Si,Al)4O10(OH)2.
(Mg,Fe,Al)3(OH)6
T
O
T
T
O
T
Na+. Ca2+.H 2O
10Å
T
O
T
Smectites form a large group of TOT
layer minerals with both cations and
water molecules between the layers.
They may be either dioctahedral or
trioctahedral and the net charge on the
TOTlayers is the result of various
substitutions in both tetrahedral and
octahedral sheets (e.g. Al3+ for Si4+ in
the tetrahedral sheets, or Mg2+ (or Fe2+)
for Al3+ in the octahedral sheet.). The
O
14Å
T
O
T
In clays chlorite can have a more
complex composition than the larger
chlrite crystals in rocks, and can be
dioctahedral or trioctahedral.
54
FRAMEWORK SILICATES
In the framework silicates all [SiO4] tetrahedra share their corners with others, generally
forming rather open three-dimensional networks.
If no ion substitutes for Si, the entire framework has the composition SiO2 and all valence
bonds are satisfied.
When Al substitutes for Si in the tetrahedra, interstitial cations are required to maintain
charge balance. The openness of these framework structures results in rather large
interstitial cation sites compared to those in the chain silicate minerals. Thus Na+ and
Ca2+ will be considered as 'small' cations compared to K+, while ions such as Mg2+ are
too small to play a role in these structures.
The aluminosilicate framework minerals are by far the most abundant minerals in the
Earth's crust, the feldspars making up about 65% by volume.
In this course we will consider only the two most important groups of framweork
silicates: the silica minerals and the feldspars.
The silica minerals
Silica, SiO2, occurs in a number of different forms in the Earth. Quartz, the most
common crystalline polymorph is stable up to 857oC; tridymite is the stable form from
857oC to 1470oC, and then cristobalite from 1470oC up to the melting point at 1713oC.
The high pressure forms of silica are coesite, stable in the deep crust of the Earth, and
stishovite which is thought to be stable in the Earth's mantle. Stishovite has the rutile
structure and is one of the very few known materials in which Si occurs in octahedral
coordination with oxygen. The stability relationships of the SiO2 polymorphs are shown
below.
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The crystal structures of quartz, tridymite and cristobalite are related by reconstructive
phase transitions, in other words, to convert one to another as the temperature increases
requires breaking Si-O bonds and creating a new structure. This is a slow process,
especially when the transitions take place on cooling, and both cristobalite and tridymite
can be preserved metastably in volcanic rocks.
Crystal structure of quartz.
The crystal structure of quartz can be considered as being made up of 6-fold helices,
spiralling around the c axis. The figure on the left shows one such helix. Looking along
the helix axis, this gives the appearance of a hexagonal ring (right). The interlinking of
such helices gives the full three-dimensional structure (bottom).
1 /3
2 /3
1 /3
0
2 /3
0
The structure shown above is that of high quartz which is stable above 573oC. Below that
temperature the structure undergoes a displacive phase transition to low quartz. This
transition is fast and is unavoidable, no matter how fast the quartz is cooled. No breaking
of bonds is involved, just a twisting of the tetrahadra around the ‘joint’ which is the
shared oxygen atom between tetrahedra.
High quartz is hexagonal. Low quartz is trigonal.
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Twin boun dary
The structure of low quartz
One consequence of the displacive transition is that there are two equivalent ways in
which the structure can distort to trigonal symmetry. If different parts of the crystal
distort in different ways, then the boundary between these regions is related by a simple
symmetry operation and is called a twin boundary. The process of the formation of such
boundaries is called transformation twinning and is a common phenomenon in many
minerals.
The structures of tridymite and cristobalite.
These structures are related to one another, but are quite different from quartz. Both are
made up of the same structural unit which is a layer of tetrahedra, with alternate
tetrahedra pointing up and down.
In both structures these layers are stacked one on the other.
In tridymite they are stacked so that there is a two-layer repeat : ABABAB….. Tridymite
is hexagonal.
In cristobalite the layers are stacked with a three-layer repeat : ABCABC …. Cristobalite
is cubic.
57
When these structures are cooled they should transform cristobalite  tridymite 
quartz. As mentioned above, these are reconstructive transitions, and so very slow and
difficult. If they do not occur, the crustobalite (and tridymite) will cool to lower
temperatures (~ 200oC) and then distort by a displacive transition to low cristobalite and
low tridymite, which have lower symmetry than the high forms.
When tridymite and cristobalite are found in rocks they are always the low forms. These
do not appear on the phase diagram because they are not thermodynamically stable. They
are metastable, just as diamond is relative to graphite at room P,T.
The feldspars
Feldspars make up approximately 70% of the Earth’s crust, yet are structurally the most
complex mineral group because of the many phase transitions which occur on cooling.
In the feldspars some Al3+ substitutes for Si4+ in the framework, and charge balance is
achieved by cations sited in the open spaces of the framework structure.
The most common feldspars are the alkali feldspars, with compositions between
KAlSi3O8 and NaAlSi3O8 , and the plagioclase feldspars with compositions between
NaAlSi3O8 and CaAl2Si2O8.
At high temperatures there is a complete solid solution between the alkali feldspar endmembers. At lower temperatures the solid solution exsolves (or unmixes) into regions that
are Na-rich and regions which are K-rich. The result is that a single crystal of alkali
feldspar contains an intergrowth at lower temperatures. This is called a perthitic texture.
The scale of this perthitic texture depends on the cooling rate of the rock, but can usually
only be seen by using a microscope.
There is also a complete solid solution in the plagioclase feldspars at high temperatures.
This solid solution is more complex because it involves the coupled substitution of Na for
Ca and Al for Si:
Na+ + Si4+  Ca2+ + Al3+
58
At lower temperature this solid solution also breaks down, but the situation is more
complex than alkali feldpars and the intergrowths can only be seen by electron
microscopy.
The structure of the feldspar minerals.
The basic high temperature structure of the feldspars is shown below.
(a)
The basic building unit of the feldspar structure is the four-membered ring of
tetrahedra with a pair of tetrahedra pointing up and a pair pointing down. Al and Si
occupy these tetrahedra. (b) The four-fold rings are joined to form a layer in which the
rings are related by mirror planes parallel to (010) and diads parallel to the b axis. Two
sets of individual tetrahedra are distinguishable in this layer, and are labelled T1 and T2.
The T1 tetrahedra are all related to one another by symmetry, as are the T2 tetrahedra. In
the third dimension these layers are joined to each other by the apices. The K, Na and Ca
cations occupy the large oval-shaped cavities between the rings.
At lower temperatures the feldspar structures undergo:
(i) displacive transitions, (ii) exsolution processes and (iii) Al,Si ordering processes.
These processes are not independent and result in major complexities in the structures,
which are not yet fully understood.
The tendency for Al and Si atoms to become ordered in the tetrahedral sites at lower
temperatures is a very general phenomenon. Understanding such Al,Si ordering
transitions in minerals is a major topic in modern mineralogy because it has important
consequences to the stability (thermodynamics) of minerals and rocks. It will be
discussed in more detail in later Mineralogy courses.
59
CARBONATE MINERALS : CALCITE, DOLOMITE, ARAGONITE
Trigonal and orthorhombic carbonates
The geologically important carbonate minerals may be divided into two structural groups.
The trigonal structure is adopted by carbonates of small cations while the orthorhombic
structure is formed by carbonates of larger cations. The table below lists some members
of each group with the ionic radius (in Å) of the cation.
Trigonal (Calcite group)
Orthorhombic (Aragonite group)
Calcite CaCO3 (0.99)
Magnesite MgCO3 (0.66)
Siderite FeCO3 (0.74)
Rhodocrosite MnCO3 (0.80)
Smithsonite ZnCO3 (0.74)
Dolomite CaMg(CO3)
Ankerite Ca (Mg,Fe)(CO3)
Aragonite CaCO3 (0.99)
Witherite BaCO3 (1.34)
Strontianite SrCO3 (1.13)
Cerussite PbCO3 (1.20)
The most important carbonate minerals are calcite, dolomite and aragonite.
The structure of calcite CaCO3
CO3
Ca
The structure of calcite, CaCO3, can be described in terms of a rhombohedron
(essentially a cube which has been shortened along one of the triad axes). The Ca atoms
have a face-centred distribution on this rhombohedron, and the triangular CO3 groups lie
at the centres of each edge. This results in layers of CO3 groups lying normal to the c axis
of calcite, with layers of Ca atoms lying between them .
Calcite is the main constituent of limestone rocks.
60
The structure of dolomite Ca,Mg(CO3)2
The structure of dolomite is very similar to calcite but layers of Ca cations alternate with
layers of Mg cations.
The structure of aragonite
The aragonite structure is preferred by carbonates when the cation radius is larger than
about 1Å. Because Ca2+ lies on this boundary, it can fit into either structure.
The structure also contains layers of triangular CO3 groups with the Ca atoms in between,
but arrabged in a different way to that in calcite. The layers lie perpendicular to the c axis
of the orthorhombic unit cell.
The calcite-aragonite phase diagram.
From this diagram we can see that aragonite is stable at high pressures and that calcite is
the stable polymorph of CaCO3 at the earths surface. Aragonite does form in high
pressure rocks, but also forms metastably under low pressure near-surface conditions. As
well as forming the shells of some marine invertebrates, aragonite also forms in hotspring and cave deposits, and may also be directly precipitated from warm sea water.
61
When a mineral forms outside its stability field it must mean that the growth occurs under
non-equilibrium conditions, and that there is some kinetic reason for its formation, i.e. it
is easier to form aragonite than calcite under those particular conditions.