NETTVERK SILIKATER More than three quarters of the Earth’s crust is composed of framework silicates. By far the most common are quartz and feldspars. The structure of all framework silicates is based on a network of TO 4 tetrahedra, in which T is Si 4+ or Al 3+ , and all four O atoms are shared with other tetrahedra. Si S Si, Si Si O O O O O O O OO O O O O OO O
NETTVERK SILIKATER. More than three quarters of the Earth’s crust is composed of framework silicates. By far the most common are quartz and feldspars. - PowerPoint PPT Presentation
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NETTVERK SILIKATER
More than three quarters of the Earth’s crust is composed of framework silicates. By far the most common are quartz and feldspars.
The structure of all framework silicates is based on a network of TO4 tetrahedra, in which T is Si4+ or Al3+, and all four O atoms are shared with other tetrahedra.
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SiSi,AlSi,AlSi,AlSi,Al
OOOOOOOO OOOOOOOO
The fact that all O2- ions are shared, together with the repulsion of the highly charged cations, means that the structure of the framework silicates is more open than the other silicates.
This has two consequences:
(i) large cations can fit in the open structure of the framework silicates e.g. Ca2+, Na+, K+.
(ii) lower density than the other silicates e.g. quartz has density 2.65, olivine has density 3.3, even though Mg has a lower atomic mass than Si.
Low density means stable at relatively low pressures i.e. crustal rocks
The silica group minerals, SiO2
By far the most common silica mineral is quartz.
It is the only thermodynamically stable phase of silica at room T,P
Quartz
More quartz
0 400 800 1200 1600
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Temperature oC
Pre
ssur
e (k
bar)
stishovite
coesite
low () quartz
high () quartz
melt
cristobalitetridymite
Stability fields of the silica polymorphs
Low quartz is the only
thermodynamically stable phase of
silica at room T,P
The structure of high () quartz
The structure can be built up from 6-fold spirals of tetrahedra.
The spiral axis is the c axis
c
The same spiral looking along the c axis
c
The structure of high () quartz - hexagonal
Part of the high quartz structure - the 6-fold and 3-fold spirals
Part of the high quartz structure - the 6-fold and 3-fold spirals
View perpendicular to the c axis
c
Quartz structure
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The low () quartz high () quartz phase transition
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are needed to see this picture.
High () quartz - hexagonal and Low () quartz -trigonal
573oC
The phase transition from high to low quartz is displacive.
No bonds broken. Only a distortion of the structure.
The symmetry change is from hexagonal to trigonal
Screw Triad axes
Transformation twinning in quartz
There are two equally likely possibilities for distorting the hexagonal high quartz structure to the trigonal low quartz structure.
When both orientations of the trigonal structure exist in the same crystal, the crystal is twinned. The process of forming a twinned crystal in this way is called transformation twinning.
Transformation twinning in quartz - the twin plane
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Twin boundaryTwin boundary (plane)
In quartz, this type of twinning is called Dauphiné twinning
Twinned crystals can be also be formed during crystal growth – growth twinning.
In a twinned crystal there always must be a definite crystallographic relationship between the two different orientations e.g. they may be related by a mirror plane or a rotation.
Quartz growth twin - “Japan” twinning
The structures of tridymite and cristobalite
Both share the the same structural unit - a layer of tetrahedra, with alternate tetrahedra pointing up and down
In tridymite these layers are stacked one on top of the other, so that there is a two-layer repeat ….ABABAB … giving a hexagonal structure.
A
B
A
In cristobalite these layers are stacked one on top of the other, so that there is a three-layer repeat ….ABCABC … giving a cubic structure.
A
B
C
A
The cubic structure of cristobalite
The cubic structure of cristobalite
The transformations from cristobalite - tridymite - quartz on cooling
• These transformations are reconstructive and involve breaking strong Si-O bonds
• Unless cooling is very slow, these transformations will not take place
• When cristobalite cools down to about 200oC it undergoes a displacive transformation from cubic high cristobalite to tetragonal low cristobalite (i.e. a distortion of the structure which lowers the symmetry
• The same is also the case for tridymite. If it fails to transform to quartz, then at around 200oC there is a high - low tridymite transition ( a distortion from hexagonal to orthorhombic symmetry)
The distortion of the silicate tetrahedral layer in the high - low transformations in cristobalite and tridymite
High form Low form
Cristobalite and tridymite may be found in volcanic igneous rocks which have cooled too quickly for the transformations to quartz to take place.
At room temperature cristobalite and tridymite always exist as low cristobalite and low tridymite because the displacive transformations take place even with very fast cooling.
Cristobalite always shows very fine cracks because the high - low transformation involves a volume decrease of ~3%
Cristobalite with fine cracks
Glass
This is an example of cristobalite in a silica ceramic brick - optical micrograph
0 400 800 1200 1600
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40
60
80
100
Temperature oC
Pre
ssur
e (k
bar)
stishovite
coesite
low () quartz
high () quartz
melt
cristobalitetridymite
Stability fields of the silica polymorphs
Low cristobalite and low tridymite do not appear on the equilibrium phase diagram – they are metastable
Other natural low temperature forms of SiO2
1. Agate
Agate is made from very fine fibrous crystals of quartz. Agate grows from Si-rich solutions in the shallow Earth’s crust.
Chalcedony is the fibrous form of quartz
Other natural low temperature forms of SiO2
2. Opal
Opal is an amorphous form of silica formed from supersaturated Si-rich solutions.
Where do the colours in opal come from?
Electron micrographs showing small spheres of amorphous SiO2, which scatter the light to produce the colours.
Diatoms also make shells from amorphous silica
Diatoms are uni-cellular algae and are extremely abundant in both marine and freshwater.
When they die the shells form SiO2 deposits on the ocean floor.
When buried by sediment, this SiO2 eventually forms a rock called chert (kieselschiefer), which is made of very finely crystalline quartz.
It is characteristic of ocean floor sedimentary rock.
Mention: the transformation sequence from amorphous silica to chert goes via cristobalite and tridymite !!
100
0 400 800 1200 1600
20
40
60
80
Temperature oC
Pre
ssur
e (k
bar)
stishovite
coesite
low () quartzhigh () quartz
melt
Coesite - stable in the earth’s upper mantle
Coesite (partly converted back to quartz) preserved inside a crystal of garnet
This rock found in the Northern Italian Alps was once 70Km deep in the Earth, where coesite is stable
100
0 400 800 1200 1600
20
40
60
80
Temperature oC
Pre
ssur
e (k
bar)
stishovite
coesite
low () quartzhigh () quartz
melt
Stishovite - stable in the earth’s lower mantle
Si in octahedral co-ordination !!
Stishovite has been found in rocks in meteorite impact craters
FRAMEWORK SILICATES II - Feldspars
More than three quarters of the Earth’s crust is composed of framework silicates. By far the most common are quartz and feldspars.
The structure of all framework silicates is based on a network of TO4 tetrahedra, in which T is Si4+ or Al3+, and all four O atoms are shared with other tetrahedra.
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SiSi,AlSi,AlSi,AlSi,Al
OOOOOOOO OOOOOOOO
Feldspars
- framework aluminosilicates which make up ~70% of the Earth’s crust
Some Al3+ substitutes for Si4+ in the framework and charge balance is achieved by cations (most commonly Na+, K+ and Ca2+) in the open spaces in the framework
Simple chemistry yet the most complex structural group because of the many phase transitions which take place
Fields of composition of the common feldspar minerals
Alk
ali f
elds
pars
Alk
ali f
elds
pars
The feldspar structure
(a)
(b)diad axisT1T1 T1T1T1 T1T2T2 T2T2T2T2 b
Idealized structureReal structure
Na,K,Ca in these large sites
mir
ror
plan
e
diad axis
In the third dimension these sheets are joined so that the downward pointing tetrahedra in one sheet are connected to the upward pointing tetrahedra in the next sheet.
aa
a
Phase transitions in the feldspars
There are three types of behaviour which take place in the feldspar structure on cooling:
At high temperatures:
(i) at high temperatures the feldspar structure is expanded and can contain Na, K and Ca in the large M-sites.
(ii) at high temperatures the Al and Si are randomly distributed in the T-sites
(iii) at high temperatures there are extensive solid solutions in the alkali feldspars and in the plagioclase feldspars.
In this ideal high-T state, feldspars are monoclinic.
(iv) at lower temperatures there is a tendency for the structure to distort by a displacive transition. This tendency depends on the size of the cation in the M-site. K is large and prevents the distortion, Na and Ca are smaller and so the structure distorts to triclinic.
(v) there is also a strong tendency for Al and Si to become ordered as the temperature is reduced. This is to avoid Al in adjacent tetrahedra (the aluminium avoidance rule or Loewenstein’s Rule).
(vi) at lower temperatures the extent of solid solution decreases i.e. exsolution processes
Fields of composition of the common feldspar minerals
Phase transitions in K-feldspar, KAlSi3O8
1. At high temperature the structure is monoclinic with Al,Si disordered. This is called sanidine.
2. As the temperature decreases Al tends to go into one of the T1 sites. This reduces the symmetry to triclinic.
This has an important consequence :
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(a)(b)(d)
MirrorplaneDiadaxis(c)AlbitetwinPericlinetwin
Tricliniccellcb cbcbcbcbcbTransformation twinning
The 2 equivalent orientations of the triclinic unit cell can form twin domains, either related by a mirror plane (albite twin) or by a diad axis (pericline twin).
When both possibilities exist in a single crystal then there are two twin planes at right angles
Phase transitions in K-feldspar, KAlSi3O8
Fully Al,Si ordered K-feldspar is called microcline.Microcline has characteristic cross-hatched twinning, seen in a polarizing microscope :
This characteristic microstructure is due to the existence of both albite and pericline twinning in the crystal which has transformed from the high temperature disordered monoclinic structure.
Orthoclase : an intermediate stage between sanidine and
microcline. It is monoclinic on average, but in an electron
microscope it looks like microcline i.e. very fine twins
1. At very high temperature the structure is monoclinic with Al,Si disordered. This is called monalbite.
But on cooling below about 1000oC monalbite undergoes a displacive transition to triclinic symmetry because the Na is too small to stop the structure from distorting. This triclinic albite is called high albite.
In most rocks albite grows as high albite because the temperature is below that where albite is monoclinic.
2. As the temperature decreases Al, Si begin to order. There is no twinning associated with this because high albite is already triclinic and cannot reduce its symmetry further.
Albite with ordered Al,Si is called low albite. It has no transformation twinning.
The ordering pattern in albite, Al:Si = 1:3 The ordering pattern in anorthite, Al:Si = 2:2
These two ordering patterns are incompatible, and so the tendency to order in albite does not ‘mix’ with the tendency to order in anorthite. So the solid solution (in which the Al;Si ratio is between 1:3 and 2:2) does not have a simple ordering scheme.
The way in which plagioclase solid solution try to solve this problem is still not well understood, but in all plagioclases there are complex intergrowths of albite-rich and albite-poor regions, only seen by electron microscopy.
As with many mineralogical problems, there is an interplay between the thermodynamics and kinetics, and the result is often a compromise.