Transcript Alkali - Feldspars

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 Al 3+ , and all four O atoms are shared with other tetrahedra.
O
Si,Al
O
O
O
O
O
Si,Al
O
O
Si
O
Si,Al
O
O
O
O
Si,Al
O
O
O
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
Stability fields of the silica polymorphs
100
stishovite
Pressure (kbar)
80
Low quartz is the
only
thermodynamically
stable phase of silica
at room T,P
coesite
60
40
high (b) quartz
20
low () quartz
melt
0
400
800
Temperature oC
1200
1600 cristobalite
tridymite
The structure of high (b) quartz
c
c
The structure can be built up from 6fold spirals of tetrahedra.
The spiral axis is the c axis
The same spiral looking along the c axis
The structure of high (b) 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
c
View perpendicular to the c axis
Quartz structure
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The low () quartz  high (b) quartz phase transition
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High (b) quartz - hexagonal and
573oC

Low () quartz -trigonal
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
Twin
bounboundary
dary
Twin
In quartz, this type of twinning is called Dauphiné twinning
(plane)
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.
Displacive
Reconstructive
Reconstructive
Melting
1470C
573C
857C
1713C
 High cristobalite 
Low quartz 
High quartz 
High tridymit e 

melt
(trigonal)
(hexagonal)
(hexagonal)
(cubic)
Cristobalite always shows very fine cracks because the high low transformation involves a volume decrease of ~3%
Glass
Cristobalite
with fine cracks
This is an example of cristobalite in a silica ceramic brick - optical micrograph
Stability fields of the silica polymorphs
100
stishovite
Pressure (kbar)
80
Low cristobalite
and low tridymite
do not appear on
the equilibrium
phase diagram –
coesite
60
40
high (b) quartz
20
low () quartz
melt
they are metastable
0
400
800
Temperature oC
1200
1600 cristobalite
tridymite
Other natural low temperature forms of SiO2
1. Agate
Chalcedony is the fibrous
form of quartz
Agate is made from very fine fibrous crystals of quartz. Agate
grows from Si-rich solutions in the shallow Earth’s crust.
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 !!
Coesite - stable in the earth’s upper mantle
Pressure (kbar)
100
stishovite
80
60
coesite
40
20
low () quartz
high (b) quartz
melt
0
400 800 1200 1600
Temperature oC
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
Stishovite - stable in the earth’s lower mantle
Si in octahedral
co-ordination !!
Pressure (kbar)
100
stishovite
80
60
coesite
40
20
low () quartz
high (b) quartz
melt
0
Stishovite has been found in
rocks in meteorite impact
craters
400 800 1200 1600
Temperature oC
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 Al 3+ , and all four O atoms are shared with other tetrahedra.
O
Si,Al
O
O
O
O
O
Si,Al
O
O
Si
O
Si,Al
O
O
O
O
Si,Al
O
O
O
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
The feldspar structure
mirror plane
Na,K,Ca in these
large sites
diad axis
Idealized structure
Real structure
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.
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.
Phase transitions in the feldspars
(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
K - Feldspars
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 :
Transformation twinning
c
c
Diad
axis
b
(a) Mirror
plane
b
(b)
Triclinic
cell
c
c
b
c
b
b
b
c
(c) Albite
twin
(d)
Pericline
twin
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.
KAlSi3O8
Microcline
Sanidine
Monoclinic
Al,Si disordered
Found in volcanic
(fast cooled)
rocks
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
Found in rocks with intermediate
cooling rate
Triclinic
Al,Si ordered
Found in plutonic
(slowly cooled)
rocks
Na - Feldspars
Phase transitions in Na-feldspar, NaAlSi3O8
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.
Alkali - Feldspars
The alkali feldspar phase diagram
The disordered solid
solution can only exist at
high temperatures.
Below the solvus the solid
solution breaks down to 2
phases - one Na-rich, the
other K-rich.
Na-feldspar + K-feldspar
“Perthite”
This exsolution process
results in a 2-phase
intergrowth, called perthite
The alkali feldspar phase diagram
Early stages of exsolution in alkali feldspars I
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Early stages of exsolution in alkali feldspars II
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Perthite microstructure - an
intergrowth of Na-feldspar
and K-feldspar
Na-feldspar
Cross-hatched twinning
in K-feldspar
Plagioclase Feldspars
Plagioklas
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Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
At high temperatures there is complete solid solution, involving the coupled
substitution:
Na+ + Si4+  Ca2+ + Al3+
1553
1500
Temperature oC
melt
1400
melt +
plagioclase
1300
1200
plagioclase
solid solution
1100
NaAlSi3O8
albite
20
40
60
80
CaAl2Si2O8
anorthite
Note: In albite the Al:Si ratio is 1:3. In anorthite it is 2:2
Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
1553
1500
Temperature oC
melt
1400
melt +
plagioclase
1300
1200
1100
plagioclase solid solution
What happens to the
plagioclase solid solution at
low temperatures?
? ? ? ? ? ?
NaAlSi3O8
albite
20
40
60
80 CaAl2Si2O8
anorthite
Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
As before, there is a strong tendency for Al,Si ordering at lower temperatures
Al
The ordering pattern of Al, Si in albite (Al:Si = 1:3)
Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
As before, there is a strong tendency for Al,Si ordering at lower temperatures
Al
The ordering pattern of Al, Si in anorthite (Al:Si = 2:2)
Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
The ordering pattern in anorthite, Al:Si = 2:2
The ordering pattern in albite, Al:Si = 1:3
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.
Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)
1553
1500
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.
Temperature oC
melt
1400
melt +
plagioclase
1300
1200
1100
plagioclase solid solution
Complex intergrowths on a
nanometre scale
As with many mineralogical
problems, there is an interplay
between the thermodynamics and
kinetics, and the result is often a
compromise.
By eye and optical
microscopy all
plagioclases appear to be
homogeneous
NaAlSi3O8
albite
20
40
60
80 CaAl2Si2O8
anorthite