NETTVERK SILIKATER

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

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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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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 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 MeltingLow quartz

573 C

⏐ → ⏐ ⏐ High quartz 857 C

⏐ → ⏐ ⏐ High tridymite1470 C

⏐ → ⏐ ⏐ ⏐ High cristobalite1713 C

⏐ → ⏐ ⏐ ⏐ melt (trigona)l (hexagona)l (hexagonal) (cubi )c

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

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stishovite

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low () quartz

high () quartz

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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 !!

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coesite

low () quartzhigh () quartz

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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

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low () quartzhigh () quartz

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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

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KAlSi3O8NaAlSi3O8 albiteCaAl2Si2O8 anorthitePlagioclase feldsparssanidineorthoclasemicrocline

Fields of composition of the common feldspar minerals

Alk

ali f

elds

pars

Alk

ali f

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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.

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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

Phase transitions in the feldspars

K - Feldspars

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Alk

ali f

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Alk

ali f

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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

Found in rocks with intermediate cooling rate

Sanidine

Microcline

Monoclinic

Al,Si disordered

Found in volcanic (fast cooled)

rocks

Triclinic

Al,Si ordered

Found in plutonic (slowly cooled)

rocks

KAlSi3O8

Na - Feldspars

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Alk

ali f

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Alk

ali f

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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

Alk

ali f

elds

pars

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Alk

ali f

elds

pars

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.

This exsolution process results in a 2-phase intergrowth, called perthite

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MMTT

Composition020406080100K-FeldsparNa-Feldspar2004006008001000HighAbLowAbDisordered solidsolution

Na-feldspar + K-feldspar

“Perthite”

Al,Si ordering

solvus

The alkali feldspar phase diagram

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Early stages of exsolution in alkali feldspars I

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Early stages of exsolution in alkali feldspars II

Perthite microstructure - an intergrowth of Na-feldspar and K-feldspar

Na-feldspar

whi

te

Cross-hatched twinningin K-feldspar

Plagioclase Feldspars

Alk

ali f

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Alk

ali f

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Plagioklas

Plagioclase feldspars : NaAlSi3O8 (albite) – CaAl2Si2O8 (anorthite)

At high temperatures there is complete solid solution, involving the coupled substitution: Na+ + Si4+ Ca2+ + Al3+

NaAlSi3O8 albite

CaAl2Si2O8 anorthite

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plagioclase solid solution

Note: In albite the Al:Si ratio is 1:3. In anorthite it is 2:2

NaAlSi3O8 albite


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What happens to the plagioclase solid solution at

low temperatures?


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)


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)


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.

NaAlSi3O8 albite


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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.

Complex intergrowths on a nanometre scale

By eye and optical microscopy all

plagioclases appear to be homogeneous

NETTVERK SILIKATER

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