NETTVERK SILIKATER

1. 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.

2. 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

3. 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

4. Quartz

5. More quartz

6. 100 stishovite 80 coesite 60 Pressure (kbar) 40 high (b) quartz 20 low () quartz melt 0 400 800 1200 1600 cristobalite tridymite Temperature oC Stability fields of the silica polymorphs Low quartz is the only thermodynamically stable phase of silica at room T,P

7. c c The same spiral looking along the c axis The spiral axis is the c axis The structure of high (b) quartz The structure can be built up from 6-fold spirals of tetrahedra.

8. The structure of high (b) quartz - hexagonal

9. Part of the high quartz structure - the 6-fold and 3-fold spirals

10. Part of the high quartz structure - the 6-fold and 3-fold spirals c View perpendicular to the c axis

11. Quartz structure

12. The low (a) quartz  high (b) quartz phase transition

13. 573oC  High (b) quartz - hexagonal and Low () quartz -trigonal

14. Screw Triad axes 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

15. 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.

16. Twin boundary (plane) Transformation twinning in quartz - the twin plane In quartz, this type of twinning is called Dauphiné twinning

17. 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

18. The structures of tridymite and cristobalite Both share the the same structural unit - a layer of tetrahedra, with alternate tetrahedra pointing up and down

19. 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

20. 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

21. The cubic structure of cristobalite

22. The cubic structure of cristobalite

23. 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)

24. The distortion of the silicate tetrahedral layer in the high - low transformations in cristobalite and tridymite High form Low form

25. 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.

26. Glass Cristobalite with fine cracks Cristobalite always shows very fine cracks because the high - low transformation involves a volume decrease of ~3% This is an example of cristobalite in a silica ceramic brick - optical micrograph

27. 100 stishovite 80 coesite 60 Pressure (kbar) 40 high (b) quartz 20 low () quartz melt 0 400 800 1200 1600 cristobalite tridymite Temperature oC Stability fields of the silica polymorphs Low cristobalite and low tridymite do not appear on the equilibrium phase diagram – they are metastable

28. 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.

29. Other natural low temperature forms of SiO2 2. Opal Opal is an amorphous form of silica formed from supersaturated Si-rich solutions.

30. Where dothecolours inopalcome from? Electron micrographs showing small spheres of amorphous SiO2, which scatter the light to produce the colours.

31. 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 !!

32. 100 stishovite 80 coesite 60 Pressure (kbar) 40 high (b) quartz 20 low () quartz melt 0 400 800 1200 1600 Temperature oC 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

33. 100 stishovite 80 coesite 60 Pressure (kbar) 40 high (b) quartz 20 low () quartz melt 0 400 800 1200 1600 Temperature oC Stishovite has been found in rocks in meteorite impact craters Stishovite - stable in the earth’s lower mantle Si in octahedral co-ordination !!

34. 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.

35. 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 Alkali feldspars Alkali feldspars Fields of composition of the common feldspar minerals Feldspars - framework aluminosilicates which make up ~70% of the Earth’s crust Simple chemistry yet the most complex structural group because of the many phase transitions which take place

36. Na,K,Ca in these large sites Real structure The feldspar structure mirror plane diad axis Idealized structure

37. 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.

38. 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.

39. 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

40. Alkali feldspars K - Feldspars Alkali feldspars Fields of composition of the common feldspar minerals

41. Transformation 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 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 :

42. 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.

43. KAlSi3O8 Microcline Sanidine 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 Monoclinic Al,Si disordered Found in volcanic (fast cooled) rocks Triclinic Al,Si ordered Found in plutonic (slowly cooled) rocks

44. Alkali feldspars Na - Feldspars Alkali feldspars

45. 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.

46. Alkali feldspars Alkali - Feldspars Alkali feldspars

47. solvus Al,Si ordering Na-feldspar + K-feldspar “Perthite” 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

48. The alkali feldspar phase diagram

49. Early stages of exsolution in alkali feldspars I

50. Early stages of exsolution in alkali feldspars II