Framework Silicates (Tectosilicates)

1. Framework Silicates(Tectosilicates)

2. FRAMEWORK SILICATES 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.

3. SiO2 3-D frameworks of tetrahedra: fully polymerizedTectosilicates quartz and the silica minerals feldspars feldspathoids zeolites other tectosilicates

4. 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. minerals of the earth crust

5. 4.35 Specific gravities of SiO2 polymorphs Low quartz is the only thermodynamically stable phase of silica at room T,P 3.01 2.53 2.65 2.32 2.26

6. 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

7. c c Two spirals are connected with each other forming six-membered rings 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, point group: 622

9. Part of the high quartz structure - the 6- and 3-membered rings

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

11. The structure of high (b) quartz 1/3 0 1/3 0 1/3 2/3 2/3 2/3 1/3 0 1/3 1/3 0

12. 573oC  High (b) quartz - hexagonal and Low () quartz –, point group: 32

13. 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 a 6-fold axis to a 3-fold axis

14. Transformation twinning in quartz There are two equally likely possibilities for distorting the high quartz structure to the low quartz structure. Left quartz Right quartz When both orientations of the low quartz structure exist in the same crystal, the crystal is twinned. The process of forming a twinned crystal in this way is called transformation twinning.

15. Right and left quartz

16. Twin boundary Transformation twinning in quartz - the twin plane Twin symmetry element 2-fold rotation axis In quartz, this type of twinning is called Dauphiné twinning

17. Twinned crystals can be also 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.

18. Citrine Amethyst

19. Rose quartz Smokey quartz Milky quartz

20. Aventurine quartz Rutilated quartz

21. 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.

22. Chalcedony is the fibrous form of quartz Petrified wood Onyx

23. Jasper

24. 4.35 Specific gravities of SiO2 polymorphs Low quartz is the only thermodynamically stable phase of silica at room T,P 3.01 2.53 Low cristobalite and low tridymite do not appear on the equilibrium phase diagram – they are metastable 2.65 2.32 2.26

25. High tridymite

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. 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.

28. The structures of tridymite and cristobalite Both share the a layer of tetrahedra, with alternate tetrahedra pointing up and down

29. 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

30. 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)

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

32. 4.35 Specific gravities of SiO2 polymorphs Low quartz is the only thermodynamically stable phase of silica at room T,P 3.01 2.53 Low cristobalite and low tridymite do not appear on the equilibrium phase diagram – they are metastable 2.65 2.32 2.26

33. 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

34. 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 !! (technicallynot a silicate, but an oxide)

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

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

37. Alkali feldspars 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

38. Na,K,Ca in these large sites

39. 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. _ (201) c Double Crankshaft

40. Structure of sanidine KAlSi3O8 Two types of tetrahedral sites T1 and T2. There are very similar, but symmetrically distinct sets of tetrahedra. Two cleavages develop through the cavity sites at 90° to each other parallel to (010) and (001).

41. 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.

42. 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

44. 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 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

45. Transformation twinning in alkali feldspars

46. Twinning in alkali feldspars Albite twinning is caused by the presence of pseudo-mirror symmetry parallel to (010): Twin plane is // (010) Components are related by reflection in (010) Polysynthetic twinning

47. Twinning in alkali feldspars Pericline twinning is caused by the presence of pseudo-diad symmetry parallel to y axis: Twin plane is parallel to (h0l). Exact orientation depends on precise values of lattice parameters and varies greatly in different types of feldspar.

48. 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.

49. Twinning in alkali feldspars Carlsbad twinning is a growth twinning caused by the presence of pseudo-diad symmetry parallel to the z axis: Twin plane is // (010) Components are related by 180° rotation about z

50. Sanidine