1. Minerals with isolated [SiO4]4- tetrahedra
Nesosilicates
Minerals with isolated [SiO4]4- tetrahedra

2. The garnet picture gallery

3. Garnets: A large group of cubic minerals with general formula
The garnet minerals
Garnets: A large group of cubic minerals with general formula
A32+B23+(SiO4)3
A – Ca2+, Mg2+, Fe2+ or Mn2+
B – Al3+,Fe3+ or Cr3+
There are also many other synthetic compositions possible

4. Spessartine Mn3Al2(SiO4)3
Garnet compositions
 Grandite group
Grossular Ca3Al2(SiO4)3
Andradite Ca3Fe3+2(SiO4)3
Uvarovite Ca3Cr2(SiO4)3
Pyralspite group
Pyrope Mg3Al2(SiO4)3
Almandine Fe2+3Al2(SiO4)3
Spessartine Mn3Al2(SiO4)3
Solid solutions within each group are typical.
At high T (above 700oC) there is also solid solution between the 2 groups.
Note: Solid solutions are always more extensive at higher Temperatures

5. The garnet structure I

6. The garnet structure II - isolated tetrahedra

7. The garnet structure III - corner-sharing with octahedra

8. The garnet structure IV - the large A polyhedra added

9. The garnet structure V - cluster around the octahedron

10. The garnet structure VI - large A polyhedra added

11. The garnet structure - building out further……..

12. The garnet structure - the whole unit cell

13. Garnet

14. Spessartine Mn3Al2(SiO4)3
Garnet compositions
 Grandite group
Grossular Ca3Al2(SiO4)3
Andradite Ca3Fe3+2(SiO4)3
Uvarovite Ca3Cr2(SiO4)3
Pyralspite group
Pyrope Mg3Al2(SiO4)3
Almandine Fe2+3Al2(SiO4)3
Spessartine Mn3Al2(SiO4)3
Solid solutions within each group are typical.
At high T (above 700oC) there is also solid solution between the 2 groups.
Note: Solid solutions are always more extensive at higher Temperatures

15. Grossular

16. Andradite

17. Uvarovite

18. Pyrope

19. Almandine

20. Spessartine

21. Olivine
The name ‘olivine’ refers to a range of compositions between Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite), which form a solid-solution.

22. The structure of olivine (Fe,Mg)2SiO4

23. Olivine compositions
No compositions exist here

24. Olivine is orthorhombic and often forms a characteristic crystal shape in a rock because it is often the first mineral to crystallise from the melt.
001
101
100
010

25. Peridot - gem quality olivine
This is a cut crystal
Olivine

26. monticellite

27. Humite Group
The humite group contains four members whose general formula is nMg2SiO4∙ Mg(OH,F)2 where n is 1, 2, 3 and 4. Thus, we get:
Norbergite Pbnm Mg3SiO4(OH,F)2 n=1
Chondrodite P21/b Mg5(SiO4)2(OH,F)2 n=2
Humite Pbnm Mg7(SiO4)3(OH,F)2 n=3
Clinohumite P21/b Mg9(SiO4)4(OH,F)2 n=4

28. norbergite

29. chondordite

30. clinohumite

31. The aluminium silicates : kyanite, andalusite and sillimanite Al2SiO5
Al2SiO5 ???? Why this formula for a silicate with isolated tetrahedra?
Could be written Al.AlO(SiO4) because there are two different Al structural sites as well as isolated [SiO4] tetrahedra.
They also have been considered as belonging to a separate silicate subclass – subsaturates, because there are conceptually too few oxygens, a naïve viewpoint.
Kyanite, andalusite and sillimanite are polymorphs, because they have the same chemical composition but can exist with different crystal structures.

32. The crystal structures of the Al2SiO5 polymorphs
All three structures have straight chains of edge-sharing AlO6 octahedra along the c axis

33. The crystal structures of the Al2SiO5 polymorphs
All three structures have straight chains of edge-sharing AlO6 octahedra along the c axis.
These chains contain half of the Al in the structural formula.
In kyanite the remaining Al atoms are in 6-fold coordination (octahedra)
In andalusite the remaining Al atoms are in 5-fold coordination
In sillimanite the remaining Al atoms are in 4-fold coordination (tetrahedra)
Density: kyanite > sillimanite > andalusite
Therefore kyanite is stable at the highest pressures and lowest temperatures, while sillimanite is stable at high temperatures and lower pressures

34. The stability of the Al2SiO5 polymorphs
The Al2SiO5 polymorphs form in metamorphic rocks
Pressure (kbar)
3.23

35. The structure of kyanite
1. The AlO6 octahedral chains

36. The structure of kyanite
1. The AlO6 octahedral chains
2. Add the other Al polyhedra

37. The structure of kyanite
1. The AlO6 octahedral chains
2. Add the other Al polyhedra
3. Add the [SiO4] tetrahedra

38. The structure of andalusite
1. The AlO6 octahedral chains

39. The structure of andalusite
1. The AlO6 octahedral chains Add the other Al polyhedra

40. The structure of sillimanite
1. The AlO6 octahedral chains

41. The structure of sillimanite
1. The AlO6 octahedral chains Add the other Al polyhedra

42. The structure of sillimanite
1. The AlO6 octahedral chains Add the other Al polyhedra
3. Add the [SiO4] tetrahedra

43. Sillimanite

44. Andalusite

45. Kyanite

46. Staurolite
Fe2Al9O6(SiO4)4(O,OH)2

47. Chloritoid (Fe,Mg)Al2SiO5(OH)2

48. Titanite (sphene)
CaTiSiO5

49. Titanite, CaTiO[SiO4] Monoclinic, 2/m
Tetrahedral Si, Octahedral Ti and 7-fold Ca

50. Topaz
Al2SiO4(F,OH)

51. Zircon

52. Silicates with isolated tetrahedra (cont.)
Zircon ZrSiO4
an important industrial mineral : main source of Zr and ZrO2
found in small amounts in many different types of rocks, usually as microscopic crystals
it is resistant to weathering, and so can form significant concentrations in sand deposits
its high density makes it easy to separate from the sand
Zircon almost always contains radioactive isotopes (e.g. hafnium, thorium, uranium.
The decay of U to Pb is the basis for radiometric dating of rocks, and so zircon is a very important mineral for geochronology.

53. The structure of zircon
Blue tetrahedra contain Si
Green polyhedra contain Zr in 8-fold coordination
Zircon is tetragonal. This is a view down the c axis

54. The structure of zircon
Blue tetrahedra contain Si
Green polyhedra contain Zr in 8-fold coordination
Zircon is tetragonal. This is a view down the c axis with the top layer of Zr polyhedra removed

55. The structure of zircon
Blue tetrahedra contain Si
Green polyhedra contain Zr in 8-fold coordination
Zircon is tetragonal. This is a view down the a axis

56. The structure of zircon
Blue tetrahedra contain Si
Green polyhedra contain Zr in 8-fold coordination
Zircon is tetragonal. This is a view down the a axis