1. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
The culprit: the [SiO4]4- tetrahedron

2. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
[SiO4] Independent tetrahedra Nesosilicates
Examples: olivine garnet
[Si2O7] Double tetrahedra Sorosilicates
Examples: lawsonite
n[SiO3]2- n = 3, 4, Cyclosilicates
Examples: benitoite BaTi[Si3O9]
axinite Ca3Al2BO3[Si4O12]OH
beryl Be3Al2[Si6O18]

3. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
[SiO3] single chains Inosilicates [Si4O11] Double tetrahedra
pryoxenes pyroxenoids amphiboles

4. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
[Si2O5] Sheets of tetrahedra Phyllosilicates
micas talc clay minerals serpentine

5. Silicates are classified on the basis of Si-O polymerism
Mineral Structures
Silicates are classified on the basis of Si-O polymerism
low-quartz
[SiO2] D frameworks of tetrahedra: fully polymerized Tectosilicates
quartz and the silica minerals feldspars feldspathoids zeolites

6. Nesosilicates: independent SiO4 tetrahedra
Mineral Structures
Nesosilicates: independent SiO4 tetrahedra

7. Nesosilicates: independent SiO4 tetrahedrab c
projection
Olivine (100) view blue = M1 yellow = M2

8. Nesosilicates: independent SiO4 tetrahedrab c
perspective
Olivine (100) view blue = M1 yellow = M2

9. Nesosilicates: independent SiO4 tetrahedrab
M1 in rows and share edges
M2 form layers in a-c that share corners
Some M2 and M1 share edges a
Olivine (001) view blue = M1 yellow = M2

10. Nesosilicates: independent SiO4 tetrahedrab c
M1 and M2 as polyhedra
Olivine (100) view blue = M1 yellow = M2

11. Nesosilicates: independent SiO4 tetrahedra Olivine Occurrences:
Principally in mafic and ultramafic igneous and meta-igneous rocks
Fayalite in meta-ironstones and in some alkalic granitoids
Forsterite in some siliceous dolomitic marbles
Monticellite CaMgSiO4
Ca  M2 (larger ion, larger site)
High grade metamorphic siliceous carbonates

12. Nesosilicates: independent SiO4 tetrahedra
Garnet: A2+3 B3+2 [SiO4]3
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites
Garnet (001) view blue = Si purple = A turquoise = B

13. Nesosilicates: independent SiO4 tetrahedra
Garnet: A2+3 B3+2 [SiO4]3
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Pyralspites in meta-shales
Ugrandites in meta-carbonates
Some high-Al igneous
Also in some mantle peridotites a2 a1 a3
Garnet (001) view blue = Si purple = A turquoise = B

14. Inosilicates: single chains- pyroxenesb
Diopside: CaMg [Si2O6]
a sin
Where are the Si-O-Si-O chains??
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

15. Inosilicates: single chains- pyroxenesb
a sin
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

16. Inosilicates: single chains- pyroxenesb
a sin
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

17. Inosilicates: single chains- pyroxenesb
a sin
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

18. Inosilicates: single chains- pyroxenesb
a sin
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

19. Inosilicates: single chains- pyroxenesb
a sin
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

20. Inosilicates: single chains- pyroxenes
Perspective view
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

21. Inosilicates: single chains- pyroxenes
SiO4 as polygons
(and larger area)
IV slab
VI slab
IV slab
a sin
VI slab
IV slab
VI slab
IV slab b
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

22. Inosilicates: single chains- pyroxenes
M1 octahedron

23. Inosilicates: single chains- pyroxenes
M1 octahedron

24. Inosilicates: single chains- pyroxenes
(+)
M1 octahedron
(+) type by convention

25. Inosilicates: single chains- pyroxenes
M1 octahedron
This is a (-) type
(-)

26. Inosilicates: single chains- pyroxenesM1
Creates an “I-beam” like unit in the structure.

27. Inosilicates: single chains- pyroxenes
(+) T M1
Creates an “I-beam” like unit in the structure

28. The pyroxene structure is then composed of alternating I-beams
Inosilicates: single chains- pyroxenes
(+)
(+)
The pyroxene structure is then composed of alternating I-beams
Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation
(+)
(+)
(+)
Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

29. The pyroxene structure is then composed of alternation I-beams
Inosilicates: single chains- pyroxenes
(+)
(+)
The pyroxene structure is then composed of alternation I-beams
Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation
(+)
(+)
(+)

30. Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms
Inosilicates: single chains- pyroxenes
Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms

31. Inosilicates: single chains- pyroxenes
The tetrahedral chain above the M1s is thus offset from that below
The M2 slabs have a similar effect
The result is a monoclinic unit cell, hence clinopyroxenes
(+) M2 c a
(+) M1
(+) M2

32. Inosilicates: single chains- pyroxenes
Orthopyroxenes have alternating (+) and (-) I-beams
the offsets thus compensate and result in an orthorhombic unit cell
This also explains the double a cell dimension and why orthopyroxenes have {210} cleavages instead of {110) as in clinopyroxenes (although both are at 90o) c
(-) M1
(+) M2 a
(+) M1
(-) M2

33. Pyroxene Chemistry The general pyroxene formula: W1-P (X,Y)1+P Z2O6
Where
W = Ca Na
X = Mg Fe2+ Mn Ni Li
Y = Al Fe3+ Cr Ti
Z = Si Al
Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles

34. Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Wollastonite
pigeonite
1200oC
orthopyroxenes
clinopyroxenes
1000oC
Diopside
Hedenbergite
clinopyroxenes
Solvus
800oC
pigeonite
(Mg,Fe)2Si2O6
Ca(Mg,Fe)Si2O6
orthopyroxenes
Enstatite
Ferrosilite

35. Ca-Tschermack’s molecule
Pyroxene Chemistry
“Non-quad” pyroxenes
Jadeite
Aegirine
NaAlSi2O6
NaFe3+Si2O6
0.8
Omphacite
aegirine- augite
Spodumene: LiAlSi2O6
Ca / (Ca + Na)
Ca-Tschermack’s molecule
0.2
CaAl2SiO6
Augite
Diopside-Hedenbergite
Ca(Mg,Fe)Si2O6

36. Pyroxenoids
“Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites
7.1 A
12.5 A
17.4 A
5.2 A
Pyroxene
2-tet repeat
Wollastonite
(Ca  M1)
 3-tet repeat
Rhodonite
MnSiO3
 5-tet repeat
Pyroxmangite
(Mn, Fe)SiO3
 7-tet repeat

37. Inosilicates: double chains- amphiboles
Tremolite:
Ca2Mg5 [Si8O22] (OH)2
a sin
Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)
yellow = M4 (Ca)

38. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
a sin
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

39. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
Same I-beam architecture, but the I-beams are fatter (double chains)
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe)

40. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
(+)
(+)
(+)
Same I-beam architecture, but the I-beams are fatter (double chains)
a sin
(+)
(+)
All are (+) on clinoamphiboles and alternate in orthoamphiboles
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

41. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
M1-M3 are small sites
M4 is larger (Ca)
A-site is really big
Variety of sites 
great chemical range
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

42. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2
(OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra
(OH)
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = H

43. Amphibole Chemistry See handout for more information General formula:
W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2
W = Na K
X = Ca Na Mg Fe2+ (Mn Li)
Y = Mg Fe2+ Mn Al Fe3+ Ti
Z = Si Al
Again, the great variety of sites and sizes  a great chemical range, and hence a broad stability range
The hydrous nature implies an upper temperature stability limit

44. Amphibole Chemistry
Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)
Tremolite
Ferroactinolite
Ca2Mg5Si8O22(OH)2
Actinolite
Ca2Fe5Si8O22(OH)2
Clinoamphiboles
Cummingtonite-grunerite
Anthophyllite
Mg7Si8O22(OH)2
Fe7Si8O22(OH)2
Orthoamphiboles
Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite  gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions

45. Amphibole Chemistry Hornblende has Al in the tetrahedral site
Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member.
Sodic amphiboles
Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2
Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2
Sodic amphiboles are commonly blue, and often called “blue amphiboles”

46. Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates
Actinolite occurs in low-grade metamorphosed basic igneous rocks
Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstones
The complex solid solution called hornblende occurs in a broad variety of both igenous and metamorphic rocks
Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called “blueschist” in reference to the predominant blue sodic amphiboles
Riebeckite occurs commonly in sodic granitoid rocks

47. Inosilicates - - - - - - - - - - - -+ + + + + + a + + + + + + + + + + + + - - - - -
Clinopyroxene -
Clinoamphibole + + a + + + + - - - - - -
Orthopyroxene
Orthoamphibole
Pyroxenes and amphiboles are very similar:
Both have chains of SiO4 tetrahedra
The chains are connected into stylized I-beams by M octahedra
High-Ca monoclinic forms have all the T-O-T offsets in the same direction
Low-Ca orthorhombic forms have alternating (+) and (-) offsets

48. Inosilicates pyroxene amphibole
Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them)
Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  o cleavages in amphiboles

49. Phyllosilicates SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]
Apical O’s are unpolymerized and are bonded to other constituents

50. Phyllosilicates Tetrahedral layers are bonded to octahedral layers
(OH) pairs are located in center of T rings where no apical O

51. Phyllosilicates
Octahedral layers can be understood by analogy with hydroxides
Brucite: Mg(OH)2
Layers of octahedral Mg in coordination with (OH)
Large spacing along c due to weak van der waals bonds c

52. Phyllosilicates a2 a1 Gibbsite: Al(OH)3
Layers of octahedral Al in coordination with (OH)
Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons
Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral

53. Phyllosilicates T O - T O - T O Kaolinite: Al2 [Si2O5] (OH)4
Yellow = (OH)
vdw
Kaolinite: Al2 [Si2O5] (OH)4
T-layers and diocathedral (Al3+) layers
(OH) at center of T-rings and fill base of VI layer 
vdw
weak van der Waals bonds between T-O groups

54. Phyllosilicates T O - T O - T O Serpentine: Mg3 [Si2O5] (OH)4
Yellow = (OH)
vdw
Serpentine: Mg3 [Si2O5] (OH)4
T-layers and triocathedral (Mg2+) layers
(OH) at center of T-rings and fill base of VI layer 
vdw
weak van der Waals bonds between T-O groups

55. Chrysotile does not do this and tends to roll into tubes
Serpentine
Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999).
Antigorite maintains a sheet-like form by alternating segments of opposite curvature
Chrysotile does not do this and tends to roll into tubes

56. Serpentine
Veblen and Busek, 1979, Science 206,
S = serpentine T = talc
Nagby and Faust (1956) Am. Mineralogist 41,
The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates

57. Phyllosilicates T O T - T O T - T O T Pyrophyllite: Al2 [Si4O10] (OH)2
vdw
Yellow = (OH)
Pyrophyllite: Al2 [Si4O10] (OH)2
T-layer - diocathedral (Al3+) layer - T-layer
vdw
weak van der Waals bonds between T - O - T groups

58. Phyllosilicates T O T - T O T - T O T Talc: Mg3 [Si4O10] (OH)2
vdw
Yellow = (OH)
Talc: Mg3 [Si4O10] (OH)2
T-layer - triocathedral (Mg2+) layer - T-layer
vdw
weak van der Waals bonds between T - O - T groups

59. Phyllosilicates T O T K T O T K T O T
Muscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV)
T-layer - diocathedral (Al3+) layer - T-layer - K
K between T - O - T groups is stronger than vdw

60. Phyllosilicates T O T K T O T K T O T
Phlogopite: K Mg3 [Si3AlO10] (OH)2
T-layer - triocathedral (Mg2+) layer - T-layer - K
K between T - O - T groups is stronger than vdw

61. Phyllosilicate Structures
A Summary of
Phyllosilicate Structures
Fig Klein and Hurlbut Manual of Mineralogy, © John Wiley & Sons

62. Phyllosilicates
Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6
= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -
Very hydrated (OH)8, so low-temperature stability (low-T metamorphism and alteration of mafics as cool)

63. “Biopyriboles”
Why are there single-chain-, double-chain-, and sheet-polymer types, and not triple chains, quadruple chains, etc??

64. “Biopyriboles”
It turns out that there are some intermediate types, predicted by J.B. Thompson and discovered in 1977 Veblen, Buseck, and Burnham
Cover of Science: anthophyllite (yellow) reacted to form chesterite (blue & green) and jimthompsonite (red)
Streaked areas are highly disordered
Cover of Science, October 28, 1977 © AAAS

65. HRTEM image of anthophyllite (left) with typical double-chain width
Fig. 6, Veblen et al (1977) Science 198 © AAAS
anthophyllite
jimthompsonite
chesterite
HRTEM image of anthophyllite (left) with typical double-chain width
Jimthompsonite (center) has triple-chains
Chesterite is an ordered alternation of double- and triple-chains

66. “Biopyriboles”
Fig. 7, Veblen et al (1977) Science 198 © AAAS
Disordered structures show 4-chain widths and even a 7-chain width
Obscures the distinction between pyroxenes, amphiboles, and micas (hence the term biopyriboles: biotite-pyroxene-amphibole)

67. Tectosilicates
After Swamy and Saxena (1994) J. Geophys. Res., 99, 11,787-11,794.

68. Tectosilicates
Low Quartz
001 Projection Crystal Class 32

69. Tectosilicates
High Quartz at 581oC
001 Projection Crystal Class 622

70. Tectosilicates
Cristobalite
001 Projection Cubic Structure

71. Tectosilicates
Stishovite
High pressure  SiVI

72. Tectosilicates
Low Quartz Stishovite
SiIV SiVI

73. Tectosilicates Feldspars
Substitute Al3+ for Si4+ allows Na+ or K+ to be added
Substitute two Al3+ for Si4+ allows Ca2+ to be added
Albite: NaAlSi3O8