1. Sheet Silicates
Abundant and common minerals throughout upper 20 km of crust
Felsic to intermediate igneous, metamorphic, and sedimentary rocks
All are hydrous
Contain H
Bonded to O to form OH-
Z/O ratio of 2/5
2 Major groups: Micas & Clays

2. Groupings Based on structure Two kinds of “layers” within the “sheets”
“T” layers – tetrahedral layers
Tetrahedral coordination of Si and Al
“O” sheets – octahedral layers
Octahedral coordination of mostly Al and Mg, occasionally Fe

3. T and O layers join to form sheets
The sheets are repeated in vertical direction
The spaces between the sheets may be:
Vacant
Filled with interlayer cations, water, or other sheets
Primary characteristic - basal cleavage
Single perfect cleavage
Occurs because bonds between sheets are very weak

4. Octahedral Sheets Two planes of OH- anionic groups
Cations are two types:
Divalent (Fe2+ or Mg2+)
Trivalent (Al3+ or Fe3+)

5. Divalent cations fill 3 of 3 sites
Form trioctahedral sheets
Ideal formula is Mg3(OH)6
This formula is brucite
A hydroxide, not a silicate mineral
All sites filled with divalent cations
Charge neutral

6. Trivalent cations fill 2 of 3 sites
Form dioctrahedral sheets
Ideal formula is Al2(OH)6
Mineral called gibbsite
A hydroxide, not silicate mineral
2/3 of sites filled with trivalent cations
Charge neutral

7. Tetrahedral sheets Sheets of tetrahedrally coordinated cations
Formula represented by Z2O5: Z/O = 2/5
Z usually Si4+, Al3+, less commonly Fe3+
Symmetry of rings is hexagonal
Symmetry of sheet silicates is close to hexagonal
Depends on arrangement of stacking
Fig. 11-2

8. Tetrahedron are meshes of 6-fold rings
Three basal oxygen on each tetrahedron shared by adjacent tetrahedron
The fourth, unshared oxygen is the apical oxygen
Tetrahedral layers are two oxygen thick

9. Tetrahedral sheet composition is Si2O52-
May have Al3+ or Fe3+ substitute for Si4+
Increases net negative charge
Fig. 13-1

10. Tetrahedral and octahedral sheets always joined
Apical oxygen of tetrahedral sheets formed part of octahedral sheets
Apical oxygen replaces one of the OH- in the octahedral sheets
Sheets joined in two ways
TO layers, called 1:1 layer silicates
TOT layers, called 2:1 layer silicates

11. Al3+ (dioctahedral) or
Mg2+ (trioctahedral)
OH in middle of rings
Basal Oxygen
T layer on top
(an example of 1:1 layer type)

12. 1:1 layer summary Consists of 3 planes of anions
One plane is basal plane of shared tetrahedral oxygen
Other side is the OH- anionic group of the octahedral sheet
Middle layer is the OH- anionic group with some OH- replaced by oxygen

13. OH- only
OH- + oxygen
Oxygen only
Al2Si2O5(OH)4 = kaolinite, dioctahedral 1:1 sheet silicate
Mg3Si2O5(OH)4 = serpentine, trioctahedral, 1:1 sheet silicate

14. 2:1 layer silicates
2 tetrahedral layers on both sides of octahedral layer
TOT structure has 4 layers of anions
Both sides (outermost) are planes of basal, shared oxygen
Middle planes contain original OH- from octahedral layers and apical oxygen from tetrahedron

15. Oxygen only
OH- + oxygen
OH- + oxygen
Oxygen only
Al2Si4O10(OH)2 = Pyrophyllite, dioctahedral 2:1 sheet silicate
Mg3Si4O10(OH)2 = Talc, trioctahedral 2:1 sheet silicate

16. How are layers stacked? 1:1 layer 2:1 layer
…T-O…T-O…T-O…
2:1 layer
…T-O-T…T-O-T…T-O-T…
c…T-O-T…c…T-O-T…c…T-O-T…c…
O…T-O-T…O…T-O-T…O
Four types of layers, each dioctahedral or trioctahedral

17. 1:1 layer silicates Kaolinite and Serpentine
Bonding between layers very weak
Electrostatic bonds – van der Waals and hydrogen
Results in very soft minerals
Thickness of TO layers around 7 Å

18. C unit cell dimension about 7 Å

19. 2:1 layer silicates Unit structure is repeating TOT layers, two ways:
(1) TOT layers can be electrically neutral
(2) substitution in TOT layers can give a net charge
Most common substitution is Al3+ for Si4+ in tetrahedral layers

20. TOT structure Only Si4+ in T layers (no Al3+ or Fe3+)
Electrically neutral, no interlayer cations
TOT layers weakly bonded by van der Wall and hydrogen bonds
Soft (Talc), greasy feel

21. C unit cell dimension about 9 to 9.5 Å
Nothing in interlayer site

22. c…T-O-T…c…T-O-T…c These are the mica minerals
Also less common are “brittle micas”
Structure is TOT layers with some tetrahedral sites occupied by Al3+

23. Micas Al/Si ratio in the tetrahedral layer is 1/3
Dioctahedral TOT layer = Al2(AlSi3O10)(OH)21-
Trioctahedral TOT layer = Mg3(AlSi3O10)(OH)21-
Negative charge balance by large monovalent cation, usually K+
Bonds are ionic, fairly strong, harder minerals

24. C unit cell dimension about 9.5 to 10 Å
K+ in interlayer site

25. Dioctahedral mica – muscovite
KAl2(AlSi3O10)(OH)2
Trioctahedral mica – Phlogopite
KMg3(AlSi3O10)(OH)2

26. Brittle Micas Similar to micas, but more Al3+ substitution
Charge balanced by Ca2+
Margarite – half of tetrahedral sites have Al3+ substitution
Clintonite – ¾ of tetrahedral sites have Al3+ substitution

27. Margarite Clintonite Dioctahedral CaAl2(Al2Si2O10)(OH)4 Trioctahedral
CaMg2Al(Al3SiO10)(OH)2

28. T layers with small negative charge
…O…T-O-T…O…T-O-T…O…
Most common members are in the chlorite group
Structure like Talc, but with brucite (Mg3(OH)6) interlayer
T layers with small negative charge
Substitute small amounts of Al3+ for Si4+
O layers often have net positive charge
Substitute Al3+ or Fe3+ for divalent cations
Minerals harder than expected

29. C unit cell dimension about 14 Å
TOT layers have slight negative charge, substitute Al3+ for Si4+
O layers often have net positive charge

30. Varieties of sheet silicates
TO structures
Serpentine (var. Antigorite, Chrysotile, Lizardite)
All are trioctahedral
Trioctahedral sheets, a = 5.4 Å; b = 9.3 Å
Tetrahedral sheets, a = 5 Å; b = 8.7 Å
Mismatched size leads to variations

31. Chrysotile (curved tubes)
Antigorite (reversed direction)
Lizardite (distorted tetrahedral mesh)
Fig. 13-5

32. Clay Minerals Clay has two meanings:
Particles < 1/256 mm, or mm
A group of sheet silicate minerals that are commonly clay-sized
Original description from not being able to identify small grain size material
Now can use X-ray diffraction to determine clays

33. Problems
Clay size fraction can contain other minerals (quartz, carbonates, zeolites etc.)
Clay minerals used to define size fraction – size not mineralogical
Several clay minerals can be larger than the size requirements

34. Clay classification 1:1 layer clays
7 Å type, TO layers
2:1 layer clays – 10 Å or 14 Å type
Have net negative charge, but less than one per formula
Requires less interlayer cations to charge balance
Mixed layer clays – combined 1:1 and 1:2

35. Charge imbalance controlled by
Three types of 10 Å clays
Low charge imbalance – smectite clays
High charge imbalance – illite clays
Intermediate charge imbalance – vermiculite
Charge imbalance controlled by
“interlayer” cations
They move in and out – Cation Exchange Capacity (CEC)
Surface adsorption

36. Low charge
Smectite
Net negative charge is 0.2 to 0.6 per formula unit, typically 0.33
Ca and Na are typical interlayer ions
May be dioctahedral or trioctahedral
Charge results from
Al substitution for Si in tetrahedron
Mg for Al in octahedron (in dioctahedral)

37. Water moves in and out depending on moisture in environment
Low charge means water and cations (Na, K, Ca, Mg) easily move in and out of interlayer sites
No water = 10 Å
One water layer = 12.5 Å
Two water layer = 15.2 Å
Water moves in and out depending on moisture in environment

38. High charge Illite/glauconite
Net negative charge of 0.8 to 1 per formula
Almost mica
Mostly substitute of Al3+ for Si4+
All are dioctahedral
Interlayer ion is K+
Very similar to muscovite – called mica-like
High K concentration means strong bond
Difficult for water to enter
non=swelling clay

39. Intermediate charge Vermiculite About 0.6 charge per formula unit
Comes from oxidation of Fe2+ to Fe3+ in biotite
Reduces the negative charge on TOT layer from -1 to -0.6
Less K+ than mica, can exchange for Ca2+ and Mg2+ and water
Swell clay
With water interlayer spacing is 14.4 Å

40. Mixed layer clays Natural clays rarely similar to the end members
Typically contain parts of different types of clays
Actually mixtures at unit cell level – not physical mixtures
Nomenclature – combined names
Illite/smectite or chlorite/smectite

41. 7 Å
1:1 layer clays
2:1 layer clays – low charge, smectite
10 Å
2:1 layer clays – high charge, illite
2:1 layer clays – Chlorite gp
14 Å
Mixed layer

42. Burial Diagenesis Smectite converts to illite with burial
Most conversion at 50 to 100 C
Conversion requires K, usually comes from dissolution of K spar

43. Mineralogy of Miocene/Oligocene sediments Gulf Coast
Release structural water of smectite; corresponds to “oil window”