PHYLLOSILICATE CLASSIFICATION
• The phyllosilicates are classified based upon:
1. the number of tet. and oct. in a sheet
2. the octahedral site occupancy (di or trioctahedral)
3. charge per formula unit for each layer.
General Classes (layer build-up) of Phyllosilicate Minerals:
Layer Type
Charge†
Trioctahedral
Dioctahedral
1 octahedra
0
brucite, Mg(OH)2
gibbsite, Al(OH)3
1 tet. : 1 oct.
0
serpentine, Mg3Si2O5(OH)4 kaolinite, Al2Si2O5(OH)4
2 tet. : 1 oct.
0
talc, Mg3Si4O10(OH)2
2 tet: 1 oct.
1
phlogopite
KMg3(AlSi3O10)(OH)2
1
biotite
KFe3(AlSi3O10)(OH)2
0.6-0.8
illite (hydrous mica)
K(Na,Ca) Al1.3Fe0.4Mn0.2Si3.4Al0.6O10(OH)2
0.6-0.9
vermiculite
0.25-0.6
smectite
pyrophyllite, Al2Si4O10(OH)2
muscovite
KAl2(AlSi3O10)(OH)2
† The layer charge per formula unit
SOIL MINERAL PROPERTIES
Octahedral Minerals:
Brucite and Gibbsite: Gibbsite is a common secondary mineral, abundant in wellweathered soils such as Oxisols. Brucite is less commonly observed in soils (weathers
rapidly).
-
Layers are bound together by van der Waals forces
-
Gibbsite is common in highly weather soils (Oxisols)
1:1 Minerals, Serpentine and Kaolinite:
Serpentine and Kaolinite: The structural sheets composed of 1 tet. to 1 oct. are held
together by H-bonds. Because of the numerous bonds these sheets are held rather
tightly together and are thus not expandable.
Serpentine (Mg3Si2O5(OH)4)
- pH > 8, (Mg) > 0.001 M needed for stability
- weathering sequence,
Serpentine --> Smectite + Chlorite ---------> .....
Kaolins:
oct --tet-Kaolinite: Al2Si2O5(OH)4
- probably the most ubiquitous silicate mineral in soils
- dioctahedral, 1:1 mineral
- forms from the weathering of 2:1 minerals (often from re-precipitation)
- SA is also low, 7 to 30 m2/g
- low cation exchange capacity: 5-15 cmol/Kg
- charge from unsatisfied bonds ==> pH dependent
- no isomorphic substitution
- forms hexagonal plates (joined by H-bonds)
usually less than < 2 µm
2:1 Minerals
Talc and Pyrophyllite: 2:1 Minerals with no layer charge. These minerals are the
structural basis for the permanently charged clay minerals.
- ideal cation site occupancy in both TET and OCT sheets
- no isomorphic substitution
- adjoining layers are bonded via van der Waals forces  weak!
- secondary alteration products of pre-existing silicates
(usually formed from low temp. metamorphism or hydrothermal alteration)
-minimal CEC, developed only at terminal bonds (i.e., edges)
Micas (phlogopite, muscovite, and biotite): 2:1 Mineral with 1 Al substitutes for every
4th Si in the tetrahedral sheet.
oct
tet
oct
- Muscovite, KAl2(AlSi3O10)(OH)2: dioctahedral
- Phlogopite, KMg3(AlSi3O10)(OH)2: trioctahedral
- Biotite, KFe3(AlSi3O10)(OH)2: trioctahedral
- in TET units; 1 Al subst.. in every 4th tet. site
- a charge of (-1) per formula unit (1/2 unit cell)
- additional charge is satisfied by K+ ions that reside in the interlayer
- K+ ion is not exchangeable
Steric 'FIT' of K in cavity (along with Cs, NH4+)
- strong assemblage of the layers ==>non-expandable
- no appreciable CEC
- MICAS ----------------------> expandable 2:1 minerals
-K + hydration of interlayer
Altered Mica Minerals: “non-expandable”
Illite (hydrous mica): “nonexpanding, dioctahedral, K-bearing mica-like minerals”
- ≈ KAl1.3Fe0.4Mg0.2Si3.4Al0.6O10(OH)2
- layer charge: 0.6 - 0.8 per formula unit
- derived from the weathering of the muscovite
- divalent cation substitutes in the octahedral layer for Al
- quantity of Si is increased in the tetrahedral layer
- CEC is approximately 30 cmol / Kg (30 meq/100g)
- high affinity for K, Cs, and NH4+ (steric factor, i.e., FIT)
EXPANDABLE 2:1 LAYER PHYLLOSILICATES
• The layer charge of these minerals is reduced relative to mica.
-
Interlayer:
Charge:
As micas weather the interlayer will progressively become hydrated as K+ is
released from the structure.
Mica ----->
dehydrated
-1
Illite ---->
paritally hydrated
0.6-0.8
Vermiculite-->Smectite
fully hydrated
0.6-0.9 / 0.25-0.6
Vermiculite: Can occur in either di- or trioctahedral forms.
Dioctahedral: Nax(Al,Fe)2(AlxSi4-x)O10(OH)2•4H2O
Trioctahedral: Nax(Mg,Fe)2(AlxSi4-x)O10(OH)2•4H2O
- layer charge: 0.6-0.9 per formula unit
- Charge originates primarily in tetrahedral layer (Al substitution for Si). The
charge in the tetrahedral layer may be somewhat offset by Al substitution for
Mg in the octahedral layer.
-The cations in the interlayer, which balance the layer charge, are exchangeable
- Ions of the correct size can lead to a layer collapse of the structure; such ions
include K+, NH4+, and Cs+. Once these ions collapse the structure they are no
longer exchangeable.
- not very expandable
- The CEC and surface area (SA) are high: CEC = 120-150 cmol/Kg, SA = 600800 m2/g.
External SA ≈ 50 m2/g
Smectites:
- Most expandable of the clay minerals  shrink-swell properties in soils
- Very high water holding capacity as a result of such swelling
- Most common smectite:
Montmorillonite: My•nH2O (Al2-y,Mgy)( Si4O10(OH)2
y= 0.25 to 0.45
*commercially available and known as “bentonite” clay.
- Abundant in moderately weathered soils
* especially in poorly drained soils
- Reason for expandable interlayer:
- Low charge and large, low-valent hydrated ions
- CEC ranges from 800 to 1200 mmol / Kg with a surface area of 600-800 m2/g.
• variable charge a minor component of mineral edges
Accessory Minerals: The Hydrous Oxides
• Less abundant, but very influential
- high surface areas
- ability to coat other particles
- high reactivity
- coordinatively unsaturated  very chemically reactive
- variable charge ( ZPC)
Allophane: A generic term for amorphous hydrated aluminosilicate with a Si:Al ratio of
≈1-2.
• Aluminum enters octahedral coordination
• Si is present in tetrahedral coordination.
• Commonly observed in volcanically derived soils and sediments: Andisols (and some
Spodosols).
• Exchange capacity of allophanes is very large; values of up to 1500 mmol/Kg have been
reported.
• Surface area is also large (70-300 m2/g).
• Allophane forms a spheroid surface topography and has short to mid-range order
Aluminum Oxides:
• Most abundant of the secondary minerals is gibbsite, -Al(OH)3.
- Very stable mineral at low temperatures and is the building block for other
phyllosilicates (the dioctahedral class)
- SA of gibbsite, 5 – 20 m2/g
- Hexagonal sheets are bound by van der Waals bonds
- ubiquitous in well developed soils
• Al-oxides have high ZPC that ranges from 8 to 9.5.
Iron Oxides: Ferric hydrous oxides are abundant in many soils and due to their strong
pigmentation they are easily recognized; the yellow and red soil colors are due to this
class of minerals. Based on radius ratios, Fe(III) should enter octahedral coordination,
and this is observed in nature. Accordingly, the Fe oxides are similar to the Al-oxides.
• Ferrihydrite: Fe2O3•nH2O (n = 5 to 9, usually): an amorphous iron hydroxide.
• Goethite (-FeOOH) is the most abundant of the iron oxides
- yellowish color
• Hematite (-Fe2O3)
- bright pink color
- favored in high temperature low moisture areas
- high pH favors the formation of hematite relative to goethite
• Form concretions and coatings in many soils
Manganese Oxides:
- MnO2
- Very reactive; high sorption capacity
- Very strong oxidants
- Form black coatings or nodules
ORGANIC MATTER
• Organic matter may only compose a small weight fraction of soils but it has a
pronounced effect on both chemical and physical properties
-
- >20% organic matter (by wt.) produces an “organic soil”
even a few weight percent of OM can add considerably to the properties of soil
charge originates from reactive function groups that are similar to terminal bonds in
minerals.
Variable charge (but very appreciable total charge).
Properties:
high CEC, high water holding capacity, increases aggregation
(increased water infiltration), nutritional properties.