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Colloids and Clay Mineralogy (Clay (Phyllosilicate (2:1 clays…
Colloids and Clay Mineralogy
Organic Matter
Not mineral material - mainly C, N, O, H
Ability to hold cations/nutrients depends on pH
Charge decreases as soil pH decreases because protons in solution neutralise charges on organic material
Near neutral pH, charge per weight of organic matter greater than silicate clays
Clay
Mainly secondary minerals
Synthesized from weathering products
Sand and silt are the main primary minerals
Clay fraction may include small sized primary minerals
Phyllosilicate
Layered silicate clays/minerals, highly structured
Hydrated silicates of Al, Fe and Mg in various combinations of layers - 'layer silicates'
Basic structural units are arranged in sheets
Tetrahedral sheets - Si surrounded by O
Octahedral sheet - Al or Mg surrounded by hydroxyl or oxygen groups
Tetrahedral and octahedral sheets bound together by mutual sharing of O ions of tetrahedral sheet
Combinations of structural layers and central cation - different mineralogies
1:1 layer clays
One tetrahedral sheet attached to one octahedral sheet
Kaolins
Kaolinite
Distinctive layering
Generally hexagonally shaped flakes
d spacing 0.7nm
Little isomorphic substitution
Al3+ as central cation
Low Cation Exchange Capacity (CEC) - 2-5 cmol/kg
No water or cations in interlayer space
Strong H bonds, layers don't separate easily - non-expanding clay
Halloysite
Similar structure to kaolinite
Varying amount of water in interlayer space - expanding clay (shrink/swell)
Max d spacing 1nm - varies between 0.7-1 depending on degree of dehydration
Similar amount of isomorphous substitution to kaolinite, similar CEC
Dickite
Nacrite
Serpentines
Chryolite
Antigorite
Lizardite
2:1 clays
Mica
Made of primary minerals, but important because:
weather to expandable 2:1 clays
Source of K in the soil
Present in many NZ soils
Cations in interlayer spaces - usually K+ but also Na+ and Ca2+
Up to 25% of Si4+ replaced by Al3+ in tetrahedral sheets
Negative charge from isomorphous substitution often balanced by inter-layer cations which are non-exchangeable
d spacing 1nm
Vermiculites
Similar to micas
Inter-layer cations balance the negative charge (usually Ca2+ and Mg2+)
Inter-layer cations are exchangeable
d spacing 1-1.5nm
High CEC - commonly 100-150 cmol/kg
Derived from weathering of micas
Limited expansion
Two tetrahedral sheets attached to one octahedral one
Smectites
Variety of cations in inter-layer space - all exchangeable
High CEC 80-150 cmol/kg
Considerable expansion
d spacing 1-2nm depending on dehydration
Chlorites
Extra inter-layer hydroxide sheet instead of K+ as in micas
Not bonded directly with 2:1 layers
Non-exchangeable
d spacing 1.4 nm
Small expansion, lowish CEC 10-40 cmol/kg
Short range order aluminosilicates
Imogolite
Tubular Structure - external Al-octahedral sheet and internal Si-tetrahedral sheet with tetrahedra pointing inwards
Each Si in imogolite is attached to 3 O atoms in octahedral sheets instead of 1
Al:Si ratio 2:1
Allophane
Important clay mineral in many NZ soils
Less ordered than imogolite, and more varied elemental composition - Al:Si ratio 1-2:1
Weathering product of volcanic and non-volcanic parent materials
High CEC, surface area etc.
Stable for thousands of years
Hydrous oxides
Ultimate weathering products - abundant in old, weathered soils
Octahedral sheets bonded together by hydrogen bonds
Low negative charge
Present in most soils
Aluminium
Gibbsite Al(OH)3 most common
Iron
Goethite FeOOH - yellow-brown
Haematite Fe2O3 - red
Ferrihydrate Fe5HO8 - orange
Importance
Large surface area and generally negatively charged
Kaolinite 10-30 m2/g
Smectite 650-800 m2/g
Some have variable charge - depends on pH
Soil structure
Soil pH buffering
Nutrient storage
P (adsorption, occlusion)
Ammonium storage
Cation exchange Ca2+, Mg2+ etc.
Identification
Elemental composition - ratio of Al:Si:Mg:Fe
XRD (X-ray diffraction)
Most common
Identifies d-spacing
Compare under different conditions
Electron microscopy
IR spectroscopy
DTA - differential thermal analysis