Soils I - Part 2 - The Solid Phase

Things are always evolving and changin. When living things die, they can become sediments. Diagenesis makes sedimentary rock from loose materials which then either build soil, go through metamorphosis again or sinks down, melts and becomes magma.

properties: homogenous components of the earth's crust in terms of physical and chemical properties

composition: solid chemical compounds such as salts, oxides, hydroxides. They occur in pure form.

structure: with a specific crystal structure or without a crystal structure (= amorphous minerals)

importance: important soil components, starting products for weathering and the formation of secondary minerals

47% Oxygen, 27% Silicone (Si), 8% Aluminum, 5% Iron, 5% Calcium, 2% Magnesium, 2% Sodium, 2% Potassium, Other elements

Silicates (SiO₄) and Silica (SiO₂; also known as quartz) mainly originate from magmatic rocks (80%). They are the most important chemical (= base unit) for the building of secondary/ pedogenic minerals. They come in different shapes and sizes (isolated, chained/ banded, tetra-/ octahedral layers, etc.) which depends on temperature, pressure, cooling rate and chemical composition.

Silicates connect to other molecules via oxygen bridges. Together they build bands, layers and chains. Because the molecule is negatively charged, it also attracts positively charged ions like K⁺, Na⁺, Al³⁺, Fe^2/3+, Mg²⁺ and Ca²⁺. Because of the charge there can be isomorphic substitution and e.g. Al³⁺ takes the place of Si⁴⁺. One extra charge then needs to be compensated by other cations entering the structure. 

Olivine (Gg, Fe)₂SiO₄

• greenish
• easily weathered due to structure
• no oxygen bridges: silicates are loosely bound via Mg²⁺ or Fe²⁺.
• no isomorphic substitution
• nutrient-rich (cations)

Characteristic: networking via O atoms in corners of the tetrahedrons.

Mineral groups

• Pyroxene: Chain silicates (Ca,Mg,Fe,Al,Ti)₂(Si,Al)₂O₆ → representative; Augite (very sensitive to weathering, easily builds on volcanic rock, good nutrient supplier to soil)
• Amphibole: Band silicates Ca₂(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂ → representative: Hornblende

1. flat 2D structure
2. aluminum (octahedral structure) is not an isomorphic substitution (→ negative charge)
3. the negative charges in the structure are stabilized by the inclusion of potassium (K⁺)
4. layers can bind together to form a specific structure
5. Example minerals in this group: Muscovite (light mica) and Biotite (dark mica)

Importance for soils

• K content 5-9%
• easily weatherable, biotite weathers more easily than muscovite

Occurrence

• Magmatic rocks: Mostly biotite
• Sediments and metamorphic rocks: Mostly muscovite

• Quartz (SiO₂)
• Feldspars

Features

• Si tetrahedral framework, SiO₂
• Primary, lithogenic mineral (generated from rock)
• Very resistant to weathering (Quartz is likely to be the last mineral remaining)
• Mostly as larger grains (sand and silt fraction) in soils and sediments
• Does not supply any nutrients

Recognisable in rock and soils: cloudy, milky, fracture surfaces not smooth, hard.

Features

25-50% of tetrahedral centres occupied with Al3+ for charge balancing K+, Na+, Ca2+ in gaps (many nutrients can attach to feldspar in the gaps)

Recognisable in rock and soils: Smooth cleavage surfaces, light minerals

Occurrence (loss due to weathering)

• Percentage in magmatic rocks: 60%
• In soils and sediments: <10%

Significance for Soils

Release of nutrients: K, Ca; Formation of secondary minerals (clay minerals)

"Secondary minerals" are formed after a complete or partial dissolution of the primary mineral (in the parent material) also through the process of soil formation.

Examples of "secondary minerals" are:

• clay minerals (layer silicates such as mica)
• Fe oxides and hydroxides, Mn oxides, Al oxides
• secondary carbonates and salts

Origin

Clay minerals originate from silicates. They look like mica with tetrahedral and octahedral layers. Their particle size is mostly < 2 μm (so-called clay fraction, very tiny particles, minerals are bigger: main difference)

Identification

• Formula unit = smallest unit of the crystal, repeating itself in all directions
• Basal spacing = distance between the 'lower' boundaries of two consecutive formula units

Classification

• Clay minerals exist in many different forms
• Si tetrahedra and Al octahedra layers: 1:1, 2:1, 2:1:1 and special ones (sphere and tubes)
• Different possible interlayers

This process is called isomorphic substitution. It takes place where the interchangeable cations are (= partly in the interlayer).

The mineral does not change its form due to the process. The higher the negative charge of the molecule, the higher the ion exchange capacity. The higher the ion exchange capacity, the more highly reactive the molecule/ mineral.

Structure

• 1:1 clay minerals (1 octahedral + 1 tetrahedral layer)
• No intermediate layer (→ layer cohesion through hydrogen bonds (OH - O))
• Formula: Al₂(OH)₄Si₂O₅
• No excess negative charge (no exchange of ions)

Origin

• Si-poor milieu of the tropical regions
• At the end of the weathering process
• The structure of this mineral is made from very small structures of weathered granite.

Features: Mostly white, cannot be expanded by water, cannot swell or shrink, therefor it has great properties for ceramics, i.e. porcelain.

Importance for soils

• No nutrients in the crystal
• Low nutrient retention capacity, as hardly any isomorphic replacement (= no cation binding)
• Soils with low fertility (→ no ion exchange capacity)

Illite, Smectite, Vermiculite

Importance

• Illite: Potassium, as a very important nutrient, can bind easily and therefor be stored for when the plants need it
• Smectite and Vermiculite: very absorbant (use as packing material, binding agent, seal in deep drilling and plugging holes)  

Illite is a three-layer clay mineral.

Origin: Mica + physical weathering = Illite

Structure

• 2:1 clay mineral: 1 Octahedron + 1 Tetrahedron
• Basal spacing of 1 nm

Features

• Isomorphic substitutions
• Firm layer cohesion through K+: almost no swelling
• Important for K nutrition, in our soils often 5-6% K
• Completely expandable by K removal (reversible)

Importance: If you have smectite in the soils and you add potassium, the potassium can bind to the structure of the smectite. If the added potassium doesn’t have anywhere to bind, it is lost rather quickly.

Smectite is a three-layer clay mineral.

Origin: often from basic igneous rocks

Structure

• = 2:1
• Basal distance from 1 to 2 nm
• Interlayer

Features

• Expandable (it can double its size): strong water retention: swelling/ shrinking. The big water retention capacity can lead to landslides, soil deformation, etc.
• Cations are hydrated
• Loose binding: cations are exchangeable

Vermiculite is a three-layer clay mineral.

Origin: biotite and muscovite.

Structure: 2:1.

Features

• expandable up to 2nm
• heavy water retention capacity: swelling and shrinking

1. Island-like inclusions of Al-hydrocide layers in the interlayer
2. Al not fully hydroxylated = positive charge
3. Not expandable → no ion exchange

Imogolite

• Al-OH outer layer + Si tetrahedron inner layer
• Forms very fine tubes (2 nm outer diameter)
• High specific surface: 100-1000 m2/g

Allophane: hollow sphere, formed from volcanic glasses

• O-OH - bridges (e.g. Kaolinite)
• Potassium ions (e.g. Illite)
• exchangeable cations + H₂O (e.g. Smectite, Vermiculite)
• (Mg,) Al hydroxide layer (e.g. Chlorite)

• Phyllosilicate (layer silicate) esp. Mica are the most important for clay formation
• Physical weathering = opening of the edges and K replacement by bigger Ca²⁺ and Mg²⁺
• “M” stands for any “Metal”

1st line: removal and replacement of the interlayer (K⁺), basic structure remains intact. Mica is a phylosillicate which is made by layers. It is broken down, some potassium is lost and Illite is formed. If you remove Potassium to the interlayer Vermiculite is formed.

2nd line: complete disintegration of the structure, formation of new minerals from decay products depending on which minerals are around.

• important secondary new formations in the soil
• subject to constant change (usually much more stable than the primary silicate from which they originate)
• Importance for soils
  • components of the finest fraction of the soil (clay fraction < 2 μm)
  • storage for nutrients
  • microstructure-makers: often adhere to each other, bind to themselves and to other coarse particles, they also form structures through expanding and shrinking

The formation of oxides and hydroxides can be broken down into two steps:

1. Al / Fe have to be released from the primary silicate: the Al-O-Si / Fe-O-Si bond has to be broken. As a result Fe²⁺, Mn²⁺ and Al²⁺ are freely available in the solution.
2. Oxidation or precipitation then lead to the formation of oxides and hydroxides.

Oxides and hydroxides are therefor the stable end-products of weathering of the primary silicates. There are many different types of oxides. 

Properties

Gibbsite forms a crystal structure (γ-Al(OH)3) constructed from octahedrons, whose centers are only 2/3 occupied by Al. It is colorless to white (if you google "gibbsite", it has a blueish/greenish color in many pictures).

Occurrence: only at very low Si-concentrations in the soil solution, i.e. at intensive weathering (especially in the tropics).

• Fe²⁺ in silicates is oxidized upon release to Fe³⁺, forming Fe(III) oxides and giving the soils a brown-red color
• The ratio of Fe3+/Fe2+ can be used as a weathering indicator
  • 0.2/0.3 in young soils
  • 0.8/0.9 in old tropic soils
• Typical concentration in soils: 0.2-20% normally, 80-90% if accumulated
• Not easily soluble, not easily crystallized
• Particle size: 2-100 nm (= nanoparticle)
• Very large specific area: 50 -200 m²/g! (for actions, storing nutrients, etc.)
• Stable under aerobic conditions
• Dissolved under anaerobic conditions
  • Released Fe2+ transported from cm to km
  • Forms hardened concretions (mottles) upon reoxidation
• Binds trace elements and influences their behavior: Phosphate, arsenate, chromate, selenate, etc.

Haematite (α-Fe₂O₃)

This (blood)red molecule forms at higher temperatures. Its appearance is a hexagonal platelet structure. And it occurs in the subtropics and tropics.

Goethite (α-FeOOH)

This molecule has yellow brownish color. It preferably forms at medium temperature. It is very stable and gives temperate soils a brown color. It occurs in all climate zones and is the dominate oxide in mid-latitudes.

Lepidocrocite (γ -FeOOH)

This formation is predominant under reducing conditions. It is Metastable and occurs on a small scale mostly in waterlogged soils.

Ferrihydrite (5Fe₂O₃ – 9H₂O)

This formation is very common in mid-latitudes. It occurs as young iron oxide (“rust”). Its characteristics are: rapid oxidation, amorphous mineral, form aggregates of 2-5 nm.

Schwertmannite (Fe16[O16/(OH)10(SO4)3])

This brownish yellow formation can be found in acidic and sulphate-rich waters. It occurs as a frequent product of pyrite weathering (FeS₂). Very important, also when there is acidic mine draining.

Carbonates

Origin

• weathering of plagioclases, secondary carbonates
• from calcite (easily soluble) and dolomite (source of Mg)

Importance for soils: Buffers soil acidification through the neutralization of CO₂ and other acids. It forms smaller individual accumulations within the soil.

Sulphates/ Phosphates

→ P-supply to the pedo- and biosphere

Sulphates (salts of sulphuric acid): Anhydrite and Gypsum

Phosphates (salts of phosphoric acid): Apatite, Vivianite and Strengite

Weathering is the change of minerals and rocks in contact with the atmosphere (reaction with gases), biosphere ((micro-) organisms, etc.) and hydrosphere (chemical weathering). It is responsible for the new formation of secondary clay minerals, salts, oxides and hydroxides which make up the Pedosphere.

Short: The process where the Lithosphere becomes Pedosphere. Weathering is more than erosion. It also includes chemical reactions and other processes.

Physical weathering is the crushing of material. Consequently the surface area of the material is enlarged relatively.

Mechanisms

• pressure release: when for example the overlying rock is removed due to erosion.
• gravity: crushing after impact
• temperature exposure: 
  • directly through insolation (sun)
  • indirectly through frost, fires and precipitation (different temperatures between night/ day, summer/ winter, etc.)
  • can be subject to locally different heating (can depend on color differences)
• applying pressure (e.g. salt crystal growth and frost-effect/ ice-blast)
  • Root growth: 10 bar
  • Salt growth: 100 bar
  • Pressure relief: 250 bar
  • Thermal stress (direct heat, night/day, etc.): 500 bar
  • Frost weathering: 2000 bar
• transport and abrasion:
  • by water, wind, ice
  • transported distance relies on the material