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Clay
Clay
Georgius Agricola (1494–1555) was seemingly the first who gave the definition of clay in 1546.
Clay acts according to its physical particles - their size and shape - and also as a mineral - its chemical and crystal structures.
Clay Hypothesis
Clay Hypothesis
Clay has been around a long time, and may be the birthplace of biochemical models, where clays could have formed templates for the development of complex organic molecular precursors to what we now call ‘life’.
The 'clay hypothesis' looks at the role of clay and the origin of life (Kloprogge & Hatmann 2022), particularly, "the catalytic role of Fe-clays in the origin and development of metabolism here on Earth" 3-4 bya
The main groups of clay minerals in temperate soils are: kaolinites (like in china clay and ball clay), chlorites and vermiculites and other mixed-layer clays like illite-smectites. Kaolinite, gibbsite, and iron oxides dominate hot equatorial soils due to the extreme weathering conditions typical of these climates.
Clay Mineral Alteration Index (CMAI) as an improved indicator of climate change.
(Warr, Grathoff & Haberrzetti 2024)
(Warr, Grathoff & Haberrzetti 2024)
Particles
Particles
'Clay' is often used to describe a particle with a size fraction (< than 0.002 mm diam) with various shapes. They are not visible with a normal microscope, but the size of the particles in a soil determines its gardening characteristics. The proportion of each particle size grade in a soil determines its texture. Clayey soil feels smooth and sticky when wet. They have excellent water retention properties due to their small particle size and high surface area and can become very hard and compacted when dry, and some swell again into life with rain.
Clay particles have a greater surface area generally compared to silt and sand. Their flat shape contributes to their cohesive and plastic properties. This plastic texture means it's a highly workable. Plastic clay (wet clay) is the type of clay generally used for pottery and for throwing or moulding. Plastic clay can be made from dry clay, using the dry clay mix and water to create the plastic stage consistency.
There are several important properties that set clay particles apart
1. Texture and Consistency: Clay has a smooth and cohesive texture when wet, which allows it to stick together (thixotropic property) Silt and sand, on the other hand, feel more granular and do not hold together as well, until ground down further.
2. Plasticity: Clay is highly plastic, meaning it can be moulded and shaped when wet. It can be formed into various shapes, making it suitable for pottery and ceramics. Silt and sand are not as plastic and do not hold their shape when molded.
3. Cohesion: Clay particles have a strong tendency to stick to one another due to their electrostatic forces and molecular structure. This cohesion gives clay its plasticity and ability to retain water. Silt and sand do not exhibit the same level of cohesion.
Clay soils often have a distinctive colour, which can vary depending on the type of clay and its mineral composition and can vary from grey to red, more noticeable with high organic content
There is much more to clay than its particle size and shape. There are lots of crystals and chemistry...
Mineralogy
Mineralogy
Clays are a subdivision, or class, of phyllosilicate minerals - or phyllosilicates. Clay minerals are referred to as fine-grained parts of geology composed essentially of hydrous aluminosilicates. The building block for the structure are layers of silica tetrahedral sheets and an alumina octahedral sheets.
Books have been written about the structures and surface properties and the peculiarities of architecture and organisation that influence how these layers behave in relation to organic compounds. See 'The Chemistry of Clay-Organic reactions', by Theng 2024.
We will explore these as we go along and those organic compounds evolve.
There are different types of layer structures, e.g Tetrahedral (T) can alternate with or sandwich the Octahedral (O) sheet.
Clay Mineral formation
Clay Mineral formation
Clay minerals most commonly form by prolonged chemical weathering of silicate-bearing rocks. They can also form locally from hydrothermal activity. Chemical weathering takes place largely by acid hydrolysis due to low concentrations of carbonic acid dissolved in surrounding water
Clay minerals are often produced through the chemical and physical breakdown of parent rocks. Granite minerals like feldspars and micas can break down into clays but not the whole rock. Quartz, a primary mineral in granites, cannot break down into clay minerals, but ends up as sand or silt grains. Granite as a rock breaks down into smaller particles of the parent minerals, and some can chemically degrade and transform to form clay minerals in various ways. These processes involve the interaction of water, carbon dioxide, and various chemical reactions.
Feldspar the most abundant (60%) 'of the Earth's crust. It's found in many types of rocks, including Igneous rocks where: Feldspar crystallised from cooling magma, Metamorphic rocks: where feldspar can form in veins of other rocks through heat and pressure and sedimentary rocks as feldspar can weather into sediments. It reacts with water and carbon dioxide to form clay minerals and ions in solution, and along the way potassium and some silicon from the feldspar are removed in solution.
Clay lattice by
Transmission electron microscopy
Transmission electron microscopy
Lattice
Lattice
The lattice structure and degree of order and disorder is hugely variable in clay minerals.
Minerals appearing in clays generally have layer lattice structures formed from (1) uncharged layers with hydroxyl ions on both surfaces, (2) hydroxyl ions on one surface and oxygen ions on the other, (3) oxygen ions on both surfaces; (4) charged layers with oxygen ions on both surfaces; or (5) two types of layers, one with hydroxyl ions on both surfaces and the other with oxygen ions on both surfaces.
The layered structure means there is as large surface area and ample sites for cations to be attracted and held by the slightly negatively charged clay lattice. Which means that when we add - say calcium, as lime - the positive charge of the Ca2+ means it quickly locks on the clay lattice. Iron, as ferrous Fe2+ or ferric ions Fe3+ and Aluminium Al3+ become attached. These positive ions - or cations - then attract anions - negatively charged ion. Phosphate - one of the big three fertilisers, is such an anion PO4- . When applied as a fertiliser, about 3/4 of these negative phosphate anions attach themselves to the positive cations - Ca, Fe and Al on the clay lattice. This aDsorption onto the surface is weak, not like ionic or covalent chemical bonding.
As well as its layered structure, there is often a process called isomorphous substitution, that occurs when one atom in a crystal is replaced by another atom that is similar in size, valency, and electronegativity.
Mineral Properties
Mineral Properties
Water: Many clays, but not all, have excellent water-retention properties. It can hold a significant amount of water within its structure, which is why clay soils tend to retain moisture for extended periods. In contrast, sand and silt have lower water-holding capacities and drain more quickly. There are some clays called 'swelling' clays like montmorillonites and vermiculites, which take water into the inter-lattice spaces and the 'c' direction lattice spacing gets bigger, and there are 'non-swelling' clays which do not physically swell, they just hold water.
The thickness of this Smectite clay TOT layer is 0.96 nm and the interlayer space can range from zero to complete separation as cations like aluminum, silicon, magnesium are attracted to the oxygen in water.
Electrical Charge: Most clay particles have a both positive and negatively charged surfaces. If you imagine a sheet, then the negative is the sheet's surface and the positive the sheet's edges. Overall soil clay minerals tend to be negatively charged, thus attracting positively charged ions (+) on their surfaces by electrostatic forces. Called cations, they include Calcium (Ca++), Magnesium (Mg++), Potassium (K+), Sodium (Na+), Ammonium (NH4+), and Hydrogen (H+) the most common in soil). As a result, the cations remain within the soil and are not easily lost through leaching. The adsorbed cations may easily exchange with other cations in the soil solution, hence the term "cation exchange." The adsorbed cations replenish the ions in the soil solution when concentrations decrease due to uptake by plant roots.
CEC measures the total negative charges within the soil that adsorb plant nutrient cations such as Ca2+ Mg2+ & K+ and thus a measure of the soils ability to balance and hold positively charged elements, exchangeable cations. This property of a soil describes its capacity to supply (like 'K ') nutrient cations. and makes most clays fertile and suitable for plant growth as silt and sand have less CEC. While many clays have highh CEC this can vary enormously and is hugely affected by pH. Clay soils with a high CEC (<20 meq/100g) are considered inherently more fertile than lighter sandy soil with low CEC (2-3 meq/100g). Most clay minerals have a high CEC because they can adsorb and release cations readily. The structure of the clay layers (mineral property) and high surface area (particle property) provide a greater capacity to attract and hold cations.
The CEC varies with the type of clay present. The chocolate montmorillonite clay soils have the highest CEC. Highly weathered clays such as kaolinite found in kraznozem soils have variable surface charge and are generally more acidic which has the effect of lowering the CEC.
But how does CEC measure nutrients when two most important nutrients - phosphates and nitrates - come as anions?
Only indirectly. Clays with higher CEC also tend to have better water retention and buffering capacity, helping to reduce nutrient leaching overall.
Silicon
Silicon
Silicon (Si) is the second most abundant element in the Earth’s crust. We are finding more roles in both soils and plants, although it is inaccessible to plants in its native state. All silicates are soluble to some degree - that's how chemical weathering occurs. "Si alleviates various biotic and abiotic stresses in plants by enhancing tolerance mechanisms at different stages of uptake/deposition as a monosilicic acid." (Sharma et al 2023). So how does it get into the plants? Si-solubilizing microorganisms (SSMs), introduced as bioinoculants, liberate soluble Si and thus make it available to plants. Researchers are looking into the isolation and exploration of rhizosphere-colonizing SSMs and their plant growth promoting (PGP) attributes, as these microbes are increasingly being seen as a benefit of reducing fertilisers.
“Clay on land is a by-product of chemical weathering that removes carbon dioxide from the air, this reduced the amount of carbon in the atmosphere, leading to a cooler planet and a see-sawing climate…caused by the spread of land plants keeping soils and clays on land, stopping carbon from being washed into the ocean, and by the growth in marine life using silicon for their skeletons (Kalderon-Asael et al 2021)
'Keeping clays on land' ?
‘Keeping soils and clays on land’. What does that mean? This was to be the vital phase in the origins of soil, to stabilise clay movement. A practical absence of organic detritus and fungi would have allowed the clay to move when water systems would have been 'flashy', with heavy flooding during storms, as probably occurred in mudflats in Mid-Carboniferous There would not be much binding things together, so there would have rapid physical erosion and instability of the surface, exacerbated by slope gradients. Temperatures at the surface would have fluctuated violently with changes in insulation. Shallow soils would have brought stability.
Thec lay mineral particles consist of platelets layered together. Clay minerals carry an electrical charge on their surface, giving them the plasticity and cohesion. The layer charge controls behaviour with water - and pH and the presence of flocculants, (see below), can change plasticity and cohesion. Clay soils are often rich in nutrients, as that electrical charge holds other charged particles, like calcium and potassium – and pesticides, like paraquat.
Weathering
Weathering
Clay size particles are formed by weathering, involving physical disaggregation and abrasion along with chemical decomposition.
Many soil clay minerals are the products of the weathering of many pre-existing aluminosilicate minerals – e.g., typically in UK soils illites (a clay mineral that is a type of phyllosilicate, or layered alumino-silicate ) and vermiculites dominate and are formed from from the weathering of micas and chlorites in many of the UK’s metamorphic bedrocks. Feldspars are not the only source of soil clays. Soil clay mineralogy varies hugely dependant on bedrock composition and environment – tropical soil clays are very different to higher latitude soil clays.
There had been clay particles around 'forever', and they were found as 'mud' ever since water arrived on Earth. Over millions of years, these sediments underwent compaction and lithification processes, eventually forming sedimentary rocks such as shale, siltstone, and sandstone. Claystones are the most abundant sedimentary rocks.
Weathering is not simply a chemical and physical process as the presence of organisms and organic material helps determine the rate of weathering.
"Organic matter in clays often controls their geotechnical behaviour because of its influence on the strength and strain properties in bulk. It is integrated in the clay particle matrix and serves as a weak ductile component to an extent that depends on the degree of decomposition, which is a function of the moisture conditions, temperature and microfauna. Nematodes and arthropods, bacteria and fungi feed on it and make the soil porous, allowing infiltration of air and water" (Pusch 2018). So, while ‘weathering’ had been going on for hundreds of millions of years, the addition of new organic sources will have played a role increasing clay formation around this period
Clay particles, along with colloidal mineral materials including iron, manganese, aluminium and silica rich ‘gunk’ become the bricks of the soil world. We will see how they get put to use building the citadels.
Clay clumps
Clay clumps
The various properties of clay, both in terms of particles and mineralogy, help make soil what it is. Flocculation of clay particles suspended in water, would help build sediments, while clay particles surrounded by other matter, particularly organic matter help build the soil foundations. One key soil characteristic to explain early soil formation is how the clay particles ‘clump’ together. Clay particles clump together through a process called flocculation. This occurs when the tiny, negatively charged clay particles, which normally repel each other in water, come into contact with certain substances (flocculants) or environmental conditions that neutralize their surface charges. Once the repulsion forces are reduced, the particles can come together and form larger aggregates, known as flocs.
Flocculation
Flocculation
Here’s a breakdown of how this works:
1. Charge Neutralization:
1. Charge Neutralization:
Clay particles have a large surface area with negative charges, which makes them repel each other when suspended in water.
When positively charged ions (like calcium, magnesium, or aluminium) or certain chemicals (flocculants like polyacrylamide or alum) are added, they neutralize these negative charges.
Once the repulsion is reduced, attractive forces, such as Van der Waals forces, become dominant, allowing the particles to come together.
2. Aggregation:
2. Aggregation:
After charge neutralization, the clay particles collide and form loose, irregular clusters called flocs.
The size and structure of these flocs depend on factors like the type of clay, the concentration of flocculants, and the water chemistry.
3. Bridging Mechanism:
3. Bridging Mechanism:
Some flocculants, especially polymers, work by physically bridging the gaps between particles. They bind to multiple clay particles, pulling them together into larger clusters.
4. Influence of Water Chemistry:
4. Influence of Water Chemistry:
The pH, salinity, and ionic strength of the water also affect flocculation. For example, higher salinity can lead to stronger flocculation by further reducing the repulsive forces between particles.
The particle properties, together with their mineralogy, make clay unique, but very variable. Their ability to flocculate with other materials and to aggregate with still more in complex ways are perhaps the key characteristics that lead to soil structures - and hence provide the basis for the science of soil.
How does clay react with organic matter, which evolved over hundreds of millions of years?
Let's see these clay-organic relations develop over millions of years
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