Composition and properties of clays

Anyone who has tried it will tell you: clays are very versatile natural remedies! Whether it is to relieve a tendonitis in a poultice, to clean a wound, to fight a gastralgia by oral way or to make a small beauty with a mask... clays work, they even run. But where do the multiple properties of clays come from? What are the mechanisms that make them so powerful? How do they act on the body? And what major properties do they offer us?

Where do clays come from?

Rock, mineral, what about clays?

In our everyday language, when we talk about clay, we immediately think of this earth or this rock with a sticky and malleable aspect, such as modeling clay, in contact with water...

If we examine these rocks more closely under the microscope, we will be able to distinguish the constituents of the rock i.e. the clay minerals. They are invisible to the naked eye and even to a magnifying glass, but these minerals contained in clayey rocks are called "phyllosilicates" in reference to their organization in sheets, from the Greek phyllon, and their silica composition. To the naked eye they are then organized in superimposed lamellae. It is these minerals that give the rock its special properties...

By abuse of language, we speak of clays but keep in mind that it is indeed the clay minerals that interest us!

The clays, these majority constituents of our soil...

Clays are permanently produced by the Earth. Our Earth, and particularly its crust, has undergone real evolutions forming various minerals over time. Among them, the most known and found in abundance are feldspar (60%), the magnesium-iron group (17%), quartz (12%), or micas (4%) to name only the best known. Via mechanical but also chemical processes, these rocks that make up the earth's crust produce clays.But how?

The alteration of rocks, also called erosion, can be due to the mechanical action of wind or water and even temperature. The phenomena of freezing and thawing disintegrate the rocks by splitting them into particles. In addition, rainwater, sometimes acidic, causes a reorganization of the rock via chemical reactions.

Thus, weathering causes the erosion of the eruptive rock, particularly of the feldspars, thus creating a sedimentary rock of which our famous clays are a part! We find them nowadays in the form of deposits, more or less extensive and of quite different colors... As an example, the Winikunka mountain, also called rainbow mountain, which is found in the heart of the Andes Mountain Range in Peru.

Clays are truly witnesses to the evolution of the earth's crust...

Composition and classification of clays

We often talk about clay... But should we talk about clay or clays?

Under this general term of clay, often used, hides in reality a numerous family...
Clay, or should I say all clays, are earthy sedimentary rocks resulting from the decomposition of mineral species. They are all composed of alumina silicates on which minerals from the environment have been grafted. The different composition of the latter offers to the clays their variety of colors!
White, Green, Red and even Blue... they make us dream in colors!

3 families of structural clays

Composed mostly of alumina silicates, the clays still have some pretty big differences in getting to their respective colors...

Depending on the minerals it possesses but also on its structure in sheets, the clay is classified in a very particular clay family. The conformation of the minerals is done in the form of sheets of the order of the nanometer. Imagine that with a microscope you could distinguish different layers of superimposed flakes... The composition of these flakes, their thickness, their structure define the clay family and more precisely the type of clay we are dealing with.

Let's not forget that clays are mostly composed of alumina silicates... Thus, the sheets are formed of two types of layers, either they are composed of silica (SiO4), or they are composed of alumina (Al2O3). In the first case, silica and oxygen (O) form a tetrahedron, i.e., a volume with 4 faces. In the second case, 8-sided octahedra are composed of alumina in the center, hydroxyl (H) and oxygen in the corners. In addition to these layers forming sheets, the clay structure is interspersed with interfoliar spaces which as their name suggests, are the spaces between separating the sheets.

Clay soils then fall into three structural families:

  • the 1:1 family corresponding to a tetrahedral layer overlying an octahedral layer. This arrangement repeats itself in this way... We are talking more clearly here about kaolinites in particular.
  • the 2:1 family, here an octahedral layer is literally sandwiched between two tetrahedral layers as is the case for illite, glauconite or montmorillonite for example. Special cases exist in this family, if the aluminum is replaced by another atom, as is the case for talc, or depending on the composition of the inter-pillar space.
  • The 2:1:1 family, these clays consist of two tetrahedral sheets framing an octahedral layer but in this case the interfoliar space, i.e., the space intersecting the sheets, is filled by an octahedral sheet. Chlorite has this conformation, for example.

These differences in structure and thickness will have a role on the cohesion between the layers, the stability of the clay and its ability to swell with water. For example, kaolinite swells less than montmorillonite because the latter has a disorganization in the stacking of its sheets facilitating their separation; water can more easily be lodged in the "empty" spaces.

Legend

An X-ray measurement of the structure of clays

The structure of a clay is measured by a method using X-ray diffraction. K├ęsako?

This technique is based on the diffraction of X-rays by the material they pass through. A beam of X-rays meets a material, here clay, which deviates the rays from their initial path. This deviation varies according to the thickness of the clay structure, the number of layers, etc...

Thus, by measuring the angles of the diffracted rays, it is possible to determine the clay family or even the clay we are dealing with!

Clay and water, a great love story...

Clay + water = colloidal solution

Let's take a look at a totally chemical aspect of clays... Have you ever done the test? In a container filled with water, pour a little bit of powdered clay and observe... The clay particles behave like drops of oil in water: suspended micelles form and, you'll see it's very amazing, they bind to each other like a magnet! It is the presence of minerals with negative and positive charges that causes these forces of attraction and repulsion. Thus, the clay particles naturally agglomerate but the slightest agitation of the water changes this state and then redisperses the clay in the water... We talk about colloidal solution.

Collo what? A colloid is a macromolecule or mineral that, when placed in water, does not form a solution as is the case when sugar is dissolved in water for example, but forms a suspension. How is this explained? The size of clay minerals is larger than the empty spaces left by water molecules (H2O)... While salt or sugar manage to slip into these "holes", we talk about dissolution, clay minerals are not able to do so, we talk about dispersion.

Dispersed state and flocculated state

Let's take our vocabulary one step further... When the exchange of negative and positive charges stabilizes, the clay aggregates flocculate. I flocculate, you flocculate, we flocculate...but yes of course! Flocculation corresponds to the deposit of clay that forms at the bottom but is able to redisperse when agitated.

You will have understood, two states of clay in water are observed: the dispersed state or the flocculated state! These are two reversible states except under particular conditions... heat, degradation, hydration... These states explain in particular the different reactions of the soil to climatic events. When clays are flocculated, they appear welded, they allow soil particles like sand to form very resistant aggregates, even to heavy rains. However, if the clays are dispersed, there is no "structure" of the soil as such... The clay loses its role of "cement" and the soil will be disturbed and sensitive to erosion and climatic factors.

In conclusion, a soil that is too waterlogged will deagglomerate the clays from the other mineral elements (sand, silt...). The soil will then be less stable. A too dry soil on the contrary can create faults in clayey soils and thus also weaken it. Finally, it is all a question of e-qui-libre... The desirable water content in an ideally constituted soil (sands 50% + silts 30% + clays 15% + humus 5%) must be around 15% to 20%.

Clay is the cement of... soil! The soil, the soil ok, but you will see later, the colloidal properties of clay explain its amazing therapeutic properties... (the suspense is unbearable..!)

Texture ring

For clay soils, you can estimate the clay content by making a "texture ring"... Yes, yes, yes, you will do the experiment. Take a ball of soil and knead to make a pudding.

  1. It holds? Estimate that your soil is 10% clay.
  2. It can round off? there are 15% clays.
  3. Can you close it in a ring despite some cracking? Theclays are nearly 30% present.
  4. In a final case, if the ring remains quite smooth, clays are present at 50%!

The properties of clay

Remember above we were talking about colloid, water and clay properties? Tell me yesiiiiiiii...Well we're getting there this is it! If clay can not dissolve in water, it is able to fix water by absorption, water but also a lot of things in suspension ...

Absorbent power of clays

This is one of the most important powers of clay: its absorbency! Absorption is a passive phenomenon that, like a blotter or sponge, allows clay to absorb water. We have seen that clays cannot dissolve in water, their molecules being too big to fit into water molecules... However, water can occupy the spaces available in the clay molecular structures!

We have also seen above, each type of clay is constituted differently. Thus, if we were to classify them, taking into account their different structures, the montmorillonites would have the strongest absorption power, followed by the illites and finally the kaolinites.

This absorptive capacity gives clays a power that justifies its use in many cataplasms to heal wounds by absorbing pathological fluids such as pus for example. Another use is practical in a home to absorb bad odors.

To better visualize it, imagine a clay soil drying out... it cracks! But you only need to add a little water to make it smooth and malleable again, as it is for the potter who kneads his material.

Absorbent power

The phenomenon of aDsorption is different from aBsorption. It is an active phenomenon, it manifests itself by the capture of molecules on active sites or by the existing attraction between positively charged molecules (cations) and negatively charged ones (anions).

Thus, clays are capable of fixing substances and chemical compounds to their surface. This is calculated through the cation exchange capacity of clays (CEC).

For example, studies have shown that clays attract bacteria or even toxins, proving their interest during digestive problems for example.

The company of the senses and its teams do not encourage self-medication. The information and advice delivered are taken from a reference bibliographic base (books, scientific publications, etc.). They are given for information purposes, or to propose avenues of reflection: they should in no case replace a diagnosis, a consultation or a medical follow-up, and cannot engage the responsibility of The company of the senses.