Kaolin comes from the Earth’s continental crust. Its formations started about 80 to 100 million years ago, caused by feldspar hydrothermal weathering. Kaolin chemical composition mostly consists of aluminum silicates. The most abundant member composed of the kaolin group is kaolinites or the hydrous aluminosilicates.
What is Kaolin?
Kaolin is a clay mineral. It is also a group consisting principally of the mineral kaolinite. These minerals are the result of the hydrothermal decomposition of granite rocks formed over a million years. In addition to kaolinites, other minerals also make up the kaolin chemical composition. They are, among others, quartz, feldspar, anatase, and muscovite.
Kaolin bears two Chinese characters in its name, “Kao-Ling” or “Gaoling”. The meaning is “high ridge” or “high hill”, referring to the hill or mountain in Jingdezhen of Jiangxi Province, China. It was the place where the first kaolin mining took place. The first discovery leads to kaolin other labels, namely china clay or white clay.
For centuries, kaolin has been the most significant component in porcelain and fine china making. It was also true even many years before its discovery in other parts of the globe. As for today’s world, kaolin still has a vital role in many industrial and commercial applications. Paper coating, rubber filler, paints, and cosmetics are some of the most common examples.
What is the Structure?
1. Chemical Composition
Some indicators for kaolin formation are volcanic rocks (volcanics) and hydrothermal alteration. These naturally occurring deposits of minerals mainly consist of the mineral kaolinite. Other accompanying minerals are feldspars, quartz, anatases, and muscovites.
Kaolin is a hydrated aluminum silicate mineral with its known chemistry symbolized as Al2Si2O5(OH)4. It is a mineral in silicate layers, with one tetrahedral silica sheet (SiO4) through oxygen atoms, linked to one of the octahedral alumina sheets (AlO6).
In more detail, kaolin has the composition of richly 1:1 clay mineral of the Al2Si2O5(OH)4 for every alumina-silicate. The structure states that kaolin contains silicate sheets (Si2O5) with a close bonding with aluminum oxide (hydroxide) layers (Al2 (OH)4). Together they produce what-so-called gibbsite layers.
As an aluminum oxide mineral, gibb sites have the same (or similar) structure with aluminum layers in kaolin. They form, alongside silicates, paired layers (s-g layers). These s-g layers create tightly yet weak bonding that accounts for the softness and cleavage properties of kaolin.
In the ceramic industry, the chemical formula for the mineral kaolinite is in oxide terms, Al2O3·2SiO2·2H2O. As a pure clay mineral, the ablaze chemistry for kaolin is one (1) part Al2O3 and two (2) parts SiO2.
Since Al2O3 is fundamental chemistry to the vast majority of coating (glazes) applications, kaolin is undoubtedly an ideal source of the material. Their secret of plasticity comes from their hydrated raw clay crystals, with their 12% crystal-bound water properties.
2. Chemical Analysis
A chemical analysis expresses the chemistry of raw materials. Also called a percentage analysis, it typically states the oxide content of the materials in percent. Each number will represent the oxides in their proportion-by-weight of the raw materials.
Generally, there are ten (10) primary oxides as parameters such as silicon dioxide, aluminum oxide, iron oxide, potassium oxide, sodium oxide, and more. They will have their spec measured by various analysis methods. To name some of them are X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Spectrophotometry, Atomic Absorption Spectroscopy (AAS), or Gravimetric.
The following is an example of kaolin chemical analysis by the XRF method.
| Parameter | Unit | Spec | Method |
| Silicon Dioxide (SiO2) | % | 45 – 47 | XRF |
| Aluminium Trioxide (Al203) | % | 33 – 39 | XRF |
| Iron Trioxide (Fe2O3) | % | 0.50 – 1 | XRF |
| Titanium Dioxide (TiO2) | % | 0.25 – 0.37 | XRF |
| Magnesium Oxide (MgO) | % | 0.01 – 0.10 | XRF |
| Calcium Oxide (CaO) | % | 0.01 – 0.07 | XRF |
| Sodium Oxide (Na2O) | % | 0.01 – 0.17 | XRF |
| Potassium Oxide (K2O) | % | 0.25 – 0.80 | XRF |
| Magnesium Dioxide (MnO2) | % | Less than 0.01 – 0.05 | XRF |
| Chromium Trioxide (Cr2O3) | % | Less than 0.01 – 0.05 | XRF |
Source: Company Profile Yukami 2021
3. Physical Properties
In its natural state, kaolin is odorless, white to yellowish, and soft clay minerals. It contains most kaolinites and other various associative minerals like quarts, muscovites, feldspars, and anatases. Kaolin basic properties are as follow:
| Property | Value | Unit |
| Hardness (Mohs) | 2.0 – 2.5 | [-] |
| Refractive index | nα = 1.553–1.565
nβ = 1.559–1.569 nγ = 1.569–1.570 | [-] |
| Relative Density | 2.16 – 2.68 | [kg/dm3] |
Source: Kaolinite – Wikipedia
As such, some of the further kaolin basic physical characteristics are:
- odorless, but creates an earthy odor when moistened or wet
- relatively low in hardness
- has an approximately 740-1785°C temperature
- low in electrical conductivity
- easy to moistened or wet and showing clay-like properties
- high level of inertness in different media
- it has layers as its structure
- unsolvable in water but darkens
Regarding kaolin chemical composition analysis, it will also present the mineral physical properties. Physical analysis usually will show the brightness level, moisture content (MC), acidity (pH), loss on ignition (LOI), viscosity, and more.
4. Other Quantitative Minerals
In addition to the most abundant kaolinites, other minerals are also composing the kaolin group. They are dickites, smectites, illites, nacrites, halloysites, and many more.
The four minerals, namely kaolin, dickite, halloysite, and nacrite are polymorphs. It means that they have the same chemistry but in different structures. Those four mineral formations are mostly from the alteration (weathering), namely the feldspar decomposition.
How to Get It?
You need the mining process to obtain this white clay mineral deposit that has its natural occurrence. Crude or raw kaolin sometimes is yellowish due to the pigments of iron hydroxide.
For various industrial applications, you have to decolorize them chemically to remove the iron pigments. You can use water to rinse and detach the kaolin from other minerals. Or else, you can delaminate kaolin – dividing them into thin layers – to formulate it for other different uses.
Kaolin indeed deposits go through numerous further processes. The mineral purification steps involve crushing, drying, pulverizing, and classifying. Meanwhile, the wet processes include the kaolin slurry formation continued by selective flocculation.
Many industries use kaolin as a main essential component. Other than the ceramics industry, various kaolin-based companies mostly have billion-dollar operations. Therefore, the ideal recommendation is by performing quality control on a routine basis. It is especially to ensure the kaolin chemical composition and analysis remain consistent.
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