The key difference between kaolinite and illite is that kaolinite is able to absorb a low amount of water whereas illite is able to absorb more water than kaolinite.
Kaolinite and illite are two types of clay minerals. These fall into the phyllosilicate category. Both these materials have tetrahedron-octahedron structure combinations in the crystal structure.
CONTENTS
1. Overview and Key Difference
2. What is Kaolinite
3. What is Illite
4. Similarities – Kaolinite and Illitee
5. Kaolinite vs Illite in Tabular Form
6. Summary – Kaolinite vs Illite
What is Kaolinite?
Kaolinite is a clay mineral having the chemical formula Al2Si2O5(OH)4. It is well-known as an important industrial material/mineral. We can identify kaolinite as a layered silicate mineral having one tetrahedral sheet of silica (SiO4) that is linked through oxygen atoms to one octahedral sheet of alumina octahedra.
Figure 01: Kaolinite
The category of kaolinite is “phyllosilicates,” and its crystal system is triclinic. The space group of this crystal structure can be given as P1. It appears in white to cream color, but it can get red, blue, or brown tints due to the presence of impurities. When considering its crystal habit, it rarely occurs as crystals. It can often be found as thin plates or stacked. It can be commonly found as microscopic pseudohexagonal plates. Kaolinite is flexible in its tenacity but inelastic. Its hardness can be given as 2-2.5 on the Mohs scale. The luster of kaolinite is pearly to dull earthy. It has a mineral streak color white.
Rocks having a high content of kaolinite are typically known as kaolin or china clay. Occasionally, it is known by the ancient Greek name lithomarge, which means “stone of marl.” At present, the term lithomarge is used to refer to a compacted, massive form of kaolin.
The shrink-swell capacity of kaolinite is low, and it also has a low cation exchange capacity. Moreover, it is a soft, earthy, usually white mineral substance that is produced by the chemical weathering of aluminum silicate minerals such as feldspar.
When compared with other clay minerals, kaolinite is chemically and structurally simple. We can describe it as a 1:1 clay mineral because it has crystals with stacked TO layers (Tetrahedral-Octahedral layers). Each TO layer contains a tetrahedral sheet consisting of silicon and oxygen ions that are bonded to an octahedral sheet consisting of oxygen, aluminum, and hydroxyl ions. In each T layer, one silicon atom binds to four surrounding oxygen atoms, which forms the tetrahedron. In the O layer, an aluminum atom binds to six oxygen atoms that surround it, making the octahedron.
There are many different uses of kaolinite such as in ceramic manufacturing, toothpaste production, light-diffusing material in white incandescent light bulbs, and as an industrial insulation material. Also, it is used in the production of cosmetics, paints, adhesives, etc.
What is Illite?
Illite is a type of clay mineral having the chemical formula (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]. It can be categorized as a mica-phyllosilicate. It has a monoclinic crystal structure, and its crystal class is prismatic(2/m).
Figure 02: Illite
The appearance of illite is grey-white to silvery-white, and the crystal habit can be described as micaceous aggregates. The hardness of this mineral is 1-2 on the Mohs scale of hardness. It displays pearly to dull luster, and its mineral streak color is white. Moreover, illite is translucent, and it has a specific gravity that ranges between 2.6-2.9.
When considering the structure of illite, it has a 2:1 sandwich structure of silica tetrahedron (T) and alumina octahedron (O). The structure comes as T-O-T layers. There is a space between the T-O-T sequence of layers that is occupied by poorly hydrated potassium cations. These cations are responsible for the absence of swelling. The structure of this mineral is similar to that of muscovite. But the latter has more silicon, magnesium, iron, and water along with slightly less tetrahedral aluminum and interlayer potassium.
What are the Similarities Between Kaolinite and Illite?
- Kaolinite and illite are clay minerals.
- Both are phyllosilicates.
- They have a crystalline structure.
- Both have a high content of silica and alumina.
- They have tetrahedron and octahedron structures.
What is the Difference Between Kaolinite and Illite?
Kaolinite and illite are important clay minerals having various different uses. The key difference between kaolinite and illite is that kaolinite is only able to absorb a low amount of water, whereas illite is able to absorb more water than kaolinite. Moreover, kaolinite has a triclinic crystal structure while illite has a monoclinic structure. In addition, the hardness of kaolinite is higher than that of illite. However, both have the TO-layered chemical structure.
Below is a summary of the difference between kaolinite and illite in tabular form for side-by-side comparison.
Summary – Kaolinite vs Illite
Kaolinite is a clay mineral having the chemical formula Al2Si2O5(OH)4, while illite is a type of clay mineral having the chemical formula (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]. The key difference between kaolinite and illite is that kaolinite is able to absorb a low amount of water, whereas Illite is able to absorb more water than kaolinite.
Reference:
1. “Illite.” An Overview | ScienceDirect Topics.
Image Courtesy:
1. “Kaolinite from Twiggs County in Georgia in USA” By James St. John (CC BY 2.0) via Commons Wikimedia
2. “Illite” By USGS – Spec Lab (Public Domain) via Commons Wikimedia