Kaolin has special deposits, alongside kaolinites and other minerals, namely halloysite kaolin. Due to their unique and attractive chemical and physical qualities, these nano-tube white clay minerals become a promising contestant in today’s industrial and commercial, as well as nanotechnology applications.
What is Halloysite?
When you open your dictionary and look for halloysite, you will surely come across countless definitions.
One says that it is a refractory clay mineral having similar composition to kaolinites. The other states it is a mineral containing a partially dehydrated halloysite. The other describes it as a member of hydrated kaolinites.
Another explanation would be that halloysite is a 瓷器-like clay of alumino-silicate mineral with the chemical formula Al2SiO5(OH)4·2H2O, containing petite and lean tubes.
All those definitions are not wrong since halloysite is a member of the kaolin group and sub-group (kaolinites). It is an aluminosilicate clay mineral with the general chemical composition Al2SiO5(OH)4. Many industrial and commercial applications appreciate this mineral’s favorable physical qualities. Unique crystal shapes, particles, high whiteness, and high translucency are most of them.
Where is The Name From?
The first finding report for halloysites came from Angleur, a district of the Walloon city of Liège, Liège Province, Belgium. It was a Belgian geologist, Jean-Baptiste Julien d’Omalius d’Halloy (1783-1875), who first observed and discovered this mineral deposit. He was a nobleman, a pioneer of modern geology in his country, and a statesman.
The first description of halloysite was in 1826. Also, in the same year, Pierre Berthier, a French geologist and mining engineer, named the mineral after the renowned Belgium geologist’s surname, d’Halloy.
What is Halloysite Chemistry?
a. Chemical Structures
埃洛石 is a clay mineral of alumino-silicates with an approximate empirical composition of Al2Si2O5(OH)4. Based on this formula, its major components are aluminum (20.90%), silica (21.76), oxygen (55.78%), and hydrogen (1.56%).
Typical halloysite formation occurs due to hydrothermal alteration of aluminosilicates. Therefore, halloysites can present alongside the mix of kaolinites, dickites, montmorillonites, and other clay minerals. Halloysites belong to the kaolinite-serpentine mineral group. Came from two distinct grandfathers and species, halloysites represent Halloysite-7Å and Halloysite-10Å.
- The Halloysite-7Å (also called Meta Halloysite) is a dehydrated halloysite mineral with reduced inter-layer spacing characteristics.
- On the other hand, European literature gives Halloysite-7Å an alternative name of endellite. It is a hydrated mineral with a chemical composition of Al2Si2O5(OH)4·2(H2O). North American literature thus calls it “hydrated halloysites”.
By this name, halloysites have a similar chemical composition as kaolin but with the water (H2O) interpolated within its clay configuration.
b. Physical Properties
Halloysites are highly fine-grained claylike masses that typically have earthy or dull to waxy or pearly appearance. They are characteristically high in whiteness, white to tan, or occasionally greenish or bluish to grayish. Also, they can be chocolate-brownish to yellowish or reddish due to iron impurities.
This mineral has a cleavage probability of f001g (probable) and shows translucent or opaque physical transparency. Its crystal system is monoclinic. The structure is specifically tubular and spherical on the microscopic scale. The optical properties are semi-transparent and biaxial (having two optical axes) in class.
The following are other physical properties for halloysites:
- High whiteness level
- High translucency level
- Tubular construction
- Nanosize particles
- High aspect ratio nanoparticles
- Low in iron and titanium elements
What is So Special About Halloysite Kaolin?
Halloysite kaolin is a special kaolin deposit. At a molecular scale, it has a distinctive formation, namely the nano-tubes. For your information, nano-tubes are cylinder-shaped molecules constructed of layers of atoms. Therefore, halloysites are also nanosized tubular white clay minerals.
Other unique yet attractive halloysite features are, among others:
- The high level of mechanical strength (modulus)
- A special structure of lumen
- Crucial non-cytotoxicity, and
- Abilities in regulating crystallization and improving thermal stability.
Halloysite vs. Kaolinite
Both kaolinites and halloysites are members of the kaolin mineral group and sub-group. And they are highly essential. A slight difference between these two clay minerals is as follows.
Kaolinites typically have platy (plate) forms and chemical composition of Al2Si2O5(OH)4.
Halloysites have a similar chemical formula, except it is partly dehydrated and hydrated (Al2Si2O5(OH)4-2H2O). There are additional water molecules found between the layers. This white clay mineral uniquely has a tubular or cylindrical morphology.
Halloysite Kaolin: What can You do with It?
Kaolin and its constituents (kaolinites, halloysites, etc.) are highly essential industrial and commercial minerals. The following are more in-depth descriptions of some of the halloysite applications.
- Since 2005, many studies and applications have emphasized pharmaceuticals and cosmetics, i.e. halloysite, as an active agent carrier. Traditionally, halloysites have become fillers in plastics (PVC in particular) and suspension agents in paints, filters or inks.
- Not to mention the ceramic tableware industries that use these highly pure clay minerals due to their nanosize properties, high whiteness, and high aspect ratio. These producers especially seek after the halloysite tube-like shape that prevents any sag before the firing process and its translucent impact on fine porcelain and bone china.
- The naturally occurring halloysite is a promising contender in manufacturing polymer-based nanocomposites. They are widely applicable for engineering and functional uses.
- In paint industries, nano-tube halloysites offer their distinct crystal shape and high aspect ratio. Acting as a suspension agent, it is highly helpful in enhancing the adhesiveness and strengths of paints and coatings. It means that halloysite-based coatings or paints will prevent or remove any fungal development on wallboards and any barnacle accumulations on vessels.
- Halloysites in Nanotechnology
Halloysites increasingly become more influential, especially in the applications of nanotechnology. Many halloysite studies also have become a guide for mineral exploration to recognize the deposits of regolith-hosted HREE (heavy rare earth element).
The nanosized tubular halloysites deposits extracted from their natural environments offer surprisingly abundant natural nano-tubes. It is good news for nanotechnology applications since many scientists and studies have used carbon nano-tubes.
The next step in this nanocarbon experiment is to create a material that can take up approximately two tonnes of CO2 for each tonne of substance. If it turns right, they can install it in numerous CO2-producing power plants or industries. The purpose is to decrease a significant number of CO2 emissions.
To sum up, kaolin’s desirable properties make it a distinctive mineral. Tubular morphology, unique habits, and natural origin are some of those attractive properties. Naturally occurring halloysite kaolin is more cost-effective and environment-friendly. It is particularly true in comparison with artificial carbon nano-tubes.