Clays consist mainly of a group of crystalline substances known as the ”clay minerals”. They are all essentially hydrous aluminium silicates. Well known minerals are of as under:
(1). Kaolinite: Al2O3 2SiO2
Physical properties of kaolinite are as under;
- Crystal System: Triclinic crystal system.
- Cleavage: Perfect basal cleavage.
- Hardness: 2 at Moh’s Scale
- Sp.gr: Specific gravity of Kaolinite is 2.6.
- Lustre: Kaolinite has dull earthy lustre.
- Colour: Kaolinite has generally white colour, but sometimes brown or gray.
- Streak: Kaolinite gives a white streak on eroding by a streak plate.
- Habit: Kaolinite is found in massive clay habit.
Kaolinite is a secondary mineral formed by the alteration of alkali felspars. It is the chief constituent of clay.
Uses of Kaolinite:
- Kaolinite is used as a filler in paper.
- Kaolinite is also used in the manufacturing of ceramics.
(2). Tale: Mg3Si4O10(OH)2
Physical properties of tale are as under:
- Crystal System: Tales have a monoclinic crystal system.
- Cleavage: Like Kaolinite, the tale also has a perfect basal cleavage.
- Hardness: The tale is softer than Kaoline. It has 1 hardness on Moh’s scale.
- Sp.Gr: The specific gravity of the Tale mineral is 2.7-2.8.
- Lustre: Tale has both pearly and greasy lustre.
- Colour: White, gray, green, or dark-brown.
- Steak: Like Kaolinite, the tale also gives white streak on rubbing with streak plate.
- Habit: Unlike the kaolinite, it has a foliated mass habit. The compact and massive variety of tale is known as ”steatite” or ”soapstone”.
- Feel: Greasy
Tale is a secondary mineral formed as a result of the alteration of olivine, pyroxene and amphibole. It is often derived from altrabasic igneous rocks. It is also found in some schists in association with actinolite.
Uses of Tale:
- Tale is used in Talcum powder.
- Tale is used as a filler in paint, paper and rubber.
- It is also used ceramics and electrical porcelain.
Other Groups of Clay Minerals:
Clay minerals are subgrouped in the following categories.
- Kaolin group: Kaolin group, which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al 2Si 2O 5(OH) 4).
- Some sources include the kaolinite-serpentine group due to structural similarities.
- Smectite Group: Smectite group which includes dioctahedral smectites, such as montmorillonite, nontronite, and beidellite, and trioctahedral smectites, such as saponite. In 2013, analytical tests by the Curiosity rover found results consistent with the presence of smectite clay minerals on the planet Mars.
- Illite Group: Illite group which includes the clay-micas. Illite is the only common mineral.
- Chlorite Group: Chlorite group includes a wide variety of similar minerals with a considerable chemical variation.
- Palygorskite: Palygorskite or attapulgite is a magnesium aluminum phyllosilicate with the chemical formula (Mg,Al) 2Si. 4O. 10(OH)·4(H. 2O) that occurs in a type of clay soil common to the Southeastern United States.
- Sepiolite: It is another clay mineral, which is a hydrous magnesium silicate formed by the precipitation of near-surface brackish or saline waters, under semi-arid climatic conditions.