Al2SiO5/Polymorphs Group of Minerals:
The three important minerals of this group are ;
These minerals occur in metamorphosed luminous rocks, such as pelitic schists.
(1). Andalusite (Al2SiO5):
Physical properties of Andalusite:
- Crystal System: Andalusite occurs in orthorhombic crystal form.
- Cleavage: Andalusite has a poor cleavage.
- Fracture: Uneven fracture
- Hardness: Andalusite has a hardness of about 7.5 on Moh’s scale.
- Colour: It has flesh red, reddish, brown, or olive green colours.
- Habit: Prismatic crystals.
Andalusite occurs typically in thermally metamorphosed argillaceous schists and in regionally metamorphosed rocks formed under low pressure conditions.
Uses of Andalusites:
Andalusite is used in the manufacture of porcelains of a high refractory nature.
(2). Sillimanite (Al2SiO5):
Physical properties of Sillimanite:
- Crystal System: Sillimanite occurs in orthorhombic crystal form.
- Cleavage: Sillimanite has perfect cleavage.
- Fracture: even fracture
- Hardness: Sillimanite has a hardness of about 6-7 on Moh’s scale.
- Luster: Sillimanite has a vitreous luster.
- Colour: It has brown, or pale, green, or white colours.
- Habit: Long slender crystals, often in parallel groups, frequently in fibrous.
Occurrence of Sillimanite:
Sillimanite occurs in schist and gneiss of high-grade metamorphism.
Uses of Sillimanite:
- Sillimanite minerals are mainly utilized in the production of mullite or high-alumina refractories. About 95% of the world’s consumption of these minerals is made for this purpose.
- The extracted aluminum is used as metals and alloys.
- Sillimanite is used to manufacture glass, ceramics, and cement.
(3). Kyanite (Al2SiO5):
Physical properties of Kyanite:
- Crystal System: Kyanite occurs in triclinic crystal form.
- Cleavage: Kyanite has perfect cleavage (100).
- Fracture: even fracture
- Hardness: Kyanite has a hardness of about 6-7 on Moh’s scale.
- Specific Gravity: 3.55
- Luster: Kyanite has a vitreous to pearly luster.
- Colour: It has blue, white and green colours.
- Habit: Flate bladded crystals and bladded aggregate.
Occurrence of Kyanite:
Kyanite occurs in regional metamorphic schist and gneisses together with garnet, staurolite, mica and quartz.
Uses of Kyanite:
- Kyanite is used in the manufacture of refractories, like the bricks, mortars, and kiln furniture used in high-temperature furnaces.
- For foundries, the molds that are used for casting high-temperature metals are often made with kyanite.
- Used in ceramics and as gemstone.
- It is also used in automotive and railroad industries, where high resistance to heat is required.
- Brake shoes and clutch faces uses a specific type of kyanite known as Mullite.
- Used in vehicle ignition plugs as an insulator, which could resist high level of heat/temperature.
- It is also used as an abrasive material for cutting and grinding metals and stones.
- Used in manufacturing of porcelain tiles, and other ceramic utensils.
- Kyanite has two types of hardnesses, one is 4.5, which is parallel to the length of the blade, while the other is 7.0 across the length of the blade. It is very difficult to cut the kyanite. Skilled cutters are required to cut and shape these stones.