Corundum is a mineral that belongs to the group of oxides, it is an aluminum oxide whose chemical formula is Al2O3, where 52.9% of the volume is Al, 47.1% is O, it is a primary mineral in igneous rocks, it originates from regional and contact metamorphic zones and is used as an abrasive and gemstone.
Characteristics and physical properties
| corundum | Characteristics and physical properties |
|---|---|
| group, class | Oxides (Aluminum Oxide) |
| Chemical formula | Al2O3 |
| Chemical composition | Al= 52.9%, OR=47.1% |
| Cleavage or exfoliation | does not have, does not present |
| mohs hardness | 9. |
| Density or specific weight | 4.02 g/cm3. |
| shine or luster | Adamantine to vitreous. |
| Color | Usually brown, pink or blue. It can be white, green, gray, sapphire blue or ruby red. |
| Stripe | colorless. |
| Fracture | conchoidal. |
| crystalline system | Hexagonal. |
| Origin and geological environment | Regional and contact metamorphism, primary mineral in igneous rocks. |
| associated rocks | Igneous (syenites, nefeliniferous syenites, pegmatite), sedimentary (rounded gravel, sandstone) and metamorphic (micaceous schists, marble and gneiss) |
| Associated minerals | Chlorite, mica, olivine, serpentine, magnetite, spinel, kyanite and diaspore. |
| varieties | sapphire, ruby |
| Applications | As abrasive, gemstone, watchmaking, scientific instrument bearings, refractories and coatings. |
The corundum in its pure state it is colorless, due to the presence of metals it presents different varieties. It is usually found brown, pink or blue. Although it can also be white, green, gray, sapphire blue or ruby red.
Due to the presence of chromium in the corundum produces a strong red color that is known as ruby, the presence of iron together with titanium produces sapphire blue.
It can also be in a gray rock, known as emery, an emery is not a mineral because it has magnetite, hematite and spinel, an emery is a black granular corundum.
The corundum of other colors it is known as “oriental” added to other gems. As the oriental amethyst is purple, the oriental emerald is green, and the oriental topaz is yellow.
The crystals have a conical hexagonal prismatic or pyramidal habit (they can be rounded like a barrel), they generally have very deep horizontal striations, their faces are usually rhombohedral.
Its hardness is very high, it is used on the Mohs scale to represent the hardness of 9, it has a relatively high specific gravity of 4.02 g/cm3 for a non-metallic mineral.
It has an adamantine to vitreous luster, is transparent to translucent, the streak color is colorless, and it is harder than streak plate.
Types of corundum
In nature there are basically two varieties of corundum, and they are divided according to color: corundums that have a blue color are called sapphires, while corundums that have a red color are called rubies.
Blue corundum stones
Corundums that have color or shades of blue are called sapphires.
These sapphires also retain all their physical and chemical properties and distinctive characteristics, however they are blue in color due to iron and titanium impurities in their crystal lattice.
Red corundum stones
The corundums that have a red color are those that are called ruby stones, they also retain their physical and chemical properties and distinctive characteristics, however their color is due to chromium impurities in their crystal lattice.
Origin, formation and geological environment.
The corundum It is found as a primary mineral in igneous rocks with low silica content such as syenite, nepheline syenites and pegmatite rocks. They can be found in large masses in the zone of separation of the enclosing peridotites.
This mineral is also found in metamorphic rocks, in areas of regional metamorphism such as schists, gneiss and marbles, and in areas where aluminous bauxites or shales were exposed to contact metamorphism.
High-quality rubies and sapphires originate from marbles in subsurface magmas along the edges.
The most important ruby and sapphire deposits on the planet are found in slope soils and sediments as a result of resistance to basalt flows.
Due to its relatively high hardness of 9 and chemical resistance has allowed it to resist weathering much more than other minerals, they are found in rounded gravels in detrital soils and sands, which is why they are concentrated in alluvial deposits.
How to identify corundum?
As mentioned before the corundum when it is pure it is colorless. Due to impurities, it can present different shades, among the best known are ruby red, sapphire blue.
An important characteristic of this mineral is its high hardness of 9, very close to that of diamond on the Mohs scale, and its relatively high density for a non-metallic mineral of 4.02 g/cm3.
It is basically identified because it has an adamantine to vitreous luster, due to its crystals with a conical hexagonal prismatic or pyramidal habit (with a rounded barrel shape), they usually present deep horizontal striations and have a conchoidal fracture.
Corundum uses
- The uses associated with this mineral are usually as a gem
- The dark red ruby is one of the most valuable gems in the world, second only to the emerald.
- Sapphire blue is another precious gem which is highly prized.
- Gems such as quality stones are used in watchmaking and bearings in scientific instruments.
- It is used as an abrasive due to its high hardness, either taken from pure solid material or from its impure form such as emery.
- Corundums are chemically inert and heat resistant, producing: Refractories, refractory bricks.
- Liners for ovens and furniture for ovens.
Like precious stone
Due to the excellent transmission and scattering of light that occurs through this mineral, in addition to its hardness, resistance to wear and its striking colors, it is extremely valued as a precious stone in the high-level jewelry industry.
In fact gems and precious stones made of corundum are very valuable in the market and would make a gift of high value.