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Corundum is a crystalline form of aluminum oxide (Al2O3) and is one of the hardest naturally occurring minerals, second only to diamond. Its exceptional hardness makes it valuable as an abrasive. When transparent and of gem quality, it is known as ruby (red) or sapphire (all other colors, including blue, pink, yellow, green, purple, orange, and colorless). Impure, granular corundum is known as emery. It typically forms in hexagonal crystals, often barrel-shaped or tabular.
How to Identify
- Color
- Highly variable: colorless, white, gray, blue, red, pink, yellow, green, purple, orange, brown, black. Ruby is red, sapphire is typically blue but can be any other color.
- Luster
- Adamantine to vitreous.
- Texture
- Typically crystalline, often with a somewhat greasy feel in massive forms.
- Crystal Form
- Hexagonal system, commonly forming barrel-shaped, tabular, or pyramidal crystals. Striations parallel to the basal pinacoid are common.
- Cleavage
- No true cleavage, but exhibits parting (pseudocleavage) parallel to the basal pinacoid and rhombohedral faces due to lamellar twinning.
- Geological Environment
- Metamorphic rocks (schists, gneisses, marbles), igneous rocks (syenites, nepheline syenites, pegmatites), desilicated environments, and placer deposits.
Key Facts
- Hardness: 9 (Mohs scale)
- Specific Gravity: 3.95 - 4.10 g/cm³
- Crystal System: Trigonal (often described as Hexagonal in older texts due to its hexagonal appearance)
- Color: Highly variable: colorless, white, gray, blue, red, pink, yellow, green, purple, orange, brown, black
- Luster: Adamantine to vitreous
- Transparency: Transparent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None (exhibits parting)
- Composition: Aluminum oxide (Al2O3)
Quick Check
- Color: Variable (red for ruby, blue for sapphire, other colors, or colorless)
- Luster: Adamantine to vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Hexagonal prisms, often barrel-shaped, tabular, or pyramidal. May be massive or granular.
- Cleavage Type: None, but exhibits parting on {0001} (basal) and {1011} (rhombohedral) planes due to polysynthetic twinning.
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Adamantine to vitreous
Formation
Corundum forms under high-temperature and high-pressure conditions, typically in metamorphic rocks (e.g., schists, gneisses, marbles), igneous rocks (e.g., syenites, nepheline syenites, pegmatites), and desilicated environments. It can also be found in placer deposits due to its hardness and resistance to weathering. The absence of silica is crucial for its formation, as aluminum would otherwise combine with silica to form feldspars or other aluminosilicates.
Usage
Gemstone (ruby, sapphire), abrasive (due to its extreme hardness), watch bearings, scientific instrument components, laser technology (synthetic ruby and sapphire), and as a refractory material.
Age Distribution
Corundum forms in a wide range of geological ages, from Precambrian to Cenozoic, depending on the specific geological environment and tectonic activity.
Where to Find
Myanmar (Burma)
Famous for high-quality rubies, particularly from the Mogok Stone Tract.
Sri Lanka
A historic source of a wide variety of sapphires and rubies, often found in placer deposits.
Thailand
Known for rubies and sapphires, particularly from the Chanthaburi and Trat provinces.
Cambodia
Produces rubies and sapphires, often associated with basaltic rocks.
Madagascar
A significant source of various colored sapphires and rubies, discovered in numerous localities.
Australia
Eastern Australia (New South Wales, Queensland) is a major producer of blue and fancy-colored sapphires from alluvial deposits.
United States
Localities in Montana (Yogo Gulch for blue sapphires, Rock Creek for various colors) and North Carolina (emery and some gem-quality corundum).
Kashmir (India/Pakistan)
Historically renowned for highly prized 'cornflower blue' sapphires, though production is now limited.
Tanzania
Produces rubies (e.g., Winza, Longido) and sapphires (e.g., Tunduru, Umba Valley).
Finding Tips
Look in Placer Deposits
Due to its high specific gravity and hardness, corundum often concentrates in alluvial (placer) deposits. Look in riverbeds, stream gravels, and ancient river terraces.
Identify Associated Minerals
Corundum is often found with minerals like spinel, garnet, kyanite, sillimanite, feldspar, and mica in metamorphic and igneous environments. The absence of quartz is a key indicator in primary deposits.
Check for Hexagonal Crystal Habit
Corundum typically forms distinctive hexagonal crystals, often barrel-shaped or tabular. Look for these characteristic forms.
Test Hardness
Corundum is extremely hard (9 on Mohs scale). It will scratch almost all other common minerals, except diamond. This is a primary identification tool.
Observe Parting
While lacking true cleavage, corundum exhibits parting. Look for flat, parallel planes of separation, especially on the basal pinacoid.
Examine Luster
Corundum typically has an adamantine to vitreous luster, which can be quite bright and reflective.
Similar Rocks
Spinel
MgAl2O4
Also known as: Magnesium Aluminum Oxide
Garnet
X3Y2(SiO4)3
Also known as: Various species (e.g., Almandine, Pyrope)
Topaz
Al2SiO4(F,OH)2
Also known as: Aluminum Fluorine Silicate
Scientific Classification
- Mineral Class
- Oxides
- Group
- Corundum Group
- Crystal System
- Trigonal
- Chemical Formula
- Al2O3
- Composition
- Aluminum oxide
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