trigonal Oxides
Corundum (Barrel)
Formula Al2O3
Crystal Structure
#! Species: Corundum
#! System: Trigonal (-3m)
#! Habit: Barrel-shaped with horizontal striations (Mogok type)
trigonal[-3m]:{10-11}@1.2[striation:horizontal] + {10-10}@0.6 + {0001}@0.2 | elongate(c:1.2)
trigonal
-3m
{10-11} {10-10} {0001}
Quick Facts
Hardness
9.0
Specific Gravity
3.99
Refractive Index
1.762
Optical Character
Uniaxial -
Physical Properties
- Crystal System
- trigonal
- Hardness (Mohs)
- 9.0
- Specific Gravity
- 3.99
- Cleavage
- None (parting on twin planes)
- Fracture
- Conchoidal to uneven
- Lustre
- Vitreous to adamantine
Optical Properties
- Refractive Index
- 1.762
- Birefringence
- 0.008
- Optical Character
- Uniaxial -
- Dispersion
- 0.018
- Pleochroism
- strong: Body color (darker) / Lighter shade (Blue sapphire shows violet-blue/greenish-blue; ruby shows purplish-red/orange-red)
- Fluorescence
- Variable depending on colour variety
Characteristics
Colours
All colours except red (sapphire)Red (ruby)Colourless (white sapphire)
Localities
MyanmarSri LankaKashmirMadagascarThailandTanzania
Common Inclusions
Rutile silk (three-direction needles)Fingerprints (healed fractures)Hexagonal colour zoningCrystal inclusions (calcite, apatite, zircon)Negative crystals
Known Treatments
Heat treatmentBeryllium diffusionLead glass fillingLattice diffusion
Crystal Forms
dipyramidprismpinacoid
Notes
Characteristic habit of Mogok rubies; barrel shape formed by dominant dipyramid faces
Related Minerals
Other minerals in the trigonal system
