Corundum

Figure 2.1 - Structure of Corundum: Side and Top views

image taken from http://jcrystal.com/steffenweber/gallery/StructureTypes/st4.html

Corundum has the chemical formula Al2O3 and consists of hexagonal close-packing oxygen anions, with 2/3 of the octahedral holes filled by aluminium cations. The cations occur two in a row and then skip a site, giving the structure shown in figure 2.1.

This structure gives rise to a number of crystal habits (forms). Click here for a popup showing these crystal habits. For sapphires, a six-sided barrel is common, but other forms exist. These tend to vary depending on where they are found; a result of the environments in which the crystals formed.

Properties

Colour variable: white, colourless, blue, red, green, yellow, brown, purple, pink. Also, different colours may be contained in the same crystal (colour zonation)
Lustre vitreous - adamantine
Transparency transparent - translucent
Crystal System trigonal (hexagonal) with 2/3 octahedral holes filled
Crystal Habit varies depending on the type of corundum, and where it is found
Cleavage none
Hardness 9 on Moh's Scale
Specific Gravity 4.0 - 4.1
Streak white

Corundum has a hardness of 9 on Moh's Scale, which makes it  the second hardest material known. When ground with haematite, magnetite and spinel it is used to make emery, which is a very good abrasive. Although corundum is second only to diamond in hardness, it makes a poor substitute. Diamond is over four times as hard as corundum.

Corundum is also particularly dense for a transparent mineral. Both this and its high hardness can be at least partially attributed to the strong aluminium-oxygen bond strength (511±3kJmol-1).

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