EOSC 118 Lecture Notes - Lecture 17: Corundum, Ruby, Red

• School: UBC
• Department: Earth and Ocean Sciences
• Course: EOSC 118
• Professor: David Turner

Document Summary

Corundum is an aluminum oxide that forms hexagonal barrel-shaped prisms that taper at both ends, or as thin tabular hexagonal plates. No dominant cleavage but sometimes has basal parting" and fractures in conchoidal manner. Has high specific gravity of ~4, so it is typically mined in secondary placer deposits through panning. Comes in all colours of the rainbow, but most commonly found as opaque crystals with dull colours. Al is the key for the striking colours of the gem varieties. Each al bonds with 6 o atoms in an octahedron. Corundum has low dispersion, so the value comes not from the fire, but the intense colour. Rubies usually have ~1 wt% of cr2o3. When cr substitutes in for al, absorption bands are generated in the violet & green- yellow ranges, and also a bit of blue. The red region doesn"t have much absorption so the resulting colour is red.