What is Riebeckite?
Riebeckite is a blue to blue-black sodium amphibole, valued for prismatic crystals, fibrous varieties and its role in alkali-rich igneous and metamorphic rocks.
Mineralogy and crystal structure
Reference formula: Na2Fe5Si8O22(OH)2. Riebeckite is monoclinic and belongs to the amphibole group. Fibrous riebeckite is historically called crocidolite and should not be cut or handled in a way that creates respirable fibres. Riebeckite is classified with silicates and has a monoclinic crystal system. Typical hardness is 5–6 on the Mohs scale and specific gravity is approximately 3.3–3.5.
Colour, lustre and habit
Typical colours include blue, blue-black, black, grey. The usual prismatic, fibrous, massive habit and vitreous, silky lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Riebeckite forms in or is associated with alkaline igneous rocks, metamorphic iron-rich rocks and hydrothermal alteration zones. Important documented localities include South Africa, Nigeria, Greenland, Russia, the United States and other alkaline-rock provinces.
Blue amphibole with a fibrous boundary
Riebeckite is attractive because the same amphibole chemistry can produce blue prismatic crystals or fibrous material with a completely different handling profile. A good specimen label should make that distinction explicit.
Collecting and care
Keep fibrous material enclosed, do not saw or grind, and use professional controls for any preparation. Handle compact crystals by the matrix and wash hands after display work.
