Riebeckite forms in two very different habits. The darker-colored forms which are individually crystallized are generally of igneous origin, such as volcanic rock and pegmatites

. The finely fibrous variety, known as Crocidolite, usually originates from altered metamorphic rocks.

What mineral is crocidolite?

Crocidolite, also called Blue Asbestos, a gray-blue to leek-green, fibrous form of the amphibole mineral riebeckite. It has a greater tensile strength than chrysotile asbestos but is much less heat-resistant, fusing to black glass at relatively low temperatures.

How is riebeckite formed?

riebeckite, a sodium-iron silicate mineral [Na₂Fe²⁺₃Fe³⁺₂Si₈O₂₂(OH)₂] in the amphibole family. It forms part of a solid-solution series that includes both magnesioriebeckite (formed when iron is replaced by magnesium) and glaucophane (formed when iron is replaced by magnesium and aluminum).