Crocidolite mineral. Crocidolite is a dark blue asbestiform (fibrous) variety of the sodium-rich mineral riebeckite that occurs in metamorphic rock. Like all asbestos minerals, crocidolite forms strong, flexible fibres that are highly heat resistant, which can be woven into insulating fabrics and materials. The mining of asbestos minerals has declined following the discovery of their link with a fatal lung condition, now known as asbestosis. This sample measures 60 millimetres across.