Crocidolite is a form of riebeckite [Na2][Fe2+3Fe3+][(OH) 2|Si8O22], which is easily apparent when you see the specimens densely-packed hair-like fibers. This variety is often called "blue asbestos" due to its deep blue-black color. This is an old-time piece from mining in South Africa. Fortunately, it was not used much commercially in the U.S., because it turned out to be perhaps the worst perpetrator of asbestos lung disease. Ex. Mullane Collection.