The structure of Bolivian crocidolite

Bolivian crocidolite, a fibrous amphibole, is shown to contain fibres approximating to single crystals. The structure, which is determined by standard methods leading to a Fourier projection on (001), is shown to be generally similar to that of tremolite. The more precise determination reveals certain departures from that structure however. The silicate chain is found to bend away from the plane of the metal ions in a lateral direction. The Si-O bond length when the oxygen atom is bonded to one silicon atom only is found to be less than that when the oxygen atom is bonded to two silicon atoms. The distribution of the magnesium and iron atoms among the available atomic sites is found to be non-uniform and is discussed with reference to the charge distribution.