Asbestiform riebeckite (crocidolite) from the Marra Mamba Iron Formation (Hamersley Basin, Western Australia) was investigated using high-resolution transmission electron microscopy (HRTEM). Crocidolite asbestos consists of bundles of fibers 0.03 to 0.5 μm in diameter that are slightly rotated with respect to each other. The fibers contain wide-chain pyriboles and sparse single-chain structures. The only sheet silicate intergrown with crocidolite has a mica structure; the Fe- and Na-rich composition of crocidolite presumably accounts for the absence of serpentine and chlorite.

Some fibrous crystals are polygonized to subgrains that are slightly rotated relative to one another; individual subgrains are connected by coherent wide-chain pyriboles at (010) interfaces. Many fibrous crystals contain (110) and (100) planar defects across which crystal misorientation occurs; such areas commonly contain Burgers vectors parallel to a*, implying that edge dislocation components are associated with these planar defects. The dislocations seem to have formed during initial crystallization, and they apparently serve as “seeds” for planar defects that eventually developed into subgrain boundaries. Imposition of tectonic stresses on defect-rich and polygonized crystals would favor the further development of fibers by separating crystals along a variety of defects and subgrain boundaries. The abundant planar defects that are extended along the fiber axes enhance the tensile strength of crocidolite fibers by mitigating the propagation of cracks and by providing sites for interplanar slip.

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