STRUCTURAL VARIATIONS IN CHRYSOTILE ASBESTOS FIBERS REVEALED BY SYNCHROTRON X-RAY-DIFFRACTION AND HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

Four samples of chrysotile asbestos have been studied by synchrotron X-ray diffraction, One of these samples exhibits asymmetrical 00l diffraction profiles, and also shows characteristics of ''Povlen-type'' chrysotile in X-ray fiber photographs. The 00/ peak asymmetry is interpreted as indicating the presence of two layer-spacings, one about 7.3 angstrom, in common with the other chrysotile samples, and one about 7.2 angstrom. TEM images of the chrysotile with two apparent layer-spacings show the presence of fibers with both curved and flat layers in a variety of disordered tubular structures having only approximate cylindrical symmetry. The flat layers in these fibers may have a smaller interlayer-spacing (7.2 angstrom) relative to the curved layer-spacing (7.3 angstrom) because of the possibility of increased hydrogen bonding between flat layers that are stacked in register with one another. In this sample, fibers with flat layers parallel to one direction only, joined at each end by curved layers are observed to occur commonly. Also present are fibers with polygonal-tube cores made up from flat layers, with outer layers that are curved in a more normal way. This type of structure appears to be the closest that has been observed to the alternative structural model proposed by Middleton & Whittaker (1976) for ''Povlen-type'' chrysotile. In the same specimen, cylindrical chrysotile fibers that possess 5-fold symmetry have been imaged, in which the layer stacking is apparently in register radially at 15 points around the circumference, arising from the b/3 repeat of the hydroxyl layer.