The complex microstructures of an anthophyllite asbestos specimen from Pelham, Massachusetts, have been elucidated by transmission electron microscopy and electron diffraction techniques. The specimen consists primarily of small amphibole crystallites that are greatly elongated in the c direction. Adjacent crystallites are crystallographically rotated with respect to each other, but are not related by a twinning operation. The anthophyllite possesses pervasive moderate chain-width disorder. Central screw dislocations are not present in the crystallites, indicating that they did not grow by a spiral growth mechanism around screw dislocations.

The grain-boundary structures between rotated crystallites have been characterized using high-resolution TEM methods. Low-angle grain boundaries may be largely structurally coherent, whereas high-angle grain boundaries are typically incoherent. Many of the grain boundaries are partially filled by talc, serpentine minerals, or chlorite. Unusual conformations of curved serpentine are present, and antigorite occurs in ordered 1- and 2-layer polytypes that are intimately intergrown with stacking-disordered antigorite. The curvature reversals in some areas of antigorite are non-periodic, with spacings much larger than those usually observed.

The microstructures strongly suggest that the primary mechanism of fiber formation in this asbestos is separation along the grain boundaries between individual crystallites. This process may be enhanced by the pervasive presence of sheet silicates along the grain boundaries. However, this is not the only mechanism of fiber formation that can occur in amphiboles, since in some other anthophyllite specimens crystals split preferentially along (100) stacking faults and (010) chain-width errors. Disaggregation along such planar defects may thus be an important secondary mechanism of fiber formation in commercial amphibole asbestos.

The correlation of chain-width errors with the asbestiform habit may result in part from the cellular structure of amphibole asbestos: hydrothermal fluids that produce these defects could diffuse much more rapidly along the incoherent grain boundaries than they could through the bulk structure of massive amphiboles. Rapid fluid conduction along these cellular grain boundaries can also account for the abundance of sheet silicates as grain-boundary fillings.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.