Indexing and Interpretation of Zone-Axis SAED Patterns of Amphibole Asbestos Minerals in the Asbestos Analysis by Transmission Electron Microscopy

Abstract

The analysis of asbestos minerals below optical resolution is conducted by the analytical transmission electron microscopy (TEM). The Asbestos Hazard Emergency Response Act (AHERA) protocol requires the measurement of three properties: the morphology by the TEM imaging, the elemental composition by the energy dispersive X-ray (EDX) spectroscopy, and the crystal structure by the zone-axis selected area electron diffraction (SAED) pattern. Among the three categories of analysis, the most challenging one is the SAED pattern analysis. After the acquisition of a zone-axis SAED pattern with EDX-derived mineral identity, the practical and effective methodology of indexing and interpretation is the comparison of the measured (observed) values of the d-spacings of two intersecting direct lattice planes and their interplanar angle against the respective reference values in precalculated d-spacing and interplanar angle lookup tables (d-θ table) based on the mineral's unit cell parameters and symmetry. This paper presents a detailed procedure of the indexing and interpretation—that is, the determination of the Miller indices of (hk0) and (hkl) lattice planes and the determination of the corresponding zone axis—the intersection line of (hk0) and (hkl). This is applicable not only to the traditional airborne asbestos analysis but also to all asbestos analysis by TEM, such as asbestos in water, dust, bulk, and human tissue samples. Pertinent issues in the execution of the procedure are also discussed in detail.