Anthophyllite Asbestos: The Role of Fiber Width in Mesothelioma Induction. Part 4: Mechanistic Considerations regarding the Failure to Observe Anthophyllite Asbestos Mesotheliomas in Humans

Anthophyllite is an amphibole mineral formed through a prograde metamorphism of magnesium-rich ultramafic talcose rocks through increasing pressure and temperature and dehydration. The talc and anthophyllite are in phase equilibrium. Anthophyllite asbestos is therefore not a ‘contaminant’ of talc but a product derived from it. Fibrous talc, or so-called transitional fibers, are anthophyllite fibers undergoing retrograde degeneration. In its fibrous asbestiform state, anthophyllite differs in several fundamental ways from other commercially exploited forms of amphibole asbestos of which there are two broad families: monoclinic and orthorhombic. The more common forms of commercial amphibole asbestos such as crocidolite and amosite are monoclinic. The anthophyllites are orthorhombic. The differences between the two crystal systems are reflected at the level of the basic amphibole-structure in a greater overall fiber width dimensional profile and a significant reduction in microstructural strength. Strength reduction most probably arises at the cellular level and is particularly pronounced within the thinner population of fibers. Here microstructural differences, due in significant part to stacking defects in the basic amphibole structure, can account for these observations. The lack of an observed attendant mesothelioma risk following exposure to anthophyllite and transitional fibers in humans is a consequencel of these microstructural features that appear to differentiate them from the equidimensional monoclinic forms of amphibole asbestos such as South African crocidolite and amosite.