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Asbestos: Selected Cancers
relationship between mineralogic properties and pathogenesis remains incomplete.
The concept of mineral species is fundamental to mineralogy. A mineral species is a crystalline solid with a specific atomic structure and a specific chemical composition (or compositional range). The specific crystal structure and chemical composition of each mineral species imparts a unique set of properties, including how the species interacts physically and chemically with its environment. In a system paralleling that for the plant and animal kingdoms, mineral species are classified hierarchically. A mineral group is roughly equivalent to the family classification and consists of minerals with similar compositions or structures. Minerals may also exhibit variability within a species with respect to a particular property. For example, some mineral species may occur with an asbestiform habit (physical form) or a non-asbestiform habit. Those are typically not given distinct mineral-species names but instead are referred to as varieties of the same species; sometimes, they are given varietal names, as in the case of crocidolite, which is the asbestiform variety of the mineral species riebeckite.
Other mineral groups may have species with occasional asbestiform varieties, but the primary mineral groups for asbestos are amphibole and serpentine. Each species of these groups has a distinct crystal structure, but chemical compositions vary between species within the group. The principal mineral species constituting asbestos are detailed below; they include asbestiform serpentine (chrysotile) and asbestiform varieties of amphibole, such as tremolite, actinolite, anthophyllite, grunerite, riebeckite (also known as crocidolite), winchite, and richterite. Table 3.1 lists the mineral species, varietal names, and mineral groups associated with the common asbestos minerals. Although the three chrysotile mineral species all have the same ideal chemical formula, these polymorphs (or polytypes) differ in the nature of the stacking relationship between successive layers, with clinochrysotile being the most abundant type (Gaines et al. 1997).
“FIBROUS” AND “ASBESTIFORM”
Many minerals may occur as small particles, including particles in the respirable size range, which is less than about 10 μm in aerodynamic diameter. Of these, some may include particles with aspect ratios (length: diameter) of 5:1 or more, usually reflecting a characteristic of the underlying crystal structure. For example, asbestiform amphiboles have fibers that are elongate parallel to the underlying silicate chains in the structure.
Fibrous is a term applied to minerals that consist of fibers, that is, exhibit a large aspect ratio. Although the minimal aspect ratio of a mineral fiber may be debated, for the purpose of definition observed aspect ratios in general are very large (for example, over 5:1 and sometimes over 100:1).