Asbestos Selected Cancers (2006) / Chapter Skim
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3 Background Information on Asbestos
3 Background Information on Asbestos
Pages 49-62
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From page 49... ...
However, in addition to those common properties, each asbestos mineral species has unique chemical and physical properties that make it distinct from the others. Details about the nature and limitations of techniques used to identify and characterize asbestos fibers will not be discussed here, but can be found in reference sources such as Spurny (1994)
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From page 50... ...
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.
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From page 51... ...
For example, consider the grain boundaries in asbestiform amphibole. Asbestos fibers typically occur as parallel bundles of fibrils (filaments consisting of individual crystals)
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From page 52... ...
is often amphibole (and not layer silicate) because the particles are formed either by growth of the original amphibole crystal in the case of acicular fibers or by fracture along weaker atomic planes in the amphibole structure.
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From page 53... ...
, and magnesium hydroxide octahedra form a sheet drawn as ball-and-stick. In chrysotile, the 1:1 units curl with the slightly smaller tetrahedral sheets to the inside, exposing an octahedral sheet to the outside of the particle.
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From page 54... ...
by cation substitution. In chrysotile, layer curvature exposes the magnesium octahedral sheet at the fiber surface, thereby reducing strain from the dimensional mismatch.
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From page 55... ...
Although most occurrences of amphibole are non-asbestiform, large deposits of some asbestiform amphiboles have been exploited commercially, particularly from deposits in South Africa, Australia, and Finland. Those that have been exploited commercially typically belong to a small subset of amphibole mineral species (riebeckite or crocidolite, grunerite, anthophyllite, actinolite, and tremolite)
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From page 56... ...
aCompositions shown for the A, B, C, and T sites are ideal simplified compositions; natural samples exhibit slight variations in composition, with typical ranges in compositional limits as shown, based on Gaines et al.
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From page 57... ...
In fact, amphibole fibers often serve as sites of precipitation in the lung, becoming coated with iron-rich material to form an asbestos body. Whether amphiboles would dissolve substantially in lower-pH physiologic fluids, as would be found in the stomach, is not known.
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From page 58... ...
· Change in particle shape or surface structures, leading to alteration in other mineral-fluid interactions · Change in fluid chemistry by release of metals and other mineral components to fluid Precipitation · Change in particle shape and/or surface structures, leading to alteration in other mineral-fluid interactions Sorption · Mineral surfaces can serve as catalysts for reactions between fluid constituents by changing their effective concentration or by changing their physical orientation to one another (latter is relevant only for molecules) Ion exchange · Buffering of activity for aqueous species, such as Na+, K+, and Ca2+ Acid-base catalysis · Mineral surfaces can transfer protons with fluid constituents Oxidation-reduction · Mineral surfaces can transfer electrons with fluid constituents tance of those properties in natural environments is well recognized, but they have not been studied in the context of the pathogenesis of cancer or other diseases by minerals.
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From page 59... ...
Of the minerals discussed here, only chrysotile is expected to dissolve substantially under most physiologic conditions, and the potential for chrysotile fibers to dissolve while amphibole fibers are far more persistent may be relevant to the relative carcinogenicity of the two main fiber types. Hume and Rimstidt (1992)
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From page 60... ...
that are loosely bound to a mineral are able to exchange with a monovalent or divalent cation in solution. The exchange capacity of the mineral is related to the proportion of exchangeable cation sites that are directly accessible by the fluid (the surface sites)
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From page 61... ...
Oxidation-reduction involves the exchange of electrons between the mineral surface and a fluid species; it results in the oxidation of the mineral site and reduction of the fluid species, or vice versa. Such processes are observed in natural environments; the mineral surface donates electrons and thereby reduces species in the fluid and commonly forms metal precipitation at structurally determined sites (Ilton et al.
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From page 62... ...
2003. Composition, Fe3+/SFe, and crystal structure of non-asbestiform and asbestiform amphiboles from Libby, Montana, USA.
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