Fibrous And Asbestiform

Many minerals may occur as small particles, including particles in the respirable size range, which is less than about 10 ^m 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).

TABLE 3.1 Asbestos Minerals

Mineral Group

Mineral Species

Asbestiform Variety

Ideal Chemical Formulaa

Serpentine

Clinochrysotile

Chrysotile

Mg3Si2O5(OH)4

Serpentine

Orthochrysotile

Chrysotile

Mg3Si2O5(OH)4

Serpentine

Parachrysotile

Chrysotile

Mg3Si2O5(OH)4

Amphibole

Riebeckite

Crocidolite

Na2Fe5Si8O22(OH)2

Amphibole

Grunerite

Amosite

(FeMg)7Si8O22(OH)2

Amphibole

Cummingtonite

Amosite

(MgFe)7Si8O22(OH)2

Amphibole

Gedrite

Amosite

(MgFe)5Al2(Si6Al2)O22(OH)2

Amphibole

Anthophyllite

Asbestiform anthophyllite

(MgFe)7(Si)8O22(OH)2

Amphibole

Tremolite

Asbestiform tremolite

Ca2Mg5Si8O22(OH)2

Amphibole

Actinolite

Asbestiform actinolite

Ca2(MgFe)5Si8O22(OH)2

Amphibole

Richterite

Asbestiform richterite

Na2Ca(MgFe)5Si8O22(OH)2

Amphibole

(Alumino)winchite

Asbestiform winchite

CaNa(MgFe)4AlSi8O22(OH)2

Amphibole

Ferriwinchite

Asbestiform winchite

CaNa(FeMg)4Fe3+Si8O22(OH)2

^Simplified representation of the overall stoichiometry of a mineral species. Mineral species typically have chemical modifications, such as substitutions of similar cations and sometimes anions (common examples are Mg2+<->Fe2+ and Si4+<->Al3+). Substitutions may cause substantiated deviations from the ideal chemical formula. Limits of chemical variation are defined for each mineral species in Table 3.2.

^Simplified representation of the overall stoichiometry of a mineral species. Mineral species typically have chemical modifications, such as substitutions of similar cations and sometimes anions (common examples are Mg2+<->Fe2+ and Si4+<->Al3+). Substitutions may cause substantiated deviations from the ideal chemical formula. Limits of chemical variation are defined for each mineral species in Table 3.2.

Asbestiform refers to a subset of fibrous minerals. Among fibrous minerals, some exhibit the additional qualities of flexibility and separability (which contribute to weavability). Such minerals are referred to as asbes-tiform. Typically, asbestiform minerals also have relatively small fiber diameters (usually under 1 ^m) and large fiber lengths (such as 5-10 ^m). The asbestiform characteristics are related to properties of the underlying crystal structures, with the specific relationship according to the mineral group. For example, it has been suggested that flexibility is related to defects in the crystal structure of the asbestiform varieties of amphibole (Veblen and Wylie 1993), whereas flexibility in asbestiform serpentine (the various forms of chrysotile) may be related to the hydrogen bonding between concentric sheets of 1:1 layers, as described below.

Some mineral species have both asbestiform and non-asbestiform varieties, and these varieties may have properties beyond just their flexibility that differ. For example, consider the grain boundaries in asbestiform amphibole. Asbestos fibers typically occur as parallel bundles of fibrils (filaments consisting of individual crystals) that are bound together along grain boundaries. The material along the grain boundaries typically is not am-

phibole but rather a layer silicate, such as talc or mica. When the material is processed, fibers are produced by the breaking apart of packets of fibrils by separation along the structurally weaker grain boundaries, which allows the layer-silicate material to become the surface of the fiber. It is this crystalline material that interacts with the biologic system after inhalation or ingestion. In contrast, the surface of a non-asbestiform variety of amphibole (either an acicular crystal or a cleavage fragment) 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. Hence, asbestiform amphibole is likely to have a different surface structure and composition from non-asbestiform amphibole. Those differences in surface material result in different surface properties between asbestiform and non-asbestiform minerals of the same species, which may in turn result in different biologic responses.

Some fibrous but non-asbestiform minerals also pose potential concern with respect to human exposure. For example, the fibrous zeolite erionite has been associated with human cases of mesothelioma after environmental exposure (Baris et al. 1987).

SERPENTINE ASBESTOS (CHRYSOTILE) MINERALOGY

Chrysotile—sometimes called white asbestos—is the most common type of asbestos to be used commercially, accounting for about 85% of world asbestos production in 1977 (Liddell 1997, Schreier 1989). At present, chrysotile is the only type of asbestos used in manufacturing in the United States (ATSDR 2001). In addition, chrysotile and other serpentine minerals are common naturally, particularly in hydrothermally altered, magnesium-rich rocks, such as altered basalt, peridotite, and dunite. Many such rocks have been almost completely altered to serpentine and are referred to as serpentinites. Although lizardite is the most common form of serpentine in these rocks, chrysotile can also be present, typically having formed as a late-stage mineral filling veins and sometimes replacing the bulk rock. Chrysotile has been commercially exploited in Canada (Quebec and Ontario), the United States (Vermont and California), Zimbabwe, Russia, South Africa, Australia, and elsewhere (Ross 1981), and it has been used in various products, including insulation, friction materials (such as brake pads), and fiber-reinforced composites (such as concrete) (Harrison et al. 1999, Ross and Virta 2001). In addition to synthetic chrysotile-bearing materials, natural deposits are possible sources of exposure to chrysotile, either by direct exposure to chrysotile-bearing rocks and soils or by redistribution of chrysotile fibers from large natural deposits, such as occurs at Coalinga, California (Klein 1993). It has been argued that atmo spheric processes have redistributed Coalinga chrysotile over the entire Northern Hemisphere from its occurrence in soils in a 50-mi2 area (Klein 1993).

Serpentine minerals belong to a family of 1:1 layer silicates, which are composed of a sheet of polymerized SiO44- tetrahedra (with silicon at the center of each tetrahedron and oxygen at each apex) that is bonded to a sheet of polymerized Mg(OH)64- octahedra (with magnesium at the center of each octahedron and oxygen at each apex) (Figure 3.1). This ratio of tetrahedral to octahedral sheets gives the 1:1 layer silicates their name. The

FIGURE 3.1 Lizardite structure viewed down the a-axis. Polymerized silica tetrahedra form a sheet at the bottom of each 1:1 unit (two units are shown stacked vertically), 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. SOURCE: Mellini (1982).

FIGURE 3.1 Lizardite structure viewed down the a-axis. Polymerized silica tetrahedra form a sheet at the bottom of each 1:1 unit (two units are shown stacked vertically), 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. SOURCE: Mellini (1982).

tetrahedral:octahedral (1:1) polymerized layers are stacked one atop another to form the chrysotile structure.

The serpentine group is based on a metal hydroxide sheet containing Mg2+ cations, giving rise to a composition of Mg3Si2O5(OH)4.

Chrysotile exhibits a smaller variation in chemical composition than other (non-asbestiform) serpentine minerals, but substitutions do occur. The most common substitutions are Si4+^Al3+, Mg2+^Fe2+, and Mg2+^Al3+; however, these substitutions typically represent much less than 10% of the atomic sites (Veblen and Wylie 1993). Other metal substitutions (such as Ni, Co, Mn, Cr, and Zn) may occur in trace amounts (Ross 1981).

Dimensionally, the octahedral (Mg) sheet is slightly larger than the tet-rahedral (Si) sheet. The two sheets are bonded to one another by the sharing of some of their oxygen atoms; the natural spacing of the atoms in the octahedral sheet is about 3.6% larger than the natural spacing in the tetra-hedral sheet (Veblen and Wylie 1993). This structural mismatch can be accommodated either by a curving of the layers (as first proposed by Linus Pauling in 1930 on theoretical grounds) by cation substitution. In chryso-tile, layer curvature exposes the magnesium octahedral sheet at the fiber surface, thereby reducing strain from the dimensional mismatch. Whittaker (1957) calculated the strain-free diameter for a single chrysotile fiber on the basis of a pure Mg octahedral sheet; his value of 0.02 ^m compares favorably with particle diameters measured from real samples (0.03-0.17 ^m), as reported by Veblen and Wylie (1993). The particles measured in the studies reported by Veblen and Wylie may consist of multiple fibers. In natural samples of chrysotile, some of the strain may also be relieved by cation substitution, which allows the particles to achieve slightly larger diameters (Gaines et al. 1997). Cation substitution in chrysotile is typically more limited than in the other magnesium-serpentine minerals (lizardite and antig-orite; chrysotile's composition is closer to the ideal Mg3Si2O5(OH)4.

Dissolution of chrysotile is likely to occur after contact with physiologic fluids. The kinetics of chrysotile dissolution have been studied extensively in experimental systems. Dissolution in the mid pH range (4-7) appears to be independent of pH (Hume and Rimstidt 1992), with Mg2+ release occurring more rapidly initially than silica release but leveling off after at most a few atomic layers of material have been removed, as consistent with the data presented in Hume (1991). At 37°C and under ionic strengths similar to those in lung fluids, Hume and Rimstidt (1992) measured a dissolution rate (k) of 5.9x10-1° mol m-2 sec-1. At lower pH, the rate would be expected to increase substantially, but no comprehensive quantitative study has been done on chrysotile dissolution rate as a function of pH in acidic environments. At the stated rate, a chrysotile particle, even as thick as 1 ^m, would be predicted to be removed from the lung by dissolution in less than a year. The process would remove the pathogenic par ticle, but it would also release into the surrounding environment any trace metals from the particle, which could be toxic in their own right, although probably in a transient fashion.

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  • shishay
    Is erionite asbestiform?
    2 years ago

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