Serpentine minerals

Chrysotile is in the serpentine mineral group aU others are amphiboles. 

The most important of these groups is the serpentine mineral group, because it includes the fibrous mineral species chrysotile, which is the most common fibrous mineral and the one most widely mined, processed, and manufactured as asbestos. 

At least seven of the minerals belonging to the serpentine mineral group, occur in fibrous forms. Table 2.2 lists the varieties, together with the crystal chemical data needed to identify them. 

Antigorite is another serpentine mineral. It is similar in composition to chrysotile except that small amounts of Fe substitute for some of the Mg" in its structure (see Table 2.2). This subtle difference in composition produces a limited sheet structure with corregated stacking of the octahedral-tetra-... 

Fig. 2.4 Chrysotile asbestos sectioned perpendicular to the fiber axis. Electron micrograph showing typical lattice images of the layers of this serpentine mineral rolled into hollow cylinders (fibrils).

Comprehensive reviews of the serpentine minerals can be found in Whittaker and Zussman (1956), Wicks and Whittaker (1975), and Zussman (1979). 

The micas are characterized by extended silicate sheets rather than chains. Their structures resemble the serpentine mineral group in that they are dom-... 

Silicate minerals that usually occur as spherulitic aggregates of fibers have formed as a result of the alteration of the many minerals subsumed within the category of biopyriboles. Alteration of the micas under hydrothermal conditions produces compositional variants on recrystallization such as hydrous muscovite. Some of these samples have been labeled asbestiform, probably because they are found in veins that criss-cross rock masses. Fibrous micaceous minerals also occur as discrete disseminated particles, although few detailed analyses of crystallites from the disperse occurrences have been made. Fibrous mica found in veins usually grades (composition-ally) into members of the serpentine mineral group, the clays or the chlorites. 

The silicate sheet in kaolinite, for example, has an 0-0 repeat distance in the sheet of 0.893 nm, whereas the octahedral or gibbsite sheet repeat is smaller, about 0.862 nm. Mismatch of the 1 1 sheets induces curvature with the smaller dimension sheet on the interior. The octahedral gibbsite layer in clays is postulated to be situated on the inside of the curve. This relationship contrasts with the hypothesis for chrysotile, in which the tetrahedral silicate sheet is smaller and is postulated to be the interior unit in the scrolled serpentine mineral. 

K. Yada and K. lishi, Serpentine minerals hydrothermaUy synthesized and their microstructures,/. Crystal Growth, 24/25,1974, 627-30... 

Serpentines. Substituting 3 Mg21 lor the 2 Al + in Lhe kaolin slruclttre results in the serpentine minerals. Mg.SrO.-tOHU In serpentines all three possible octahedral cation sites are filled. Most serpentine minerals are tubular lo fibrous in structure presumably because of misfit heiween Mg octahedral and tetrahedral layers. 

The trioctahedral two-sheet minerals are called serpentines. The serpentine minerals (chrysotile and antigorite are the most common), which are included in this subgroup, consist of a tetrahedral sheet and an octahedral sheet containing magnesium... 

Serpentinization reaction A hydrothermal reaction by which magnesium-rich silicate minerals are converted into or replaced by serpentine minerals... 

Deer, Howie and Zussman (19J give physical and optical constants for amphibole minerals in Vol. two, Rock Forming Minerals, (chain silicates), and for Serpentine minerals (chrysotile) in volume three (sheet silicates). X-ray diffraction data is obtainable from the Powder Diffraction file, Inorganic (20). This data can be used for a double check of asbestos standards. 

Regulatory agencies such as the U.S. Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA) recognize six asbestos minerals chrysotile, a serpentine mineral and five amphibole minerals, actinolite asbestos, tremolite asbestos, anthophyllite asbestos, crocidolite asbestos, and amosite asbestos. Nonasbestiform amphibole minerals are not included in U.S. health regulations regarding asbestos because there is insufficient evidence that they will produce adverse health effects of the same type and severity produced by chronic exposure to asbestos. 

Serpentinite Igneous or metamorphic rock chiefly composed of serpentine minerals such as chrysotile or lizardite (Jackson 1997). Chrysotile, when found, can occur in localities with serpentinite rock (Churchill et al. 2001). 

Ulmer and Trommsdorff (1999) reviewed thermodynamic and field constraints from Alpine ultramafic rocks, which document that lizardite and crysotile, the other two serpentine minerals, break down to form antigorite via reactions such as chrysotile + talc = antigorite, chrysotile + tremolite = antigorite + diopside, and chrysotile = brucite + antigorite. Therefore, antigorite is the appropriate serpentine mineral to study experimentally to determine the maximum pressure-temperature stabilities for serpentine sensu lato. 

Economically important metamorphic minerals such as serpentine can affect health. In the past few years, asbestos removal has had significant impact on the cost of operating schools and other public buildings, hi the confusion over illness associated with asbestos made from the amphibole mineral crocidolite, citizens demanded the removal of all asbestos, unaware that a less-hazardous form of asbestos, the serpentine mineral chrysotile, also was removed at great expense. 

Because of their alternating tetrahedral (T) and octahedral (O) layers, the two-layer phyllosil-icates are said to have T 0 structures. Listed in Table 9.1 are other T 0 minerals, including the kaolinite polymorphs nacrite, dickite, and halloysite, and the trioctahedral serpentine minerals lizardite, antigorite, and chrysotile, in which brucite layers alternate with layers of silica tetrahedra. 

Serpentine (mineral). Mg,O.Si,. Occurs in nature as the dihydrate, <0H)4MgSi203 exists in, two forms anrigorite, a plaly variety, and chrysotile. a fibrous variety. The latter is the most common form of asbestos, q.v,... 

Plasma-Chemical Extraction of Nickel from Serpentine Minerals... 

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