Serpentine Chrysotile

Chrysotile (serpentine) occurs in both clino and ortho stmctures. Both one-layer ortho and clino, and six-layer ortho (as in nacrite) stmctures have been observed. Chrysotile transforms at high temperature to forsterite [15118-03-3] and siHca. Particularly fibrous varieties are called asbestos (qv). 

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

Serpentine An asbestos mineral with a wav-y appearance, such as chrysotile. 

Fig. 1. Summary of chrysotile-steady state element fluxes (A) Mg and (B) Si as a function of pH. Filled and open diamonds are steady-state fluxes determined after the onset of an experiment and after a pH-jump, respectively. Circles are Bales Morgan (1985) element fluxes their rates were determined in C02-free solutions. The solid line is the linear least square fit to all the data. The stars are the range in fluxes determined from field serpentine weathering (Freyssinet Farah 2000).

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. 

Chrysotile is commonly fibrous whereas the other common serpentines, lizardite (a polymorph) and antigorite, are usually platy or massive, and rarely occur in fibrous forms. 

Most serpentines and other layered silicate minerals, such as micas and clays, are composed of tetrahedral and octahedral sheets that lie virtually flat. In chrysotile samples, however, the layers curl, rolling up like a carpet, to form concentric hollow cylinders (Fig. 2.4). The average diameter of a cylinder, which is a chrysotile fibril, is about 25 nanometers (25 nm = 0.025 mi-... 

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).

Fig. 2.5 Electron micrograph of Povlen (sectored) chrysotile. Each sector is a collection of flat serpentine layers.

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. 

Chrysotile A mineral in the serpentine group composed of hydrated magnesium silicate, which occurs in several crystalline modifications (see chapter 2) and usually in fibrous form. First described and named in 1834, chrysotile fibers were mined under the name serpentine-asbestos, or simply asbestos, long before that time. 

Serpentine-asbestos An early name for the fibrous form of serpentine rock that was mined. Today the term refers to chrysotile-asbestos. 

Figure 7.3 Serpentine sheet structures antigorite (left) and chrysotile.

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