Serpentine fibers

Synthetic organic fibers have been used to replace mineral fibers in some application since mineral fibers have come under attack. Natural serpentine fibers are mined from Earth and are therefore relatively inexpensive when compared to synthetic organic fibers. Aramid polymers, Kevlar , have been substituted for serpentine minerals in automobile brakes and seem to be reasonably effective, but expensive. 

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).

The fibrous forms of amphiboles, and serpentines, in addition to occurring in aggregates amenable to mining, are widespread as minor constiments of many rocks. The fibers are also widely distributed throughout the soils and waters of the world. These occurrences, together with the many long-term uses for asbestos society has devised, ensure that the ubiquitous presence of asbestos in our environment. 

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. 

It was suggested for the serpentines that cbmpositionally induced distortions favored the production of fibers, but how can this be accomplished in materials with a basic sheetlike structure Several specific examples of sheet stmcture minerals that occur as fibers (in addition to the micas) will be described before we examine some of these suggestions. 

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. 

Cross-fiber asbestos Term applied to aggregates of asbestos in veins (see Fig. 1.1 A) in which the fibers are arranged in parallel position, usually perpendicular to or sometimes at a low inclination angle to the rock surfaces. The term may be applied to either serpentine- or amphibole-asbestos. 

Figure 6.1 Schematic of laminate lay-up. Insert shows serpentine path that matrix resin and voids might take through connected pores formed by the graphite fibers. Each ply is actually many more fibers thick than is shown...

ASBESTOS. The tenn asbestos is a generic designation referring usually to six types of naturally occurring mineral fibers which are or have been commercially exploited, These fibers are extracted from certain varieties of hydrated alkaline silicate minerals comprising two families seipe.ntin.es and amphiboles. The serpentine group contains a single fibrous variety chrysotile five fibrous forms of amphiboles are known anthophyllite, amosite, crocidolite. tremolite, and actinolite... 

CHRYSOTILE. A delicately llbrous variety of serpentine which separates easily into silky, flexible fibers of greenish or yellowish color, wrlh formula MgtSUOdGHlj. It crystallizes m the monoclinic system hardness. 2.5 sp gr, 2.55. Its name is derived from [he Greek words meaning gold and fibrous. Most of the common asbestos of commerce is chrysolile. It is mined in Thelford. Province of Quebec, and in the Republic of South Africa. See also Asbestos and Serpentine. 

Figure 23 Two varieties of asbestos fibers (A) amphibole and (B) serpentine. The thin amphibole fibers present a larger toxicological threat due to their greater ability to penetrate lung tissue and reach the alveoli.

Asbestos fibers occurring naturally in minerals such as serpentine were once valued for their insulating qualities and resistance to chemical attack. Locomotive and industrial boilers, pipes, and cargo compartments of ships were once coated with asbestos. However, there have been so many cases of workers in the asbestos industry developing respiratory illnesses such as asbestosis or mesothelioma that asbestos is no longer being used and in many places it is being removed. 

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