Serpentine mineral groups

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. 

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 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

The formula of the solid product (Al2Si205(0H)4) is that for kaolinite, an important member of the serpentine-kaolin group of clay minerals (Section 4.5.1). This reaction demonstrates incongruent dissolution of feldspar, i.e. dissolution with in situ reprecipitation of some compounds from the weathered mineral. 

All of the above forms of asbestos belong to either the Serpentine or Amphibole mineral groups and have different chemical compositions and hence different associated properties. All have excellent heat insulating properties and are relatively incombustible, whilst some are flexible or have high tensile strengths, or resistance to acids and alkalis. 

Asbestos (1332-21-4) is the generic term used to describe over 30 different types of naturally occurring hydrated silicates representing two mineral groups (serpentine and amphiboles) that separate into fibres on mechanical processing. While the exact taxonomy continues to evolve, the sole serpentine variety is called chrysotile (12001-29-5) while the amphiboles include five forms ... 

It is the similarity between silicates and phosphates that suggested that phosphorus might be substituted for silicon in molecules of serpentine mineral types. If properties of silicates can be retained while adding known safety properties of phosphates, a safe asbestos -type product is possible. An early unsuccessful attempt was made by the author to prepare a safe asbestos by inserting phosphate groups within silicate chains of chiysotile. With some additional work this should still be possible and no more than two or three percent of the weight of the chrysotile product would be required as inserted phosphates, and probably inexpensively. 

FIGURE 2 [010] view of structures of major clay mineral groups. 1 1 and 2 1 T 0 layers are represented by the primary units in serpentine-kaolin and talc-pyrophyllite, respectively. The c-sin (5 distance is the basal (001) c-axis d-spacing. Layer eharge per formula unit = X. (Reproduced with the permission of the Mineralogical Society of Great Britain and Ireland from Ref. 13.)... 

The three minerals kaolinite, dickite, and nacrite differ not in the fundamental nature of their structural units, but in the ways in which these units are stacked one on top of another. The numerous forms of cronstedite investigated by Steadman and Nuttall [1963, 1964] and of chrysotile and other serpentine minerals investigated by Whittaker and Zussman in numerous publications also differ from one another only in the ways the structural units are stacked. Thus, the many minerals in this group can be considered as polymorphic variants of a few basic structures. 

The asbestos minerals are crystalline fibrous silicates, some of which have been exploited commercially, especially as an insulating material. These minerals comprise sheets, or chains of silicate tetrahedra in which oxygen is either bound to two silicon atoms or to one silicon atom, and possesses a negative charge. There are two major groups of asbestos mineral serpentines and amphiboles. White asbestos or chrysotile is a serpentine mineral, whereas the other forms of asbestos, crocidolite (blue asbestos), amosite (brown asbestos), anthophyllite, tremolite, and aetinolite, are amphiboles. Tremolite can also be found as a contaminant in other minerals, such as chrysotile, talc, and vermiculite. 

A number of serpentine group minerals have substitutions in both the tetrahedral and octahedral layer, but they stiU maintain electrostatic neutraUty. Amestite [12413-27-5] which approximates (Mg2Al)(SiAl)0 (0H)4 in composition, cronstedite [61104-43-3] (Fe " 2 Fe " )(SiFe " )0 (0H)4, chamosite,... 

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