Structural serpentine-chlorite

Fig. 2.13 The 1 1 and 2 1 layer arrangements in the sheet structure minerals and the (010) view of the structures of the serpentine, clay, talc, pyrophyllite, mica, and chlorite minerals. X = layer charge per formula unit. [From Bailey (1980), Fig 1.1, p. 3 Fig 1.2, p. 6.1...

Table 2 gives temperatures of montmorillonite stability which are established by the experiments reported. The most important criteria used is reaction reversal this lacking, length of the experiments and variety of starting material was taken into consideration. Two points are important among micas and other phyllosilicates only kaolinite, serpentine and muscovite are stable to very low temperatures. All trioctahedral 2 1 structures break down to expandable phases at low temperatures (bio-tites) or to 1 1 structures plus expandable phase (chlorites). 

Berthierine, as shown by Brindley (1982) is essentially a trioctahedral mineral, following the line of trioctahedral chlorites in Figure 7. In our simulations of the XRD spectra of odinite, we use a ferrous serpentine and a ferric dioctahedral smectite component. Translated into constituent ions of a mineral structure, this mineral combination will give a bulk average composition between nontronite (ferric, dioctahedral smectite) and berthierine (trioctahedral chlorite). 

Subclass Phyllosilicates (sheet silicates) In this structure each Si04 tetrahedron is linked to three adjacent tetrahedra to form an infinite sheet of tetrahedra. Each tetrahedron in the sheet thus shares three (out of four) apical oxygens, having the basic structural unit Si205. Some Si of the tetrahedral site may be replaced by AF, and the charge is balanced by inclusion of additional cations. The micas, clay minerals, chlorite, talc and serpentine minerals are examples of phyllosilicates. 

Clay minerals, the micas, chlorite, serpentine, talc, between them can also find within the group of minerals phyllosilicates or layered silicates. They all have different morphology, structure, and texture (Bergaya et al. 2009). The some representative layer silicates used mainly in nanocomposites-based starch are listed in Table 2. It is important to note that the some phyllosilicates do not display a normal layered strucmre, for example, the sepiolite shows a fibrous structure meanwhile halloysite has spheroidal aggregates morphology (Duquesque et al. 2007 Bergaya et al. 2009). 

TALC. 3Mg0-4Si02-H20. Talc is a hydrous magnesium silicate, with the composition 63.4% SiOj, 31.9% MgO and 4.7% HjO when found in pure form. It is an extremely soft mineral with a Mohs hardness of 1, has a platy structure and it is naturally hydro-phobic. Talc occurs as a relatively pure massive mineral in Montana, Australia and China. Elsewhere it occurs in conjunction with magnesite (Vermont, Quebec, Ontario and Finland), with tremoUte and serpentine in New York and with chlorite in France and Austria. In many ceramic applications, the presence of non-talc minerals such as chlorite and tremolite are beneficial. 

Brindley and Gillery observed that the 00/ intensities from an Fe-rich daphnite specimen from Cornwall were not in accord with a true chlorite structure. One-dimensional Fourier syntheses indicated the tetrahedral Si and O peaks to be unexpectedly low and broad and to extend closer to the interlayer sheet than normal. A model was postulated in which approximately one-third of the tetrahedra is inverted to link with the interlayer sheet instead of with the silicate octahedral sheet. This has the effect of changing a chlorite 14 A unit into a 7 A layer at the point of inversion, so that the structure can be described as a mixed layer 7-14 A structure. The inversion can be accomplished physically by shifting a Si atom to the opposite side of its basal triad and completing tetrahedral coordination with an interlayer anion. The reverse of this process may be the mechanism of the hydrothermal transformation of 7 A aluminian serpentines to chlorites. 

Hydrothermal syntheses at low temperature and pressure by Yoder [1952], Roy and Roy [1955], Nelson and Roy [1954, 1958], and Cillery [1959] have demonstrated that the 7 A serpentine structure can be formed for any composition between chrysotile and amesite. At higher temperatures and pressures, above 400 to 500°C and 10,000 to 15,000 Ib/in., aluminian serpentines between penninite and amesite in composition invert slowly to the 14 A chlorite structure. It is not certain whether the lower-temperature 7 A structures are thermodynamically stable or metastable. Phase equilibrium diagrams for both possibilities have been given by Nelson and Roy [1958], shown here as Figure 26. 

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