Pyroxenes enstatite

Fig. 2.10 Schematic structures of the pyroxenes, enstatite, and clinoenstatite. (A) Enstatite, (001) projection Mg2Si20f,. (B) Comparison of the structures of enstatite and the polymorph clinoenstatite in a (010) projection. The large circles are Mg, the small circles are Si " and the black dots are...

Figure 3D. Tetrahedra unichain silicates, for example, pyroxenes (enstatite, diopside) (smaller spheres represent coordinating cations).

Primary MORE magmas are generated by partial melting of the upper mantle believed to be composed of a rock type termed peridotite which is primarily composed of the minerals olivine, pyroxenes (enstatite and diopside), and minor spinel or garnet. 

During the ablation experiment, temperature within the char layer exceeds 1000°C and approach 2000-2500°C at the surface. At these temperatures, any carbonaceous residue from the pol3oner will contain graphite. Additionally, mica-type layered silicates, such as montmorillonite, irreversibly transform into other aluminosilicate phases. Between 600 and 1000 C, montmorillonite dehydroxylates and has been observed to initially transform into spinel, cristobolite, mullite and/or pyroxenes (enstatite) (24). At temperatures greater than 1300 C, mullite, cristobolite and cordierite form and subsequently melt at temperatures in excess of 1500 C (mullite 1850 C, pure cristobolite 1728°C and cordierite --ISSO C) (25). The presence of an inorganic that transforms into a high viscosity melt on the surface of the char will improve ablation resistance by flowing to self-heal surface flaws. This is known to occur in silica-filled ablatives (26). 

Mossbauer spectra have been measured for various tektites, as well as for both natural and synthetic iron-bearing silicate minerals. These results are reported and compared with other similar studies available in the literature. The ratios of the intensities of the appropriate Mossbauer lines have been used to determine the ferric-ferrous ratios where possible. The spectra of the ferrosilite-enstatite series of pyroxenes show four lines which are interpreted as two quadrupole split doublets, and the ratio of the intensities of these lines indicates the degree of ordering in filling the available metal ion sites. Similar studies on the fayalite-forsterite series of olivines are also reported. 

Pyroxene Name Enstatite Bronzite Hypersthene F erro-hypersthene Eulite Ferrosilite... 

Mossbauer spectrum of ferrosilite-enstatite series of pyroxenes... 

The chemical classification of pyroxene components disregards to some extent the represented structural groups, being based essentially on the chemistry of major cations. As an example, table 5.29 reports the classification of Deer et al. (1978), which subdivides pyroxene components into five groups (Mg-Fe, Ca, Ca-Na, Na, and Li pyroxenes). In this classification, the term enstatite identifies a component crystallizing in different structural classes Pbca, P2Jc, Pbcn). 

Enstatite Mg-Fe PYROXENES Mg2Si20g Pbca, Pljc, Pbcn... 

Table 5.32 Compositions (in weight %) of natural pyroxenes (samples 1-5 from Deer et al., 1983 samples 6 and 7 from Bonatti et al., 1986) (1) enstatite from a pyroxenite (2) ferrosilite from a thermometamorphic iron band (3) hedembergite (4) chromian augite from a gabbroic rock of the Bushveld complex (5) aegirine from a riebeckite-albite granitoid (6) diopside from a mantle peridotite (7) enstatite from a mantle peridotite. ...

Table 5.33 lists the structural characters of various pure components of pyroxene crystallizing in the stable state, according to the synthesis of Smyth and Bish (1988). Note the doubling of cell edge a in the transition from monoclinic to orthorhombic structure, composition being equal (i.e., clinoenstatite-enstatite clinoferrosilite-ferrosilite). This effect may be represented by the equation... 

Figure 5JO (A) Simplified Gibbs free energy curves for various polymorphs along enstatite-diopside join at T = 1300 °C. (B) Resulting solvus, spinodal field, and miscibility gap compared with experimental data of McCallister and Yund (1977) on pyroxene unmixing kinetics (part B from Ganguly and Saxena (1992). Reprinted with permission of Springer-Verlag, New York).

As might be anticipated for minerals with chain structures, pyroxenes commonly occur in columnar, prismatic, rodlike, and acicular forms. Enstatite has been found in the form of rosettes of fine-fibrous crystals. Special names such as victorite, chladnite, and shepardite were assigned to different occurrences in this distinctive morphology, possibly because the fibrous aggregates were located in iron meteorites. However, the composition and crystal... 

Si04 units share two corners to form infinite chains (Figure 1.52(c)). The repeat unit is SiOs . Minerals with this structure are called pyroxenes (e.g., diopside (CaMg(S 103)2) and enstatite (MgSiOs)). The silicate chains lie parallel to one another and are linked together by the cations that lie between them. 

Pyroxenes e.g., Mg2[Si2Ofi] = 2MgSi03, enstatite) and amphiboles (double chain silicates containing OH groups) form chains of Si04 tetrahedra. By interlinking the chains we arrive at layer silicates. 

The occurrence of minerals which show CL is highly dependent on the type of meteorite. Possibly the most common phase which occurs is feldspar. Because this mineral accepts very little Fe into the structure, quenching is not a problem however, because the feldspar structure is quite open, the Na- and K-rich feldspars are easily damaged by electron beams. In contrast anorthite, the Ca rich variety, is quite stable. Pyroxene and olivine are common phases in meteorites but because they both usually contain iron, most do not luminesce. Only in the primitive meteorites do nearly pure enstatite and forsterite occur and both show brilliant CL. Other minerals are rare but include ... 

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