Pyroxenes synthetic

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. 

The other mineral spectra shown are pyroxenes. The bronzite sample studied (Figure 10) is almost a member of the solid-solution series listed in Table V, although small amounts of Al and Ca are present. Consequently, it should be compared with the synthetic members of this series described below. Even in this natural mineral sample there is clear... 

Figure 5.22 Cell parameters of synthetic pyroxenes in the Mg2Si206-CaMg2Si206-CaFe2Si206-Fe2Si206 quadrilateral. From Turnock et al. (1973). Reprinted with permission of The Mineralogical Society of America.

Intracrystalline Fe2+-Mg2+ distributions in natural and synthetic orthopyroxenes have been determined from intensities of absorption bands in their polarized spectra (Goldman and Rossman, 1977a Steffen et al., 1988). Molar extinction coefficients of crystal field bands centred at 10,500 to 11,000 cm-1 and 4,900 to 5,400 cm-1 originating from Fe2+ ions located in pyroxene M2 sites ( 5.5.4) enabled the iron contents to be estimated from the Beer-Lambert law equation, eq. (3.7). 

Other transition metal-bearing pyroxenes Optical spectral measurements have been made on a variety of natural and synthetic pyroxenes containing a number of transition metal ions (Rossman, 1980). Crystal field splitting and stabilization energy parameters derived from these spectra are summarized in table 5.7. 

Although Cr2+ ions are rare and unstable in terrestrial minerals, their presence is suspected in olivines and pyroxenes from the Earth s Mantle and the Moon (Bums, 1975a Smith, 1971). Crystal field spectra exist for these silicates, as well as other synthetic Cr2+-bearing phases, and parameters are summarized in table 5.12. Just one spin-allowed transition, corresponding to 5Eg - 5T2g, might... 

The cause of colour in natural and synthetic chromium-bearing blue diop-sides has been widely debated and assignments of absorption bands in their polarized spectra remain controversial (Mao et al., 1972 Bums, 1975a,b Ikeda and Yagi 1977, 1982 Schreiber, 1977, 1978). One interpretation is that low-spin Cr3 ions in tetrahearal sites in the pyroxene structure are responsible for the colour and spectra of blue diopsides (Ikeda and Yagi, 1977, 1982). 

The strong preference of Mn2+ ions for the orthopyroxene M2 sites, first demonstrated indirectly by Mossbauer spectroscopy (Bancroft et al., 1967), was subsequently confirmed by X-ray structure refinements of synthetic Mg2+-Mn2+ (Ghose et al., 1975) and Mg2+-Mn2+-Co2+ (Hawthorne and Ito, 1977) orthopyroxenes. These X-ray measurements, as well as those of synthetic Mg2+-Co2+ and Mg2+-Zn2+ orthopyroxenes (Ghose et al., 1975), also showed that Co2+ and Zn2+ ions are both relatively enriched in the orthopyroxene M2 site. In a synthetic Mg2+-Cr2+ pyroxene, the Cr2 ions are strongly enriched in the M2 site (Angel et al., 1989), causing increased distortion of this site as a result of the Jahn-Teller effect ( 6.3). 

Therefore, the true configurational entropy is 0.95 J/(deg. mole) lower than the maximum value as a result of Fe2+-Mg2+ ordering in the orthopyroxene structure. The cation ordering found in other members of the enstatite—ferrosilite series, as well as the synthetic Mg2+-Ni2+, Mg2+-Co2+ and Mg2+-Mn2+ pyroxenes (Ghose et al., 1975 Hawthorne and Ito, 1977), shows that most transition metal-bearing orthopyroxenes are not ideal solid-solutions. 

The shock experiments with synthetic basaltic eucrite at room temperature reveal a similar pressure-dependent sequence of characteristic shock features in pyroxene and plagioclase to those seen in samples recovered from previous experiments (see, e.g., ]9,10]). Also, tliere is no change in the sequence of progressive shock effects at elevated initial temperatures. Amorphization of plagioclase... 

Asbestos, the first inorganic fiber material used, is currently still exclusively produced from natural mineral deposits. It is formed by the hydrothermal conversion of basic and ultrabasic volcanic rock (olivine and pyroxene) to serpentine upon which the actual asbestos formation takes place leading to two asbestos sorts with different structures serpentine asbestos and amphibole asbestos. Asbestos can be produced synthetically by several hours heating of a polysilicic acid/metal oxide mixture (e.g. Mg, Fe, Co, Ni) in water at 300 to 350°C and 90 to 160 bar. The properties of four important asbestos types are summarized in Table 5.2-2. 

Jervisite is a scandium pyroxene and a natural analogue of the synthetic phase NaScSi20e (Mellini et al. 1982). The crystal structure of NaScSi206, the synthetic jervisite, was analyzed by Hawthorne and Grundy (1973). In the crystal structure (fig. 46), the Na atom occupies the M2 site which is considered as a very distorted square antiprism, and the Sc atom occupies the Ml site which is a distorted octahedra. 

Pyroxene minerals like diopside CaMg(SiOj)2, enstatite MgSiOj, jadeite NaAl(Si03)j and synthetic silicates like Na SiOj, Li SiOj are conunon examples. 

The results of X-ray diffraction, electron microscopy and microprobe studies suggest that the phenomenon of shock metamorphism occurred in the siminals. However, because the composition of the siminals is a synthetic analog of naturally occurring, shock-induced minerals, the changes consist of those in pyroxene, not in the polymorphic transformations, and the removal and redistribution of elements between the components because pyroxene is resistant to impact. 

S.G. Eeckhout, E. De Grave, C.A. McCammon, R. Vochten, Temperature dependence of the hyperfine parameters in synthetic P21/c Mg-Fe pyroxenes along the MgSi03-FeSi03 join. Am. Mineral. 85, 943-952 (2(X)0)... 

E. Dowty, D.H. Lindslay, Mossbauer spectra of synthetic hedenbergite-ferrosUite pyroxenes. Am. Mineral. 58, 850-868 (1973)... 

The simple univariant reaction described above, although it is of great importance to geophysical interpretations, is not well understood quantitatively, in spite of several experimental determinations. The reasons are simple and two-fold. First, the kinetics of reaction in dry silicate systems become impossibly slow at temperatures below about 1200°C, so that convincing demonstration of chemical equilibrium in the form of "reversals , or growth of one assemblage at the expense of an isochemical one across the reaction boundary, is often impossible. Second, the compositions and ordering states of the synthetic phases may be quite non-equilibrium. This is particularly true of the A1 content of the pyroxenes, equilibrium with respect to which is extremely difficult to achieve experimentally. 

Fig 2 Enthalpy of solution of synthetic diopside-CaTs pyroxenes in a lead borate melt at 970 K (Newton et al, 1976), compared with trends predicted by completely ordered, unit cell charge balanced, and completely disordered models of Wood (1975). 

Okamura, Ghose and Ohashi (l9T ) showed that synthetic CaTs pyroxene has a high degree of (Si,Al) disorder. The ideal activity in the case of complete cation randomization would be ... 

The analysis of the statistical or preferred orientation of the crystallites in solid polycrystaUine materials is commonly referred to as texture analysis. Again, the diffraction technique allows the definition of the relationship between a microscopic property, i.e. the orientation of the crystallites defined as coherent diffraction domains, and the macroscopic physical properties of the crystal aggregate. Texture studies are of course crucial in the characterization of oriented synthetic materials such as cold-rolled metals or oxide thin films, but they are also of great relevance in the study of the formation processes of mineral assemblages. As an example, the texture features of olivine or pyroxene minerals in meteoritic chondrules yield information on the early condensation sequence... 

Mysen (1976) measured distribution coefficients for Sm in olivine and Ca-poor pyroxene by beta track mapping of synthetic silicates. That technique allows use of very low concentrations of a trace element. Mysen reported a strong concen-... 

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