Ferrosilite

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. 

Mossbauer Effect in Similar Glasses and Crystals. Expression of the Mg-Fe content of tektites in terms of enstatite-ferrosilite percentages suggests a comparison with silicate minerals which contain Fe and... 

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

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

Table XI. Mossbauer Parameters of a Synthetic Enstatite-Ferrosilite ( n-Fs) Series as a Function of Iron Concentration Expressed as Percent Ferrosilite Present by Subscript on Fs...

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... 

Experimental studies on C2 c pyroxenes (Cameron et al., 1973b Finger and Ohashi, 1976) at temperatures up to E = 1000 °C show that the structure remains stable without transition phenomena, and that the increase of the angle 0 is 2 to 3°. Analogous studies on Pbca pyroxenes (hyperstene, ferrosilite Smyth, 1973) show phase stability in the T range 25 to 1000 °C but a more conspicuous increase in the angle 0 (i.e., 10° for chain A and 15° for chain B in ferrosilite 5° for chain A and 10° for chain B in hyperstene). However, phase transitions have been observed in P2j c pyroxenes. Brown et al. (1972) observed a transition to the form... 

This is the case, for instance, for the enstatite-ferrosilite solid mixture [orthopyroxene (Fe,Mg)Si03] ... 

Remaining Ca, Fe, and Mg normalized to form wollastonite + enstatite ferrosilite. 

Lindsley D. H. (1981). The formation of pigeonite on the join hedembergite-ferrosilite at 11, 5 and 15 kbar Experiments and a solution model. Amer. Mineral, 66 1175-1182. 

Figure 2-3 A Mossbauer spectrum for an orthopyroxene sample with ferrosilite mole fraction of 1.1% equilibrated at 900°C. The solid curve is the fit to the spectrum, and the dashed curves are individual fit lines. The residual is also shown (shifted upward by 0.989 units so that it can be shown clearly). The area ratio of two Ml peaks to two M2 peaks is 0.2367 0.0055. Fe " is negligible. The cation distribution (or formula) of this orthopyroxene (4 cation and 6 oxygen basis) is as follows tetrahedral site, Alo.oi8Si1.982 Ml Fe0.0041Mg0.9776Ti0.0007Al0.0176 M2, Cao.ooi6Feo.oi72Mgo.9803-There is also minor amount of Na (0.0004) and Mn (0.0005) in M2 site. From Wang et al. (2005).

Chain silicate minerals with MgSiOg (En), FeSiOg (Fs), and CaSiOg (Wo) as the major components, where En is enstatite, Fs is ferrosilite, and Wo is wollastonite. 

Several polarized absorption spectra have been described for a variety of unheated and heated natural and synthetic orthopyroxenes (e.g., Bancroft and Bums, 1967a Runciman et al., 1973b Goldman and Rossman, 1976, 1977a, 1979 Rossman, 1980, 1988 Zhao et al., 1986 Steffen et al., 1988). In fig. 5.15, representative spectra are illustrated of two orthopyroxenes, enstatite Fs14 5 and ferrosilite Fs864. The a spectra show a very intense band centred near 10,700 cm-1. The P spectra consist of two bands located near 11,100 cm-1 and between 5,400 to 4,900 cm-1, with a shoulder around 8,500 cm-1 becoming well developed in Fe2+-rich orthopyroxenes. The y spectra of Mg2+-rich... 

These assignments of the crystal field bands may be used to construct the 3d orbital energy level diagrams illustrated in fig. 5.17 for Fe2+ ions in the Ml and M2 sites of ferrosilite, Fsgg 4. The polarized absorption spectra of this ferrosilite (fig. 5.15b) show that two of the M2 site Fe2+ bands are centred near 10,700 cm-1 and 4,900 cm-1. The lower-level splittings of 2,350 cm-1 and 354 cm-1 listed in eq. (5.11) for enstatite axe assumed to apply to ferrosilite. This information is... 

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. 

Steffen, G., Langer, K. Seifert, F. (1988) Polarized electronic absorption spectra of synthetic (Mg-Fe)-orthopyroxenes, ferrosilite and Fe3+-bearing ferrosilite. Phys. Chem. Minerals, 16,120-9. 

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