Iron orthopyroxene

Pure iron orthopyroxene (a-FeSiOj) is unstable at the pressures presumably existing in metamorphic processes, due to breakdown into the stable association fayalite + quartz (see Fig. 75) ... 

Recently, Evans and co-workers (11) studied the spectra of three orthopyroxenes with Fe/(Mg + Fe) ratios of 0.758, 0.532, and 0.280. They obtained two somewhat asymmetric lines, and their results are summarized in Table VIIIA. One of these authors had previously established by x-ray analysis (12, 13) that the sample with the 0.532 ratio exhibits Mg-Fe ordering, with iron preferring the larger, more distorted M2 site. Consequently it was concluded that the two-line Mossbauer spectra must consist of a superposition of two quadrupole-split doublets nearly coinciding. Mossbauer parameters for iron in these... 

A. Mossbauer parameters for two-line orthopyroxene spectra of varying iron content as determined by Evans, Ghose, and Hafner (11/... 

As figure 5.70 shows, with increasing T the miscibility gap shrinks toward the iron-free join moreover, on the Ca-poor side of the quadrilateral, miscibility relations are complicated by the formation of pigeonite over a restricted P-T-X range (cf section 5.4.6). Once the compositions of coexisting clinopyroxene and orthopyroxene are known, figure 5.70 is a practical geothermometer. 

Matsui Y. and Nishizawa O. (1974). Iron magnesium exchange equilibrium between coexisting synthetic olivine and orthopyroxene. Amer. Jour. Set, 267 945-968. 

Ganguly J. and Stimpfl M. (2000) Cation ordering in orthopyroxenes from two stony-iron meteorites implications for cooling rates and metal-silicate mixing. Geochim. Cosmo-chim. Acta 64, 1291-1297. 

Molin G.M., Saxena S.K., and Brizi E. (1991) Iron-magnesium order-disorder in an orthopyroxene crystal from the Johnstown meteorite. EarthPlanet. Sci. Lett. 105, 260-265. 

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). 

The spectra illustrated in fig. 5.15 show that absorption maxima of all spin-allowed CF bands move to longer wavelengths with increasing iron content of the orthopyroxene, forming the basis of composition determinative curves (Hazen et al., 1977b Adams, 1974) and enabling this pyroxene structure-type to be identified in telescopic reflectance spectra of surfaces of the Moon (Pieters etal., 1985 Bums, 1989a). 

In pigeonites, X-ray diffraction (e.g., Clark et al., 1971 Brown et al., 1972 Takeda et al., 1974) and Mossbauer spectral measurements (Bancroft and Bums, 1967b) of samples from lunar and volcanic rocks demonstrate that there is a strong enrichment of Fe2+ ions in the M2 sites, but the enrichments are slightly smaller than those in metamorphic orthopyroxenes with similar iron contents due, in part, to the higher Ca contents of pigeonites. 

In addition, the predicted orders in eqs (7.18) and (7.19) do not take into account phase equilibrium relationships involving coexisting minerals. For example, olivine should always be enriched in Fe2+ ions relative to orthopyroxene, according the the CFSE data plotted in fig. 7.6. Moreover, studies of the system MgO-FeO-Si02 at atmospheric pressure (Bowen and Schairer, 1935) showed that magnesium olivine crystallizes first and becomes increasingly enriched in iron before pyroxene commences to crystallize. The olivine main-... 

Burnham, C. W., Ohashi, Y., Hafner, S. S. Virgo, D. (1971) Cation distribution and atomic thermal vibrations in an iron-rich orthopyroxene. Amer. Mineral., 56, 850-76. 

Goldman, D. S. Rossman, G. R. (1977a) The spectra of iron in orthopyroxene revisited The splitting of the ground state. Amer. Mineral., 62,151-7. 

Rajamani, V. (1976) Distribution of iron, cobalt and nickel between synthetic sulfide and orthopyroxene at 900°C. Econ. Geol., 71,795-802. 

Orthopyroxenes, found only in highly metamorphosed rocks, have an iron content of 60 to 88% the latter value is close to the maximum iron content of natural pyroxenes of the ferrosilite series (Fig. 15). In the BIF of the Ukrainian shield the content of minor components in the pyroxenes is not high AI2O3 —0.15-1.60 (0.78) CaO—0.49-2.11 (1.15) MnO—0.07-1.56 (0.35) Na20 + KjO—0.08-0.47 (0.26) TiOj —0.05-0.19 (0.10) (average of 12 analyses in parentheses). Pyroxenes from other areas where BIF are developed have a low content of impurities except for the manganiferous varieties (up to 3-6% MnO) found in the Tobacco Root Mountains and in the Carter Creek area of the Ruby Mountains, Montana (Immega and Klein, 1976) the iron content of these pyroxenes is within 60-62%. 

In the case of a high iron content in the carbonate, olivine is formed instead of orthopyroxene. A 10% increase in magnesium content leads to a 5-8° shift of the P-T curve into the higher temperature region. The shift is relatively small, but it plays a definite role in the metamorphic redistribution of iron among the minerals, particularly in reactions in which iron oxides are formed in the absence of water ... 

Figure 89 shows schematic P-T curves of the upper temperature limit of stability of Mg-Fe amphiboles. The region of overlap here is arbitrarily retained. From the experimental data it follows that the iron content of cummingtonite does not fall below 30-40%, while ferroanthophyllite does not contain more than 60-70% iron molecule. Therefore the interpolation was not strictly linear, and the P-T curves obtained may contain an error of the order of 5-15°. From these approximate formulations it follows that a 10% increase in the magnesium content of cummingtonite would cause a 10-15° rise in dehydration temperature. As in the carbonate reactions, the product of decomposition will be olivine in the case of a high iron content and orthopyroxene in the case of an iron content below 70-90%. 

Fig. 92. Equilibrium of olivine with pyroxene in rocks with excess silica in the absence of fluid. Figures on curves indicate maximum iron content of orthopyroxene, in mol.%.

Fig. 93. Equilibrium of magnesian-iron minerals in rocks with excess silica. A. In silicate iron-formations (aqueous fluid. Cum + Px -t- OH- Q association). B. In carbonate iron-rich rocks (carbonic acid fluid. Car -(- Px -I- 01 -t- Q association). Figures indicate maximum iron content of orthopyroxene in association with quartz and olivine.

The association pyroxene + olivine + cummingtonite + quartz, often encountered in highly metamorphosed BIF, is also very important for judging the thermodynamic parameters of mineralization. This association fixes the temperature at 700-720°C, which depends little on pressure and iron content of the silicates, while pressure can be estimated fairly precisely from the iron content of orthopyroxene. A decrease in partial pressure of water in conjunction with mechanical equilibrium P =Ff) due to dilution of the fluid by other volatiles, for instance carbon dioxide, can lead to some shift of the P-T curve into the lower-temperature region. However, the amount of such a shift, especially at high pressures (8-10 kbar) cannot be significant because Fe-Mg carbonates which are stable in carbon dioxide fluids already appear at 610-650°C (Fig. 93b), and their paragenesis with anhydrous silicates is not typical of iron formations. 

Val ter, A.A., 1969. On the distribution of magnesium and iron between coexisting olivine and orthopyroxene solid solutions with high iron content. Dokl. Akad. Nauk S.S.S.R. (Rep. Acad. Sci, U.S.S.R.), 187(2) 410-413 (in Russian). 

Fig. 12. Fischer-Tropsch reaction at 1 atm is first-order in CO, with an activation energy of 27 kcal/ mole (Lancet, 1972). Rate in a flow system is 10 times faster than in the static system used here. Dashed line shows extrapolation to solar nebula, assuming that the rate is proportional to (PcoIIPhj) . Reaction proceeds at an undetectable rate when the Bruderheim L6 chondrite is used as a catalyst. Apparently the high-temperature minerals in this meteorite (olivine, orthopyroxene, troilite, and nickel-iron) do not catalyze the hydrogenation of CO. Thus CO can survive in the solar nebula down to 400 K, when catalytically active minerals first from (Fig. 1 and 10)...

Folco L., Mellini M., and Pillinger C. T. (1997) Equilibrated ordinary chondrites constraints for thermal history from iron-magnesium ordering in orthopyroxene. Meteorit. Planet. Sci. 32, 567-575. 

Figure 1 Na/Al versus FeO/MnO for achondrites. The FeO/MnO for acapulcoite-lodranite clan (ale), wino-naite-IAB-iron silicate inclusion clan (wic) and aubrites are for orthopyroxene. All other data are bulk rock values. Meteorite data are from sources listed in the text, while planet data are from McDonough and Sun (1995), Taylor (1982), and Wanke and Dreibus (1994).

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