Pyroxene composition

Klock W., Thomas K. L., McKay D. S., and Palme H. (1989) Unusual olivine and pyroxene composition in interplanetary dust and unequilibrated ordinary chondrites. Nature 339, 126-128. 

There is additional astronomical information on cometary silicates that provides far more information than simply the presence of olivine. High-resolution and good signal-to-noise ratio IR spectra show additional fine strucffire on the 10 p,m silicate feature of bright LP comets. A small feature at 11.9 p.m is also due to olivine and a slope change at 9.2 pm and 9.3 pm is attributed to pyroxene and amorphous silicate with pyroxene composition (Manner and Bradley, 2003) (Figure 11). 

Figure 12 IR data from the ISO for comet Hale-Bopp compared with a hve-component dust composition model. BBl is a 280 K blackbody, BB2 is a 165 K black body. Cry 01 is 22% forsterite. Cry o-Pyr is 8% orthopyroxene, and Am Pyr is 70% amorphous silicate with pyroxene composition (source Crovisier et al., 2000).

Takazawa E., Prey P., Shimizu N., and Obata M. (1996) Evolution of the Horoman Peridotite (Hokkaido, Japan) implications from pyroxene compositions. In Melt Processes and Exhumation of Garnet, Spinel and Plagioclase Facies Mantle (eds. M. A. Menzies, J. L. Bodinier, F. Frey, F. Gervilla, and P. Kelemen). Elsevier, Amsterdam, Netherlands, vol. 134, pp. 3-26. 

The amount of water that can dissolve in orthopyroxene depends on pressure, temperamre, and composition. In a study at 1,100 °C, Rauch and Keppler (2002) found that water content in pure enstatite increased from 55 ppm H2O at 0.2 GPa to 867 ppm at 7.5 GPa, then decreased slightly to 714 ppm at 10 GPa. They also looked at the effect of changing pyroxene composition the most impressive change occurred with increasing aluminum content of the pyroxene. The addition of 1 wt.% AI2O3 increased water solubility from 199 ppm to 1,102 ppm H2O at 1.5 GPa and... 

Discfimi nation diagrams for basalts based upon cU no pyroxene composition... 

Himmelberg GR, Ford AB (1973) Pyroxene compositional trends in the Dufek intrusion, Pensacola Mountains. Antarctic J US 8(5) 260-263... 

Table I gives the activity coefficients of CaTs for various pyroxene compositions calculated by Wood (1975) from his experimental data, assuming the completely disordered and the unit-cell charge-balance models. Given also are the activity coefficients of the CaMgSi20 component, calculated from the CaTs activity coefficients by means of the Gibbs-Duhem equation.

Fig. 8.33 Left, robotic arm with MIMOS II positioned on the rock Adirondack, as seen by the navigation camera of the rover Right. Mossbauer Spectrum (14.4 keV temperature range 220-280 K) of the rock Adirondack at Spirit landing side Gusev Crater, plains. The data were taken at the as-is dusty surface (not yet brushed). The spectrum shows an olivine-basalt composition, typical for soil and rocks in Gusev plains, consisting of the minerals olivine, pyroxene, an Fe doublet, and nonstoichiometric magnetite...

Given the mineral assemblage quartz (qz)-pyroxene (px)-garnet (ga)-plagioclase (pi) compositions given in Table 5.18, discuss the possible reactions between mineral end-members. 

Present M sbauer Studies of Natural Pyroxenes and Olivines. Table IX gives the major element chemical compositions of the silicate minerals examined in this study. Table X compares the Mossbauer parameters of these minerals, while Figures 9-13 show representative Mossbauer spectra. Fayalite (Figure 9) is the only olivine in this group. The two lines are, however, somewhat broadened (0.35 and 0.39 mm./ sec.) compared with the width of natural iron foil lines observed with our source (0.24 mm./sec.) and suggest the near coincidence of two quadrupole-split doublets resulting from Mi and M2 sites. Analysis of this "two-line spectrum into a four-line spectrum in the manner described by Evans et al (11) could possibly yield parameters for the two iron sites, but this was not undertaken since both lines appear symmetric. The "two-line quadrupole splitting of 2.78 mm./sec. is somewhat smaller... 

In geochemistry it is necessary to describe the composition of pyroxene by end-members that are compositionally simple and stoichiometrically well defined. It is also opportune to distinguish the various structural classes, because P-T stabihty and reactivity vary greatly with type of polymorph. This inevitably requires the formulation of fictitious components (i.e., components that have never been synthesized as pure phases in the structural form of interest, but that are present as members of pyroxene mixtures). For example, table 5.30 gives the list of pyroxene geochemical components proposed by Ganguly and Saxena (1987) (partly modified here). 

Pyroxenes (as feldspars) occupy a position that is chemically central in the realm of rock compositions (Robinson, 1982). They are therefore found ubiquitously and virtually in any kind of paragenesis developing in the P-T-Xconditions typical of earth s mantle and crust. Table 5.31 hsts the main occurrences. 

Table 5.32 hsts chemical compositions of natural pyroxenes. Pyroxene chemistry is quite complex and analyses must follow precise stoichiometric rules based on site occupancies. Cameron and Papike (1982) follow four rules in the selection of acceptable analyses among the plethora of values reported in the literature ... 

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