Clinopyroxenes Ml-site

We conclude that it is very unlikely that any Pa enters the clinopyroxene Ml site. 

Orthopyroxene has a Vl-fold Ml site and a Vl-fold M2 site. Both are predominantly filled by Mg and Fe. The smaller Ml site shares many characteristics with the clinopyroxene Ml site. It is therefore reasonable to assume that no U-series cations, including Pa (see above) enter that site. We will confine our discussion to the octahedral M2, which is smaller than the equivalent (Vni-fold) clinopyroxene site, even after allowing for the different coordination number. Consequently most of the U-series elements have very low orthopyroxene-melt partition coefficients. 

Orthopyroxene has a large sixfold site, M2 and the smaller sixfold Ml site, which is similar to the clinopyroxene Ml site. We will confine our discussion to the octahedral M2, which is smaller... 

According to Cameron and Papike (1982), pyroxenes contain Cr " and TF" in rocks equilibrated at low fo (lunar specimens, meteorites). However, spectroscopic evidence is ambiguous and insufficient for a safe attribution (Rossman, 1982). Some authors (Bocchio et ah, 1979 Ghose et al., 1986 Griffin and Mot-tana, 1982) report the presence of Mn " in Ml sites in clinopyroxene. Davoli (1987) reexamined this hypothesis, proposing precise structural criteria to detect the presence of Mn " in the monoclinic phase (the ratio Mn /Mn may be a potential /02 barometer). 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

Only in calcic clinopyroxenes, in which Ca2+ ions completely fill the M2 sites and Fe2+ and other transition metal ions occur in the Ml sites alone, is ideal solution behaviour to be expected. This is because cation ordering is not possible in one-site atomic substitution in the pyroxene Ml site. Furthermore, there is an insignificant variation of the CFSE of Fe2+ across the diopside-hedenbergite series ( 5.5.3). 

In orthopyroxenes, the very intense Fe2+/M2 site bands near 11,000 cm-1 and 5,000 cm-1 in spectra measured at atmospheric pressure (cf. fig. 5.15), as well as the Fe2+/Ml site band located at 8,333 cm-1, show pressure-induced blue-shifts (Shankland et al., 1974 Mao and Bell, 1971). At elevated temperatures, the Fe2+/M2 site 1 micron (11,000 cm-1) band shows negligible thermal shifts (Sung et al., 1977 Singer and Roush, 1985). However, the 2 micron (5,000 cm-1) band shows a significant red-shift in orthopyroxenes and a blue-shift in clinopyroxenes (Singer and Roush, 1985). These effects, which have important applications in remote-sensed spectral measurements of hot planetary surfaces, are described in chapter 10 ( 10.7). 

Pyroxenes have the general formula X(Y)(Si03)2. They crystallize in a monochnic (clinopyroxenes) or orthorhombic (orthopyroxenes) structure. The Y cations are located in the Ml sites which are moderately distorted octahedra whereas X represents cations in highly distorted sites with six-, seven- or eight-fold coordination according to the kind of mineral. Generally, the structure is orthorhombic when X is Mg or Fe " and is monoclinic when X is Ca or Na. 

Only McDade et al. (2003b) report experimental data on the partitioning of Nb and Ta between olivine and melt. At 1.5 GPa they find Dm = 1 x lO " and Dia = 6 x 10 ", which indicates that is smaller than rNb (0.660 A). This will serve to exclude Pa (rpa = 0.78 A) from the M-sites in much the same way as it is excluded from Ml in clinopyroxene. Furthermore D h and Dxa are themselves two orders of magnitude lower in olivine than coexisting clinopyroxene (McDade et al. 2003b). For these reasons we suggest that Pa is substantially more incompatible in olivine than clinopyroxene. 

Given Equation (16) we derived, by nonlinear least-squares fitting, values of r from a total of 82 experiments in which three or more clinopyroxene-liquid REE partition coefficients were measured and in which the compositions of crystal and liquid phases were well constrained. Adopted radii values for individual REE were those of Shannon (1976) for 3+ ions in VIII coordination. Resulting values of range from 0.979 A to 1.055 A. We performed stepwise linear regression of the derived values of r against all major compositional parameters, pressure and temperature. The result was that the only important parameters appeared to be the octahedral (Ml) A1 content and the calcium content of the M2-site. Fitting Tq to these parameters yields an equation which reproduces the 82 points with a standard deviation of 0.009 A (i.e., less than the difference in ionic radius between adjacent REE) ... 

C) Specimens of clinopyroxenes separated from 10003 and 10044 showed clearly resolved Fe " " quadrupole doublets from the Ml and M2 sites [240]. The 10044 sample appeared to be exclusively augite, while 10003 also contained some pigeonite. Relative site occupancies were estimated in several samples, and again no evidence for high-spin Fe " " was found. 

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