Pyroxenes clinopyroxene

Pyroxenes are poly silicates ( inosilicates in the mineralogical classification), crystallizing, respectively, in the monoclinic clinopyroxenes spatial groups C2lc, Pljlc, P2ln) and orthorhombic systems orthopyroxenes spatial groups... 

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). 

On the basis of the experimental data of Lindsley and Dixon (1976) on the CaMgSi20g-Mg2Si205 binary join and accounting for the observed compositions of natural pyroxenes coexisting at equilibrium, Kretz (1982) proposed two empirical thermometric equations applicable to clinopyroxene and valid, respectively, for T > 1080 °C and T < 1080 °C ... 

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). 

Figure 10.5. The 1 pm versus 2 pm pyroxene spectral determinative curve widely used to identify compositions and structure-types of pyroxenes on planetary surfaces (from Adams, 1974). Circles refer to room-temperature data. Numbered squares (orthopyroxene En86Fs14) and triangles (clinopyroxene Wo42En51Fs7) represent spectral data obtained at the temperatures (1) 80 K (2) 173 K (3) 273 K (4) 373 K and (5) 448 K (modified from Singer Roush, 1985).

Figure 10.7 Reflectance spectra of mixed-mineral assemblages (modified from Singer, 1981). Left orthopyroxene (En86Fs14) - clinopyroxene (Wo41En51Fs7) mixtures right orthopyroxene - olivine (Fo85Fs15) mixtures. Mineral proportions are expressed as wt per cent. Note how pyroxene dominates the mineral-mixture spectra. Olivine causes broadening of the pyroxene 1 pm band but another olivine feature persists near 1.25 pm.

The contrasting temperature-induced shifts of the pyroxene 1 and 2 pm bands could lead to erroneous estimates of the composition and, to a lesser extent, structure-type of a pyroxene-bearing mineral assemblage deduced from the remote-sensed reflectance spectrum of a hot or cold planetary surface if room-temperature determinative curves, such as that shown in fig. 10.5, are used uncritically. For example, remote-sensed spectra of planets with hot surfaces, such as Mercury and the Moon, would lead to overestimates of Fe2+ contents of the orthopyroxenes and underestimated Fe2+ contents of the clinopyroxenes (Singer and Roush, 1985). Planets with cold surfaces, such as Mars and the asteroids, could produce opposite results. On the other hand, the room-temperature data underlying the pyroxene determinative curve shown in fig. 10.5 may impose constraints on the compositions of pyroxenes deduced from telescopic spectra of a planet with very high surface temperatures, such as Mercury. 

Figure 15 Oxygen-isotopic compositions of individual minerals in (a) aluminum-rich chondrules from ordinary chondrites (Russell et at, 2000) and (h) CR carhonaceous chondrites (Krot et ah, 2002h). (c) Oxygen-isotopic compositions of individual minerals in the CAI-hearing chondmle 17 from Acfer 094 (data from Krot et al, 2003a) (Ahhreviations chd, chondmle cpx, clinopyroxene gl, glass hih, hihonite nph, nepheline ol, olivine opx, orthopyroxene pi, plagioclase px, pyroxene sp, spinel).

Clinopyroxene shows a range of REE patterns from extremely enriched to very depleted TREE signatures (Figure 22). Noncratonic peridotites are subdivided on the basis of clinopyroxene REE patterns into LREE-depleted (type lA) and LREE-enriched (type IB Menzies, 1983 Figure 17). LREE-enriched type IB pyroxenes are the norm in most suites. LREE-depleted varieties are relatively scarce. Very few clinopyroxenes show simple LREE-depleted REE patterns that can be interpreted solely in terms of melt depletion, i.e., LREE depletion, fiat, unfractionated MREE-HREE patterns (e.g., UM-6 or 2905 Eigure 22). For peridotites that do have LREE-depleted clinopyroxenes, a correlation of HREE with other incompatible trace elements (e.g., yttrium, strontium, zirconium) in xenoliths suites worldwide requires fractional melting to be the principal means of depletion in the mantle (Norman, 2001). 

Few systematic Sr-Nd isotope studies have been performed on ocean island xenolith suites. Ducea et al. (2002) analyzed clinopyroxenes from plagioclase-spinel and spinel peridotites from Pali, (Oahu, Hawaii) and found relatively depleted strontium and neodymium isotope systematics that they interpret as representing their evolution as residues from the extraction of Pacific Ocean crust. Consistent with this is a 61 20Ma errorchron defined by the pyroxene separates that is within error of the 80-85 Ma age of Pacific lithosphere beneath Hawaii. 

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