Pyroxene orthopyroxene

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... 

Figure 5,25 Phase stability relations for natural pyroxenes in the quadrilateral. (A) Magmatic pyroxenes (not reequilibrated at low 7). (B) Three-phase region for pyroxenes of stratihed complexes and magmatic series. (C) Pyroxenes reequilibrated in subsolidus conditions. = Pbca orthopyroxene V = augite C2lc) A = pigeonite (P2jlc) 0 = olivine. From Huebner (1982). Reprinted with permission of The Mineralogical Society of America.

Ross M. and Huebner J. S. (1975). A pyroxene geothermometer based on composition temperature relationship of naturally occurring orthopyroxene, pigeonite and augite. In International Conference on Geothermometry and Geobarometry, The Pennsylvania State University. 

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). 

The crystal field spectral measurements of orthopyroxenes described in 5.5.4 also demonstrate the relative enrichments of Fe2+ ions in the pyroxene M2 sites (Goldman and Rossman, 1979). 

The strong preference of Mn2+ ions for the orthopyroxene M2 sites, first demonstrated indirectly by Mossbauer spectroscopy (Bancroft et al., 1967), was subsequently confirmed by X-ray structure refinements of synthetic Mg2+-Mn2+ (Ghose et al., 1975) and Mg2+-Mn2+-Co2+ (Hawthorne and Ito, 1977) orthopyroxenes. These X-ray measurements, as well as those of synthetic Mg2+-Co2+ and Mg2+-Zn2+ orthopyroxenes (Ghose et al., 1975), also showed that Co2+ and Zn2+ ions are both relatively enriched in the orthopyroxene M2 site. In a synthetic Mg2+-Cr2+ pyroxene, the Cr2 ions are strongly enriched in the M2 site (Angel et al., 1989), causing increased distortion of this site as a result of the Jahn-Teller effect ( 6.3). 

The Ni2+, Cr3+ and low-spin Co3+ ions do not acquire additional stabilization in distorted octahedral sites. They are expected to favour smaller sites that more closely approximate octahedral symmetry than other available sites in the crystal structures. As noted in 6.8.2, the high octahedral CFSE s acquired by these three cations in small octahedral sites in silicate and oxide structures accounts for the observed relative enrichments of Ni2+ in the olivine Ml and orthopyroxene Ml sites, the sole occupancy by Cr3+ of pyroxene Ml sites, and the occurrence and stability of low-spin Co3+ in Mn(IV) oxides. 

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. 

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-... 

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.

Figure 10.8 Diffuse reflectance of orthopyroxene (PYROX) with plagioclase feldspar (PLAG) and magnetite (MAG) (from Adams, 1974, attributed to C. M. Pieters). Mineral proportions are expressed as wt per cent. Note how the opaque oxide phase swamps the diagnostic pyroxene bands at 1 and 2 pm.

Figure 10.9 Reflectance spectra obtained from Earth-based telescopes for small (< 5 km diameter) areas within the Copernicus crater on the Moon (from Pieters et al., 1985). Left reflectance scaled to unity at 1.02 im right residual absorption after continuum removal. Spectra are offset vertically, (a) Wall and (b) floor areas containing orthopyroxene are deduced to be of noritic composition (c) floor containing pyroxene and glass is an area of extensive impact melt and (d) central peak containing olivine is deduced to be troctolite.

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. 

---