Calcic pyroxenes

Before discussing the dominant Fe2+/M2 site crystal field spectra of pyroxenes, it is necessary to identify the locations of absorption bands originating from Fe2+ ions in the less distorted Ml sites. Crystal field spectra of stoichiometric hedenbergite, CaFeSi206, consist of two broad bands centred at 10,200 cm-1 and 8,475 cm-1 (Rossman, 1980 Straub et al., 1991), from which the crystal field parameters are estimated to be 

When slight deficiencies of calcium occur in calcic pyroxenes, Fe2+ replacing Ca2+ ions in the M2 positions produce additional bands centred around 9,600 cm-1 and 4,400 cm-1 (White and Keester, 1966 Bums and Huggins, 1973 Hazen et al., 1978). These two bands may be more intense than the Fe2+/Ml site CF bands located near 10,200 cm-1 and 8,500 cm-1, even in spectra of subcalcic hedenbergites, such as that portrayed in fig. 5.14, even though 

In augites, Fe2+ occupancies of the M2 sites increase as the pyroxene becomes increasingly subcalcic. Significant replacement of larger Ca2+ by smaller Fe2+ ions leads to slight contraction of the M2 site (table 5.6), which causes the dominant Fe2+ CF bands located near 10,000 cm-1 (wavelength of 1 micron) and 5,000 cm-1 (2 microns) to move to shorter wavelengths (Adams, 

5 Crystal field spectra of transition metal ions 

1974 Hazen et al 1978), in accord with the A a R 5 relationship (eq. 2.17). The positions of the 1 micron and 2 micron bands (cf. fig. 10.5) provide a powerful means of identifying pyroxene structure-types and compositions on surfaces of terrestrial planets by telescopic reflectance spectral measurements discussed in chapter 10. 

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Trivalent titanium has been positively identified by optical spectral measurements of a green calcic pyroxene from the meteorite that fell near Pueblo de Allende, Mexico, in 1969. The chemical analysis of this titanian pyroxene (Dowty and Clark, 1973) revealed it to be an iron-free subsilicic diopside (fas-saite) containing coexisting Ti3+ and Ti4+ ions and having the chemical formula Ca1.0lM 0.38,n3+0.34,n4+0.14Alo.87Sil.2606-... 

Figure 5.13 The calcic pyroxene structure projected onto the (001) plane showing the locations of the Ml and M2 cation sites (based on Cameron Papike, 1981). Note the chains of edge-shared Ml octahedra extending along the c axis.

On Earth, the Ti(m) oxidation state is unstable. However, Ti3+-bearing minerals are well-characterized in some meteorites and Moon rocks, generally coexisting with Ti4 in such phases as calcic pyroxene, ulvospinel, hibonite and ilmenite. In hibonite, CaAlI20I9, a refractory phase in carbonaceous chondrites, EPR and optical spectral data indicate that Ti3+ ions are present (Hunger and Stolper, 1986 Live et al., 1986). The trivalent Ti ions may be stabilized in the five-coordinated trigonal bipyramidal M5 site of the hibonite structure... 

Figure 10.1 Comparisons of visible to near-infrared spectra of calcic pyroxene in transmitted and reflected light. Polarized absorption spectra of single crystals are correlated with the reflectance spectrum of a powdered sample of the same mineral (cf. fig. 5.14).

Brearley and Jones (1998) have given an exhaustive review of the long list of minerals that have been found in CAls, along with all of the chemical variations found in those minerals, and there is no need to repeat their treatment here. However, a large number of the known CAl minerals are rare in occurrence or minor in abundance. In fact, the mineralogy of most CAls in most chondrite types can be defined mainly in terms of relatively few essential minerals spinel, melilite, perovskite, hibonite, calcic pyroxene, metal, anorthite, and olivine. Somewhat rarer but important phases are grossite and corundum. Table 1 lists the principal chemical variations of abundant and rarer phases along with selected comments on their occurrences. Table 2 lists the most common secondary minerals that are found in CAls from a variety of chondrite types. 

Calcic pyroxene CaMgSizOg-CaAbSiOg- A primary phase only in Common in CAIs in all... 

Like calcic pyroxene, sodium-free plagioclase occurs in two fundamentally distinct settings in CAIs it is a primary igneous phase in type B CAIs, and it occurs as a secondary phase that replaces melilite in Wark-Lovering rim sequences and in CAI interiors. The phase shows very little chemical variation (see Brearley and Jones, 1998). 

Anorthite has an importance far out of proportion to its simple chemistry. First, because of its very low MgO content (hence, high AI/Mg ratio) and the fact that it occurs not just in many CAIs but also in aluminum-rich chondrules, it is a prime analytical target in the search for evidence of extinct A1. Second, like calcic pyroxene, anorthite is one of the principal phases that CAIs share in common with aluminum-rich chondmles. In fact, MacPherson and Huss (2000) showed that the trends defined within CMAS (CaO-Mg0-Al203-Si02) space by silicate-bearing CAI and aluminum-rich chondrule bulk compositions converge on a region intermediate between... 

Figure 9 A type B1 inclusion from the Allende CVS chondrite. This centimeter-sized marhle consists mainly of melilite (bluish-white), titanium-aluminum-rich calcic pyroxene (bright colors), and anorthite and spinel (not readily visible in photo). Type B1 inclusions figure prominently in the early petrologic, trace element, and isotopic studies of CAIs, in part because of the richness of information about physicochemical histories available from petrologic, chemical and isotopic properties. Ironically, because type B inclusions occur only in CVS chondrites, they are nonrepresentative of CAIs in general. Photograph taken in cross-polarized transmitted fight. The colors are not the true colors of the crystals they are artifacts of the polarized fight.

In terms of sheer numbers, probably the most abundant variety of CAl in most chondrites consists largely of spinel and calcic pyroxene. Some of these contain lesser amounts of anorthite, melilite, hibonite, and perovskite, but pyroxene-spinel-rich assemblages are almost universal. These objects range in size from a few tens of micrometers up to 1-2 cm (in CV3 chondrites). Large or small, from whatever kind of chondrite, the essential structure consists of small spinel grains or dense nodules of spinel grains or even chains of spinels, enveloped in a continuous thin rim of aluminum diopside that binds the entire structure together. 

The CAIs in CH chondrites show httle secondary mineralization rare anorthite replaces melihte in some CAIs (e.g.. Figure 20(c)), but aUcali-rich phases, grossular, and phyUosUicates have not been found. Wark-Lovering rims occur on some but not all CAIs those on the microspherules in ALH85085 tend to consist mainly of a wispy layer of calcic pyroxene. 

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