Extraterrestrial pyroxenes

The pyroxene M2 site, on the other hand, is very distorted and metal-oxygen distances span wide ranges as demonstrated by the data summarized in table 5.6 chosen for a variety of extraterrestrial pyroxenes. Distances from the M2 cation to the four non-bridging oxygens are considerably shorter than those to oxygens bridging two silicon atoms. Moreover, distances to, and numbers... 

Photomicrograph of the ALH84001 Martian meteorite, field of view = 1.3 cm. Large, broken grains of pyroxene form this breccia. This sample created a stir when it was proposed to contain evidence for extraterrestrial life. Image from Lauretta and Kilgore (2005), with permission. 

Pyroxenes from extraterrestrial sources provide unequivocal examples of Ti3+ —> Ti4+ IVCT and Fe2+ —> Ti4+ IVCT bands. For example, the iron-free green titanian pyroxene in the Allende meteorite discussed in 4.4.1 is the one irrefutable example of a mineral showing a Ti3+ — > Ti4+ IVCT transition. The position of the band at 666 nm (15,000 cm-1) shown earlier in fig. 4.2 is insensitive to pressure, but it does intensify at high pressures (Mao and Bell, 1974a), consistent with it representing a Ti3+ —> Ti4+IVCT transition between adjacent Ti3+ and Ti4+ ions located in edge-shared Ml octahedra in the pyroxene structure (fig. 5.13). 

Pressure-induced intensification and blue-shifts of absorption bands around 470 to 478 nm (21,280 to 20,920 cm-1) in extraterrestrial titanian pyroxenes enabled Ti3+ ions to be identified in meteorites (Mao and Bell, 1974a) and specimens from the Moon (Abu-Eid, 1976). These effects are described in 4.4.1 and 4.7.2.5. 

Two Other factors unique to these extraterrestrial samples must also be accounted for. The first is that tracks from heavy cosmic ray particles (mainly Fe-group nuclei) and other nuclear interactions with cosmic rays may also be present, in addition to fission tracks. The simplest method for correcting for the cosmic ray background is to measure the track density in adjacent silicate mineral grains, such as feldspar and pyroxene, which do not contain uranium and are therefore free of fission tracks. The second factor is that the samples are so old as to contain tracks from the spontaneous fission of now-extinct transuranic elements, particularly with a half-life of 82 Myr. Such Pu tracks will only be present in samples older than about 3.9 Gyr (Crozaz and Tasker 1981). 

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