Extremely reducing conditions for the stability of khatyrkite and cupalite

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Marco E. Ciriotti
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Extremely reducing conditions for the stability of khatyrkite and cupalite

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 11:45

Referenza:
▪ I-Ming Chou (2018): Extremely reducing conditions for the stability of khatyrkite and cupalite. IMA2018 Abstract book. Poster presentations. Meteorites and the Early Solar System, 432.

Abstract:
A rock sample, found in the Koryak Mountains in Russia that contained icosahedrite (an icosahedral quasicrystalline phase with composition
Al63Cu24Fe13), was considered by Bindi et al. (2012) as a part of a meteorite, likely formed in the early solar system about 4.5 Gya. The quasicrystal grains they observed are intergrown with diopside, forsterite, stishovite, and several metallic phases, including khatyrkite (CuAl2), cupalite (CuAl), and β-phase (AlCuFe). Even though an extraterrestrial origin was inferred from their secondary ion mass spectrometry 18O∕16O and 17O∕16O measurements of the pyroxene and olivine intergrown with the metals, the mechanism that produced this exotic assemblage was not yet understood. They claimed that the assemblage of metallic copper-aluminum alloy is extremely reduced, with the stability boundary between khatyrkite (representing the originally formed liquid CuAl2) and its oxidized solid products CuO and Al2O3 being slightly more oxidizing than that of the Si-SiO2 buffer in a log fO2-T plot (their Fig. 5; see attached image). Similarly, as shown in their Fig. 5, the stability boundary between cupalite (representing liquid CuAl) and its oxidized solid products CuO and Al2O3 is between those of Fe-FeO and Si-SiO2 buffers. Based on these phase relations, they concluded that these alloys (actually their original respective liquids) require highly reducing conditions to form relative to their stoichiometric oxides, but not so reducing as for pure Al or Si. Therefore, they thought they resolved the apparent anomaly of finding no metallic silicon at the metal-silicate interfaces in their sample. In addition, their calculated fO2 for khatyrkite (actually liquid CuAl2) is close to the estimated oxygen fugacity of the early solar nebula in the region where calcium-aluminum-rich inclusions (CAIs) formed (Grossman et al., 2008). Therefore, they concluded that, given the association of the metal with CAI phases (especially spinel) in their sample, a nebular origin could provide the necessary conditions to stabilize the metallic alloys. However, these conclusions were based on the metastable reactions, using the unstable CuO as the oxidized product for both khatyrkite and cupalite liquids instead of the stable Cu, as indicated by the Cu-CuO buffer curve shown in their Fig. 5. Based on the same thermodynamic data base used by Bindi et al. (2012), the calculated stability boundary between khatyrkite (actually liquid CuAl2) and its oxidized products Cu and Al2O3 at 0.1 MPa is only slightly more oxidizing than that of the Al-Al2O3 buffer. Also, it is slightly more reducing than that between cupalite (actually liquid CuAl) and its oxidized products Cu and Al2O3. Both of these newly calculated phase boundaries are more reducing than that of Si-SiO2 buffer, therefore, the anomaly of finding no metallic silicon at the metal-silicate interfaces still remains unresolved.
Furthermore, their conclusion that a nebular origin could provide the necessary conditions to stabilize the metallic alloys is not valid, because more reducing conditions were required. Recognizing the redox states actually required for the stability of these metal alloys is essential for resolving the remarkable puzzles posed by these rather unordinary meteorite fragments.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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