ACHONDRITE ALHA77005: ALTERATION OF CHROMITE AND OLIVINE

The olivine crystals of the 77005 achondrite are brown except for colorless shock lamellae, mottled patches, and grains adjacent to pools of impact melt. Sporadic dark alteration patches in brown olivine and Cr-rich spinel gave the following average electron-microprobe analyses: (olivine) P₂O₅ 0.9, SiO₂ 57.9, TiO₂ 0, Al₂O₃ 0.7, Cr₂O₃ 0.4, V₂O₃ 0, Fe₂O₃ (assumed oxidation state) 17.0, MgO 1.6, CaO 0.2, Na₂O 0, K₂O 1.8, SO₃ (assumed oxidation state) 9.2, Cl 0.1, sum 89.8 wt. %; (spinel) P₂O₅3.5, SiO₂2.1, TiO₂.2.2, Al₂O₃2.1, Cr₂O₃ 13.4, V₂O₃ 0.8, Fe₂O₃ 40.7, MgO 0.9, CaO 0.1, Na₂O 0, K₂O 2.0, SO₃ 11.1, Cl 0.1, sum 79.0 wt.%. Ion-microprobe analyses revealed H in both. Rare orange patches in brown olivine from another area gave SiO₂ 33–35, FeO 30-28, MgO 28–32, sum 93 wt. %. Thermal metamorphism under dry oxidizing conditions is discussed as a possible alternative to shock-induced oxidation for generation of the brown olivine (McSween and Stöffler). Because alteration patches transgress shock lamellae, and because sulfatic alteration occurs in fusion crusts of Antarctic meteorites (Gibson et al., 1983), alteration of the 77005 achondrite at the Antarctic surface is preferred to a complex series of processes needed for pre-terrestrial alteration.