Nakhlites, Martian clinopyroxenites, are believed to have experienced aqueous alteration on the Martian surface, on the basis of the presence of reddish brown "iddingsite" on the rim and along cracks of large olivine crystals. Amorphous to poorly crystalline phyllosilicates (smectites) and carbonates are major products of aqueous alteration of the Yamato 00 nakhlites. The Yamato 000593 nakhlite and its pair Yamato 000749 contain large olivine crystals having opaque black material intimately associated with iddingsite on their rims and along cracks. Synchrotron radiation X-ray diffraction, micro-Raman spectroscopy, and transmission electron microscopy revealed that the opaque material is the nonstoichiometric olivine-type mineral laihunite □0.42(Mg2+, Fe2+) 0.79Fe0.793+ Si1.00O4, typically formed as a high-temperature oxidation product under nonequilibrium conditions. Because laihunite has been decomposed into a mixture of magnetite and amorphous silicate near the fusion crust, it is preterrestrial in origin. This is the first observation of large olivine crystals in Martian meteorites being replaced by laihunite, although its presence in olivine in the mesostasis of the Miller Range 03346 nakhlite has been suggested by micro-Raman spectroscopy. Because laihunite in the nakhlites does not form nanometer-sized intergrowth with iron oxides, fluid-assisted high-temperature oxidation may have played an important role in transporting away free Fe2+ cations liberated from olivine during laihunite formation. Although the Yamato 00 nakhlites also contain jarosite, we could not confirm the presence of preterrestrial jarosite based only on its occurrences.