Northwest Africa 998

Algeria or Morocco

Purchased 2001 September

Martian meteorite (nakhlite)

A. and G. Hupé (xHupé) purchased, from dealers at the Tucson Gem and Mineral Show in 2002 February, the main mass from a 456 g stone that had been acquired at an unspecified site in western Algeria or eastern Morocco in 2001 September. Dimensions before cutting: 72 mm by 65 mm by 48 mm. Classification and mineralogy (A. Irving and S. Kuehner, UWS): a friable, dark green rock with minor orange-brown alteration products that probably are of pre-terrestrial origin. It is composed mainly of subhedral, olive-green, complexly zoned subcalcic augite (Fs22Wo39) with subordinate yellow olivine (Fa64), orthopyroxene (Fs49Wo4), interstitial plagioclase (Ab61Or4 containing 0.1 wt% SrO, and exhibiting normal birefringence), titanomagnetite, chlorapatite and pyrrhotite. The overall texture is that of a hypabyssal, adcumulate igneous rock, and the apparent crystallization sequence is olivine, orthopyroxene, titanomagnetite, augite, apatite, plagioclase. There is a weak preferred orientation of prismatic pyroxene crystals, many of which have very distinctive zoning, with cores of augite surrounded by irregular, inverted pigeonite rims (now consisting of orthopyroxene with fine augite lamellae). Trains of tiny melt inclusions are present along healed fractures within pyroxene; microprobe study confirms that most of these are K-Na-Al-bearing silicate glass, but some are intergrowths of glass and Fe-bearing carbonate, which may represent quenched immiscible silicate-carbonate liquids. Symplectitic intergrowths of titanomagnetite and low-Ca pyroxene are present at grain boundaries between large, discrete olivine and titanomagnetite grains, but are not present around chromian titanomagnetite inclusions within olivine. These observations suggest that a pre-terrestrial oxidation process produced the symplectites, and involved high temperature, deuteric fluid infiltration along grain boundaries; such fluids also may have produced the irregular pigeonitic rims on augite crystals. Secondary (probably pre-terrestrial) ankeritic carbonate, K-feldspar (some Fe-bearing), serpentine (?), calcite and a Ca sulfate are present on grain boundaries and within cracks in augite. Oxygen isotope composition (D. Rumble, CIW): replicate analyses of acid-washed augite by laser fluorination gave δ¹⁸O = +3.9 ± 0.2‰; δ¹⁷O = +2.4 ± 0.1‰; ∆¹⁷O = +0.30 ± 0.02‰. Specimens: type specimens, 20 g, UWS, 20 g, FMNH, and two polished thin sections, UWS; main mass, xHupé.