NWA 998 NAK
Meteorite Name
NWA 998
Classification
(Martian) Nakhlite
Location
Found 2001, Algeria or Morocco
Weathering &
Shock Level
 
TKW
456g
Specimen Type
dpu thin section
Links
MB87
A. and G. Hupé (Hupé) 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, Hupé.

Image nwa998-1
Copyright 2008
Copyright 2008
Copyright 2008

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