Northwest Africa 2400

Morocco or Algeria

Purchased 2003 April

Achondrite (ungrouped)

A 137 g light tan to rusty brown, fully crusted stone was purchased in Rissani, Morocco in April 2003. Description and classification (T. Bunch and J. Wittke, NAU; A. Irving and S. Kuehner, UWS): composed mainly of approximately mm-sized, randomly oriented pigeonite (Fs_64-64.8Wo_5.6-6.4) with very fine, parallel exsolution lamellae of augite (Fs_38-49.1Wo_36.7-23), and interstitial, recrystallized plagioclase (<0.10 mm in size, ~An₈₆Or_0.5) with prominent triple junctions, and rare, unrecrystallized, tabular plagioclase relicts (<1 mm, zoned from An₈₀to An₈₉). FeO/MnO ratios for all pyroxenes range from 59.5 to 68.7 (avg. 64.7). Only small amounts of pyroxene are present in the recrystallized zones. Accessory minerals occurring predominantly in the recrystallized plagioclase zones include a silica polymorph, chromite [Cr/(Cr + Al) = 83, Fe/(Fe + Mg) = 67], ilmenite (53.5 wt. % TiO₂, 43.6 wt. % FeO; with small baddeleyite inclusions), merrillite (some with minor amounts of Na₂O, MgO, and FeO), Zn-bearing troilite, pentlandite, Cr-ulvöspinel and altered Fe metal (associated with minor troilite, barite and calcite). Shock damage is not apparent and, with the exception of the oxidation of minor metal, weathering effects are very low. Oxygen isotopes (D. Rumble, CIW): replicate analyses by laser fluorination gave δ¹⁷O = - 0.15 ± 0.1, δ¹⁸O = 3.27 ± 0.2, Δ¹⁷O = -1.57 ± 0.03 per mil. In its texture, mineral mode, mineral compositions and oxygen isotope composition NWA 2400 is nearly identical to the anomalous achondrite NWA 011 (Yamaguchi et al., 2002; Promprated et al., 2003), which strongly indicates that these two stones are paired. Specimens: type specimen 20.2 g and three polished thin sections, NAU; main mass, anonymous purchaser.