Northwest Africa 7325 (NWA 7325)

(Northwest Africa)

Purchased: 2012 Apr

Classification: Ungrouped achondrite

History: Purchased by Stefan Ralew in April 2012 from a dealer in Erfoud, Morocco. The material was found reportedly in southern Morocco in early 2012.

Physical characteristics: A group of 35 very fresh, dark greenish stones (total 345 g), several of which have partial remnant chartreuse-colored fusion crust. This material is notable for the characteristic "frosty" luster of the plagioclase and bright green color of the clinopyroxene.

Petrography: (A. Irving and S. Kuehner, UWS): The studied specimen has an overall plutonic igneous (possibly cumulate) texture. It is a medium-grained aggregate of Al-Cr-bearing diopside, calcic plagioclase and lobate (possibly resorbed) grains of forsterite, with accessory Cr-bearing iron sulfide (probably troilite), very sparse ferrochromite, kamacite and taenite, and rare eskolaite (associated with Cr-Fe sulfide). In thin section the untwinned plagioclase is birefringent, but the levels of birefringence are less than normal and variable on a fine scale (suggesting the existence of compositional and/or structural microdomains); there also are sparse vesicles (suggesting at least partial melting). The pyroxene exhibits at least two sets of polysynthetic twin lamellae, which resemble those produced by moderately high shock.

Geochemistry: Clinopyroxene (Fs_1.1-2.6 Wo_45.1-44.5, FeO/MnO = 12-21, Al₂O₃ = 2.6-2.8 wt.%, Cr₂O₃ = 1.0 wt.%), plagioclase (An_88.1-89.2Or_0.0), olivine (Fa_2.7-2.8, FeO/MnO = 30-36, CaO = 0.27-0.32 wt.%, Cr₂O₃ = 0.32-0.35 wt.%). Bulk composition (G. Chen, UAb): analyses of representative bulk wire-saw cutting dust by ICP-MS gave: (in ppm) La 0.15, Ce 0.34, Nd 0.16, Sm 0.05, Eu 0.58, Gd 0.05, Hf 0.44, Th 0.27. Oxygen isotopes (K. Ziegler, UNM): laser fluorination analyses of acid-washed subsamples gave, respectively δ¹⁷O = 3.214, 3.670, 3.249; δ¹⁸O = 7.566, 8.204, 7.957; Δ¹⁷O = -0.781, -0.662, -0.952 per mil [for TFL slope of 0.528].

Classification: Achondrite (ungrouped). The combination of highly magnesian mafic silicates and highly calcic plagioclase with chromium-bearing sulfides is unique among achondrites, and the specimen is highly depleted in most trace elements.

Specimens: A total of 21 g of type material and one polished thin section are on deposit at UWB. The remaining material is held by Ralew.