Northwest Africa 7835 (NWA 7835)

(Northwest Africa)

Purchased: 2013 Nov

Classification: Ungrouped achondrite

History: Purchased by S. Ralew in November 2013 from a dealer in Zagora, Morocco.

Physical characteristics: A very fresh stone (56 g) coated in black, glossy fusion crust, which upon cutting was seen to be composed mostly of gray pyroxene and pale yellow olivine with very little metal or sulfide.

Petrography: (A. Irving and S. Kuehner, UWS) This specimen is composed predominantly of large grains (up to 6 mm) of olivine and compositionally-zoned orthopyroxene with minor clinopyroxene, chromite, rare taenite and troilite in a sparse, interstitial matrix of vesicular, feldspathic glass charged with angular fragments of the constituent minerals. No chondrules are present.

Geochemistry: Olivine (Fa_23.7-24.6; FeO/MnO = 57-60, N = 4), orthopyroxene (Fs_19.5-20.4Wo_1.4-1.6; more magnesian cores Fs_15.0-15.8Wo_0.8-0.7; FeO/MnO = 33-37), clinopyroxene (Fs_7.2Wo_45.0; FeO/MnO = 24). Oxygen isotopes (K. Ziegler, UNM): analyses of random multiple subsamples of the specimen (after acid washing) by laser fluorination gave (all in per mil) δ¹⁷O = 3.190, 1.971, 2.309, 1.889, δ¹⁸O = 5.042, 3.709, 4.044, 3.870, Δ¹⁷O = 0.528, 0.013, 0.174, -0.154 (for a TFL slope of 0.528). Bulk composition (R. Conrey, WSU; G. Chen, UAb): analyses of representative, clean bulk cutting dust by X-ray fuorescence spectrometry and ICPMS gave (in wt.%) SiO₂ = 42.9, TiO₂ = 0.16, Al₂O₃ = 2.6, Cr₂O₃ = 0.45, FeO = 17.9, MnO = 0.33, MgO = 29.9, CaO = 1.7, Na₂O = 0.85, K₂O = 0.13, P₂O₅ = 0.05; (in ppm) La = 0.45, Ce = 1.23, Nd = 0.89, Sm = 0.28, Eu = 0.10, Gd = 0.37 Dy = 0.39, Er = 0.24, Yb = 0.23, Lu = 0.04, Hf = 0.31.

Classification: Ungrouped achondrite. This unusual specimen appears to have some affinities to ordinary chondrites, yet chondrules are absent and there is hardly any metal present (and no kamacite). The bulk major and trace elements are broadly chondritic, yet only one out of four subsamples analyzed for oxygen isotopes plots near the trends for any typical ordinary chondrites (namely H chondrites). However, the compositions of the mafic minerals are inconsistent with those of H chondrites and more similar to those of L chondrites. Three other oxygen isotope analyses conducted on acid-washed subsamples of this very fresh specimen plot close to the TFL. The overall texture is that of a melt-matrix fragmental breccia, which could explain the range of Δ¹⁷O values, although no foreign clasts were observed in the polished section.

Specimens: 11.9 g and one large polished thin section are at UWB. The remaining material is held by Ralew.