Northwest Africa 4734

Rissani, Morocco

Find: October 2006

Achondrite (lunar)

History: Two pieces were purchased from nomads in Erfoud, Morocco, in October 2006 and February 2007.

Physical characteristics: A. Habibi purchased two pieces with a total mass of 477 g in Rissani, Morocco; several other pieces of the same stone totaling 895 g are with Ait Elkaid in Rissani, Morocco. Dull black/brown fusion crust is nearly complete and inside slightly tarnished where absent. It is a gray, coarse-grained, pristine magmatic rock consisting of millimeter-sized phenocrysts of mainly pyroxene and plagioclase.

Petrography: (A. Jambon, O. Boudouma, and D. Badia, UPVI). The texture is best described as shergottite-like. Pyroxene grains are highly fractured while plagioclase laths, partly transformed to maskelynite, are only affected by a small number of fractures. Silica and silica-feldspar glass are minor components. A few patches of impact melt are also observed. Ilmenite, baddeleyite, zirconolite, tranquilityite, pyrrhotite, and metal. Fayalite associated with silica probably results from the dissociation of iron rich pyroxene. Modal mineralogy (vol%): Cpx 50, plagioclase + K-feldspar 32, silica + glass 7.5, opaques (ilmenite, Ti-magnetite, pyrrhotite) + fayalite 7, voids + fractures 3.

Geochemistry: Mineralogy by EMP and SEM. (Trace and major element analyses ICP-MS and ICP-AES, J.-A. Barrat, UBO). Pyroxene grains are complexly zoned (En₆₅Fs₂₁Wo₁₃ to En₂Fs₈₃Wo₁₅; FeO/MnO = 78 [average]). A few compositions correspond to pyroxferroite. Plagioclase is normally zoned from An_75-91 (average An₈₉) with minor olivine (Fa_80-95). Chondrite normalized REE pattern with an enrichment of 53 (La) to 40 (Yb). Trace element pattern with negative anomalies of Sr and Eu. Interstitial glass is high in silica (75 wt%) and contains microcrysts of K-feldspar with a significant celsian component. The chemistry and major and trace elements are identical to NWA 032-479-773 and LAP 02205-02224-02226-02436- 03632. The texture is very similar to that of the LAP specimens. The very low abundance of olivine and the relative abundance of silica in NWA 4734 are the main differences beside the grain size and the slightly different composition of the major phases.

Classification: Achondrite (lunar); extensive shock.

Type specimens: A total of 20 g of sample and one polished section is on deposit at UPVI. Mbarek Ait Elkaid holds the main masses.

Submitted by: A. Jambon, UPVI. The description of NWA 4734 was submitted as a monzogabbro, but the Nomenclature Committee was not convinced by the materials submitted that this was a correct sub-classification for this sample.