Northwest Africa 17581 Lunar Fragmental Breccia

Petrography: NWA 17581 has a predominantly feldspathic composition with numerous macroscale clasts set in a similar microscale matrix. Clasts and matrix are composed of mono and polymineralic fragments mostly of anorthite with minor Ca-pyroxenes and rare slightly ferroan olivine. Examination of BSE images with ImageJ software gives a mode of approximately 85% anorthite, 14% pyroxene, <1% olivine, and <1% opaques. Pyroxenes often show exsolution. Several grains were observed with approximately one half of the grain composed of high-Ca pyroxene and the other half of low-Ca pyroxene with a sharp linear transition between the two. Rare accessory kamacite is present as small submillimeter grains and veinlets, with veinlets often following shock fissures transecting multiple phases. A single highly shocked pyroxene grain was observed with abundant preferentially oriented 5 µm to 10 µm rod-like kamacite microstructures transecting exsolved lamellae off-set by intragranular shearing. Shock features such as melt veins and significant fracturing of mineral grains are present throughout the matrix and clasts. Kamacite was identified using BSE imaging and EDS but was not quantitatively analyzed.

Geochemistry: Olivine Fa_54.4±0.2, (wt%) Fe/Mn=99.5±5.8, n=7; low-Ca Pyroxene Fs_46.9Wo_2.7, (wt%) Fe/Mn=68.6 n=1; Pigeonite Fs_36.6±3.9Wo_10.3±6.9 Fe/Mn=54.7±5.5 (wt%) n=2; Augite Fs_21.8±2.9Wo_38.2±4.2, (wt%) Fe/Mn=59.8±5.4, n=5; Plagioclase An_97.0±0.5Ab_2.9±0.5, (wt%) n=16.