Northwest Africa 15500 (NWA 15500)

(Northwest Africa)

Purchased: Mar 2022

Classification: Lunar meteorite

History: Purchased online in March 2022 by Craig Zlimen froma meteorite dealer in Algeria.

Physical characteristics: Single stone with no visible fusion crust. A slice of the interior reveals the stone to be divided in half into two different rock types.

Petrography: (D. Sheikh, Cascadia) Specimen represents a dimict breccia comprised of two distinct (but genetically related) lithologies separated by a sharp contact: Lith A) A feldspathic fragmental breccia composed of mm-sized angular to subangular lithic and mineral clasts set within a fine-grained matrix. Clast lithologies identified in this sample include anorthosites, norites, olivine norites, troctolites, pink spinel troctolites, pink spinel anorthosites, and impact melt clasts; clasts lithologies are predominantly Mg-suite in composition, with minor input from lunar ferrous anorthosite. Accessory phases include ilmenite, SiO₂-rich glass, and merrillite. Lith B) A clast-poor troctolitic impact melt composed of compositionally zoned olivine and minor high-Ca pyroxene dendrites (Av. grain size ~10 µm, and unzoned plagioclase laths (mix of both cracked and smooth, Av. grain size ~0.5 mm). Clast lithologies identified include anorthosites, pink spinel anorthosites, and troctolites. Accessory phases include SiO₂-rich glass and ülvospinel.

Geochemistry: Lith A [Olivine (Fa_17.8±12.1, range Fa_4.4-53.3, Fe/Mn=85±14, n=76), Low-Ca Pyroxene (Fs_23.9±13.8Wo_3.4±1.2, range Fs_10.5-52.2Wo_0.8-4.6, Fe/Mn=55±8, n=23), Pigeonite (Fs_33.0±10.1Wo_10.7±3.3, range Fs_19.1-56.6Wo_6.3-17.0, Fe/Mn=55±7, n=16), High-Ca Pyroxene (Fs_15.8±6.6Wo_38.1±5.3, range Fs_5.8-29.6Wo_26.0-45.7, Fe/Mn=50±10, n=15), Calcic Plagioclase (An_96.5±0.6, range An_94.9-98.0, n=79), Pink Spinel (Cr#=3.9±0.2, Mg#=85.5±1.3, n=70)]; Lith B [Olivine (Fa_21.6±5.8, range Fa_9.1-30.7, Fe/Mn=79±11, n=35), Low-Ca Pyroxene (Fs_15.7±0.8Wo_4.3±0.1, range Fs_15.2-16.3Wo_4.3-4.4, Fe/Mn=57±4, n=2), High-Ca Pyroxene (Fs_27.4±12.9Wo_39.5±2.7, range Fs_8.6-37.4Wo_36.8-43.2, Fe/Mn=46±5, n=4), Calcic Plagioclase (An_95.8±0.8, range An_94.4-97.9, n=40), Pink Spinel (Cr#=3.9±0.1, Mg#=82.0±1.3, n=19)].

Classification: Lunar (dimict breccia). The unusual presence of bonifide pink spinel anorthosite clasts that contain pink spinel compositionally similar to those measured by the Moon Mineralogy Mapper imaging spectrometer (M^3) suggest derivation of this specimen from a previously unsampled region on the lunar surface.

Specimens: 20.2 g at Cascadia, main mass with Craig Zlimen.