Lunar Meteorite: Northwest Africa 15500

Northwest Africa 15500. Photo credit: Daniel Sheikh and Craig Zlimen
Two sawn faces of NWA 15500. The A lithology is on top, the B lithology on the bottom. Photo credit: Daniel Sheikh and Craig Zlimen
Plane-polarized (PPL) image of a thin section of NWA 15500. The A lithology is on right, the B lithology on the left. Image credit: Daniel Sheikh
A pink spinel clast in hand specimen. Image credit: Daniel Sheikh
Left: Photomicrograph of the exterior edge of lithology A (in lower left). Right: same image with increased color saturation to show the pinkness of the spinels. On the upper right of each image is mounting epoxy with embedded quartz and feldspar crystals for thickness calibration. Image credit: Daniel Sheikh

from The Meteoritical Bulletin, No. 111

Northwest Africa 15500 (NWA 15500)

(Northwestern Africa)
Purchased: 2022 March
Mass:467 g (1 piece)

Lunar meteorite

History:  Purchased online in March 2022 by Craig Zlimen from a 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 (Fa17.8±12.1, range Fa4.4-53.3, Fe/Mn=85±14, n=76), Low-Ca Pyroxene (Fs23.9±13.8Wo3.4±1.2, range Fs10.5-52.2Wo0.8-4.6, Fe/Mn=55±8, n=23), Pigeonite (Fs33.0±10.1Wo10.7±3.3, range Fs19.1-56.6Wo6.3-17.0, Fe/Mn=55±7, n=16), High-Ca Pyroxene (Fs15.8±6.6Wo38.1±5.3, range Fs5.8-29.6Wo26.0-45.7, Fe/Mn=50±10, n=15), Calcic Plagioclase (An96.5±0.6, range An94.9-98.0, n=79), Pink Spinel (Cr#=3.9±0.2, Mg#=85.5±1.3, n=70)]; Lith B [Olivine (Fa21.6±5.8, range Fa9.1-30.7, Fe/Mn=79±11, n=35), Low-Ca Pyroxene (Fs15.7±0.8Wo4.3±0.1, range Fs15.2-16.3Wo4.3-4.4, Fe/Mn=57±4, n=2), High-Ca Pyroxene (Fs27.4±12.9Wo39.5±2.7, range Fs8.6-37.4Wo36.8-43.2, Fe/Mn=46±5, n=4), Calcic Plagioclase (An95.8±0.8, range An94.4-97.9, 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.

Randy Says…

I have not studied Northwest Africa 15500. It is apparently a unpaired stone unlike any other lunar meteorite.

More Information

Meteoritical Bulletin Database

NWA 15500

Lunar Meteorites
List – ABC123 Order
List – Al2O3 Order