Meteoritical Bulletin: Entry for Northwest Africa 13758

Writeup from MB 111:

Added notes for NWA 13758

(C. Agee, UNM) The total weight of the R chondrite described here, under the name NWA 13758, was originally stated to be 100 kg. It is possible that other R chondrites from northwest Africa with official names, listed as found or purchased since 2017, could be stones taken from this same 100 kg. According to Dustin Dickens (pers. comm. to Carl Agee, 2021), the total mass of paired stones is likely to be about 200 kg. Assuming accurate pairings of NWA R chondrites could be established, their published masses should not be added together to determine the total recovered mass from the NWA 13758 strewnfield, as the same stones would likely be counted more than once.

Northwest Africa 13758 (NWA 13758)

Adrar, Algeria

Purchased: 2017

Classification: Rumuruti chondrite (R3)

History: A large group of identically appearing stones, found together, was purchased by a Saharan dealer from a nomad in Algeria and was subsequently acquired by Mark Lyon and Dustin Dickens. The original find was reportedly in the Tanezrouft Plateau, with the nearest town being Reggane, Algeria.

Physical characteristics: Approximately 500 pieces ranging in mass from <1 kg to >5 kg, with a total known weight of approximately 100 kg. Dark brown sandblasted exterior. Saw cut reveals a breccia with fragments up to ~2 cm set in a fine-grained matrix. Abundant distinct chondrules are visible throughout. Two main lithologic domains exist within this meteorite, one has a gray, unstained matrix, and the second has a brownish-orange-stained matrix.

Petrography: (C. Agee, UNM) Chondrite with mean apparent chondrule size 350±240 µm (n=21) set in abundant matrix (~35 vol%). Electron microprobe analyses were performed separately on the two main lithologic domains. Domain "A" has compositionally unequilibrated olivine and pyroxene both in chondrules as well as single mineral fragments in the matrix. Very small grains of albitic plagioclase are present. Chromite, pentlandite, and troilite are ubiquitous. No metal or oxidized iron was detected. Domain "B" has compositionally unequilibrated olivine and pyroxene both in chondrules as well as single mineral fragments in the matrix. Domain "B" appears to have slightly more chondrules than domain "A". Very small grains of albitic plagioclase were present. Ti-rich chromite, pentlandite, and Ni-bearing troilite are ubiquitous. No metal or oxidized iron was detected.

Geochemistry: (C. Agee, UNM) Domain "A": olivine Fa_36.6±8.4, CV=23%, Fe/Mn=83±15, n=12; low-Ca pyroxene Fs_21.4±6.8Wo_0.9±0.5, Fe/Mn=39±19, n=4; augite Fs_8.8±0.7Wo_46.0±0.2, Fe/Mn=43±4, n=2; plagioclase An_8.8±0.6Ab_86.5±0.2Or_4.7±0.7, n=3. Domain "B": olivine Fa_38.7±5.8, CV=15%, Fe/Mn=87±10, n=12; low-Ca pyroxene Fs_10.0±7.7Wo_2.0±1.8, Fe/Mn=15±7, n=4; augite Fs_11.4±0.9Wo_44.8±1.3, Fe/Mn=51±11, n=2; plagioclase An_9.2±0.9Ab_85.9±1.6Or_4.9±0.8, n=3.

Classification: Rumuruti chondrite (R3). Domain "A" is approximately R3.7 and domain "B" is approximately R3.9, based on olivine coefficient of variation (CV) after Bischoff (2000).

Specimens: 1127.72 g complete stone, and a 57.69 g full slice from a different complete stone, are on deposit at UNM. Mark Lyon holds the main mass including a total of 14 pieces weighing 19.84 kg, Dustin Dickens holds ~60 kg of medium to small stones.