Northwest Africa 10769 (NWA 10769)

Western Sahara

Purchased: Mar 2015

Classification: Ungrouped chondrite

History: Collected by nomads in Western Sahara and purchased by Nicola Castellano at the Bologna Mineral Fair in March 2015 from a Moroccan dealer.

Physical characteristics: A single piece weighing 149 g partially covered by fusion crust. The sawn surface displays chondrules set in a red-brown matrix with no metal spots.

Petrography: (V. Moggi Cecchi, G. Pratesi, S. Caporali, UniFi): The thin section displays a chondritic texture, with several chondrules set in a fine-grained matrix, mainly consisting of olivine and pyroxene. Chondrules range from 0.2 to 0.7 mm in diameter and are mainly PP type, with minor RP and C. Pyroxene crystals in PP chondrules are often rimmed by pigeonite. Chondrules account for about 70% of the total surface of the section. Opaque phases account for about 5% of the total area and are mainly pentlandite and Ni-rich pyrrhotite, partially weathered to iron oxides, while kamacite is very rare.

Geochemistry: (V. Moggi Cecchi, G. Pratesi, S. Caporali, UniFi) Matrix: olivine (Fa_24.7±3.5, Fe/Mn=55±4, n=8, MD=1.76, PMD=7.76); low-Ca pyroxene (Fs_14.6±5.0En_84.4±5.2Wo_0.9±0.2, 0.2 TiO₂, 0.2 Al₂O₃, 0.2 Cr₂O₃, all wt. %; Fe/Mn=21±5, n=6 MD=2.47, PMD=25.48); Chondrules: olivine (Fa_23.4±3.6, Fe/Mn=58±4, n=6, MD=2.38, PMD=11.06); low-Ca pyroxene (Fs_14.4±5.1En_84.6±4.9Wo_0.9±0.2, 0.2 TiO₂, 0.2 Al₂O₃, 0.2 Cr₂O₃, all wt. %; Fe/Mn=21±5, n=8 MD=2.49, PMD=25.52); pigeonite (Fs_23.1±2.7En_72.8±5.1Wo_4.1±2.4, 0.2 TiO₂, 0.8 Al₂O₃, 0.9 Cr₂O₃, all wt. %); Oxygen isotopes: (I. Franchi, R. Greenwood, OU) δ¹⁷O = 3.65 ‰, δ¹⁸O = 5.95 ‰, Δ¹⁷O = 0.55 ‰ (mean of two runs, bulk rock, 1 chip weighing 0.5 g); oxygen isotopes plot in a region above the TFL distinct from ordinary chondrites and at relatively high δ¹⁸O values.

Classification: The chondritic texture together with the presence of pentlandite and pyrrhotite and the absence of troilite point to a classification as an ungrouped chondrite; isotopic data and the compositional variability of Fa and Fs contents point to petrologic type 3.8; S1; W3

Specimens: A total of 20.8 g specimen is on deposit at MSN-Fi. Castellano holds two thin sections and the main mass.