NORTHWEST AFRICA 12774


Angrite
Basaltic/Quenched, Picritic
standby for nwa 12774 photo
Purchased June 2019
no coordinates recorded

A single, relatively fresh stone weighing 454 g was acquired from a Mauritanian dealer by R. and J. Chaoui and subsequently sold to M. Jost and K. Wimmer at the Ensisheim Show. A sample of the meteorite was analyzed at the University of Washington in Seattle (A. Irving) and at Washington University in St. Louis (P. Carpenter), and NWA 12774 was classified as a quenched, olivine-phyric angrite.

The meteorite has a porphyritic texture composed of compositionally-zoned, skeletal calcic olivine phenocrysts and high-Al (up to 18 wt%), Ti-bearing augite phenocrysts arrayed in a black, quenched groundmass (Hayashi et al., 2020, #2360). The groundmass is very fine-grained and consists of nearly pure anorthite (An>99.5), olivine, kirschsteinite, and Al–Ti augite, along with minor troilite and ulvöspinel and rare kamacite. Some larger olivine clusters (~3 mm) with more magnesian compositions (up to Fo92) are considered to be xenocrysts. As determined by Raman Spectroscopy, the silicate phenocrysts contain various tiny opaque inclusions such as Ti-bearing magnetite/spinel, pyrrhotite, kamacite, carbon phases, and silico-phosphate (Hoffmann et al., 2020, #2323).

A limited number of unique angrites are represented in our collections today which can be grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample NWA 8535 (photo courtesy of Habib Naji). Another quenched angrite, NWA 7203 (photo courtesy of Labenne Meteorites), exhibits a striking variolitic texture. Portions of the angrite asteroid must be in a stable orbit (planetary or asteroid belt) from which spallation has continued to occur over the past ~56 m.y. as indicated by the broad range in angrite CRE ages.

Northwest Africa 12774 has a similar texture and bulk composition to the 46.2 g NWA 7812, but mineralogical details are inconsistent with a fall pairing (Irving et al., 2020, #2399). Additionally, a petrographic comparison between NWA 12774, NWA 1670, and LEW 87051 show that they are all closely related, but the differences that exist attest that none are fall paired (Hayashi et al., 2020). The Fe–Mg diffusion profile for NWA 12774 was utilized by Hayashi et al. (2020) to calculate the cooling rate of 3.5°C/hr (1400°C to 900°C), which is nearly the same as 3°C/hr (1400°C to 900°C) calculated for NWA 1670 by Hayashi and Mikouchi (2019). By contrast, they calculated a two-stage cooling rate for the quenched angrite NWA 7203; first a rapid cooling of 20°C/hr (variolitic texture), and thereafter ~1°C/hr (dendritic texture) possibly due to deeper burial by subsequent lava flows.

The photo of NWA 12774 shown above is a 0.32 g part slice, while the top photo below is an impressive 17.196 g full slice of this visually striking angrite, shown courtesy of Tom Stalder. The bottom photo is an excellent petrographic thin section micrograph of NWA 12774 shown courtesy of Peter Marmet.

standby for nwa 12774 full slice photo
click on photo for a magnified view
Photo courtesy of Tom Stalder—Sahara Gems

standby for nwa 12774 thin section photo
click on photo for a magnified view
Photo courtesy of Peter Marmet