Northwest Africa 16595 (NWA 16595)

(Northwest Africa)

Purchased: 2021

Classification: HED achondrite (Eucrite, monomict)

History: Purchased by Edwin Thompson in early 2021 from a meteorite broker in Mauritania, who said it was found in the Adrar region of Mauritania. Acquired in May 2021 by Cascadia as part of a trade.

Physical characteristics: Physical Characteristics: The single stone was covered with a dark fusion crust with shrinkage crack. Two areas where the fusion crust was missing showed a light gray interior. The cut face shows dark clasts containing white rectangular plagioclase grains set in a light- to medium-gray clastic matrix.

Petrography: (M. Hutson, A. Ruzicka, Cascadia): The sample is composed of clasts with sub-ophitic texture set in a clastic matrix. In addition to plagioclase feldspar and zoned pyroxene, the sub-ophitic clasts contain abundant silica and ilmenite, with small phosphate and sulfide grains. No chromite or metal was observed in the section. Ferrous olivine is present in veins and small single grains, as well as in symplectitic patches composed of ferrous olivine, low-Ca pyroxene, augite, ilmenite, and silica. The clastic matrix is composed of fragments of material found in the sub-ophitic clasts.

Geochemistry: Pyroxene analyses plot in four areas on a pyroxene quadrilateral: there is a concentration of analyses with Wo<5 and Fs_54-65 (low-Ca pyroxene); a broad horizontal swath of analyses with Wo_5-10 and Fs_32-62 (pigeonite); a concentration of augite analyses with Wo>40 and Fs_29-43; and 15 analyses for ferroan grains that lie along tie lines between low- and high-Ca pyroxenes. Individual pyroxene grains show normal zoning, with sharp boundaries between Mg-rich cores and more ferrous rims; in one grain the core is Fs_32.2Wo_5.5 and the rim is Fs_61.5Wo_6.1. Average pyroxene analyses: Low-Ca pyroxene Fs_62.4±3.9Wo_2.3±1.2, range Fs_54.2-65.1Wo_1.5-4.9, Fe/Mn (molar) = 28.7±0.7, N=14; pigeonite Fs_47.3±10.4Wo_6.9±1.5, range Fs_32.2-61.7Wo_5.1-9.2, Fe/Mn = 29.3±1.8, N=24; augite Fs_29.2±0.4Wo_43.4±0.6, range Fs_28.7-29.5Wo_43.0-44.3, N=4. Ferrous olivine found in veins and symplectitic intergrowths: Fa_83.2±3.8, Fe/Mn = 38.0±0.9, N = 21. Plagioclase feldspar: Ab_13.2±2.1An_85.8±2.4Or_0.9±0.4. N = 17.

Classification: HED achondrite (Eucrite, monomict) based on the mineral chemistry and texture of clasts and matrix. The meteorite overall has unequilibrated pyroxene grains.

Specimens: Cascadia holds 94.7 g in five pieces, as well as a polished thin section.