Northwest Africa 4222

Northwest Africa

Find: 2006

Martian (shergottite)

History: A small fragment was purchased in Erfoud by a Moroccan dealer for M. Chinellato.

Physical characteristics: The sample weighs 16.5 g and is partially covered with a black fusion crust.

Petrography: (V. Moggi Cecchi, G. Pratesi, MSP) Cumulitic fine-grained porphyritic texture consisting of few very large rounded and zoned phenocrysts of brown olivine up to 1350 μm in maximum size set in a fine-grained basaltic groundmass of twinned tabular pyroxene crystals from 80 to 240 μm wide and from 130 to 820 μm long surrounded by an interstitial glassy matrix. Pyroxene is primarily pigeonite with subordinate enstatite. Other minerals are chromite, titanian chromite and ilmenite, up to 100 μm in size, merrillite, and rare pyrrhotite grains up to 50 μm in size; shock features include strong mosaicism and planar deformation in olivine, undulose extinction and twinning in pyroxene.

Geochemistry and mineralogy: Olivine Fo_58.9–68.7; orthopyroxene (core Fs_17.2En_80.3Wo_2.4; rim Fs_31.4En_63.9Wo_4.7, FeO/MnO = 30.2–42.4); pigeonite (core Fs_24.1En_65.5Wo_10.3; rim Fs_32.6En_58.4Wo_8.9), FeO/MnO = 32.5–46.5). Plagioclase (An_61.1Or_0.2). Oxygen isotopes (I. A. Franchi, R. Greenwood, OU): δ¹⁷O = 2.85‰, δ¹⁸O = 4.91‰, Δ¹⁷O = 0.30‰ (mean on 2 analyses).

Classification: Martian (shergottite). Low weathering and high shock.

Type specimens: A total of 3.55 g of sample and one thin section is on deposit at MSP. MChin holds the main mass.