Nakhla

Nakhla is an achondritic meteorite that fell on June 28, 1911, in the village of the same name in Beheira province, Egypt. This extraordinary event produced a meteorite shower that dispersed approximately 40 fragments, the largest of which weighed 1.8 kg. The mineralogical composition is dominated by augite (pyroxene) and olivine, with traces of plagioclase and magnetite, a configuration that suggests a volcanic origin from a differentiated planetary body.

Isotopic and mineralogical studies have revealed that Nakhla belongs to the group of shergottite meteorites, a subclass of achondrites believed to originate from Mars. Radiometric dating indicates primary crystallization approximately 1.3 billion years ago, followed by a thermal shock event approximately 11 million years ago, likely caused by the impact that expelled it from the Martian surface. The discovery of traces of hydrous minerals and aqueous alteration in samples provided crucial evidence for the presence of liquid water in the ancient Martian subsurface, making it an invaluable geological archive for understanding the hydrological evolution of Mars.

Crystal system: monoclinic (for constituent phases). Mineralogical composition: augite (Ca,Mg,Fe)2Si2O6 (40–50% vol.), olivine (Mg,Fe)2SiO4 (30–40% vol.), plagioclase (An60–70) (5–10% vol.), magnetite Fe3O4 (1–3% vol.). Density: 3.24–3.35 g/cm³. Hardness: 5–6 Mohs (augite and olivine). Refractive index (olivine): nα = 1.635, nγ = 1.680. Refractive index (augite): nα = 1.680, nγ = 1.710. Optical angle (2V): augite ~60°, olivine ~90°. Isotopic composition: Sm-Nd ratio indicates a model age of 1.3 Ga. ⁴⁰Ar/³⁹Ar ratio: cooling age ~11 Ma. Average chemical composition (wt%): SiO2 44–46%, MgO 20–22%, FeO 12–14%, CaO 10–12%, Al2O3 4–6%. Traces of hydrous minerals (iddingsite, chlorites) indicate post-crystallization aqueous alteration. Porosity: 5–8%. Classification: basaltic shergottite, subclass of achondrite. Texture: ophitic to sub-ophitic structure, with augite crystals 0.5–2 mm in size embedded in a matrix of plagioclase and olivine.