Pallasite

Pallasites represent one of the most fascinating types of stony-iron meteorites, constituting approximately 8% of meteorites that reach Earth. Their unique structure—a matrix of kamacite and taenite (iron-nickel alloys) hosting yellow-green olivine nodules—suggests an origin in the outer core of a differentiated planetesimal, likely formed in the first few million years of the Solar System. When a section is polished and treated with dilute nitric acid, the celebrated Widmanstätten pattern emerges, a banded crystalline structure revealing extremely slow cooling in space.

The name derives from Count Ernst Friedrich von Chladni, who classified meteorites in 1794, while the term "pallasite" was coined in honor of Peter Simon Pallas, a Russian naturalist who studied the Krasnojarsk meteorite (Siberia) in 1772. Notable specimens come from Atacama (Chile), Imilac, and Seymchan (Russia), where desert conditions preserve these cosmic fragments for millennia. The variable hardness (4-7 on the Mohs scale) reflects the composite nature: olivine is harder (6.5-7), while the metallic matrix is softer (4-5).

Pallasite is classified as a stony-iron meteorite, subtype PAL according to international meteorite nomenclature. The chemical composition is dominated by metallic Fe-Ni (60-90% by weight) with olivine (Mg₂SiO₄) constituting 10-40% by volume. The metallic matrix exhibits a characteristic microstructure composed of kamacite (cubic Fe,Ni, 5-8% Ni) and taenite (cubic Fe,Ni, 25-35% Ni), with occasional minor phases such as troilite (FeS) and schreibersite ((Fe,Ni)₃P).

The crystal system of the olivine component is orthorhombic (not cubic as previously noted for the metallic matrix), with lattice parameters a ≈ 4.76 Å, b ≈ 10.20 Å, c ≈ 5.98 Å. Olivine hardness is 6.5-7 Mohs; the metallic matrix is 4-4.5 Mohs. Total density ranges from 5.0 to 5.5 g/cm³. Olivine exhibits weak pleochroism and moderate birefringence; refractive indices are nα ≈ 1.635, nβ ≈ 1.651, nγ ≈ 1.670. The Widmanstätten pattern, visible in polished section after acid etching, reveals the lamellar structure resulting from extremely slow cooling (approximately 1-10 °C per million years) in the planetesimal core. Oxygen isotope analysis (δ¹⁸O vs δ¹⁷O) places pallasites in a distinct group, consistent with origin from a single parent body or a few similar bodies. Radiometric dating (Pb-Pb, Ar-Ar) indicates a crystallization age of 4.5-4.56 billion years.