Meteorite basics: fusion crust, streak, magnet, density

A fresh meteorite has a fusion crust: a thin (under 1 mm), dull black to brown skin formed by ablation, often with flow lines and shallow thumbprint hollows (regmaglypts). Weathered finds lose it to rust-brown. Slag has a glassy, bubbly surface; a fresh break through a meteorite shows no vesicles at all — gas bubbles rule a specimen out.

Streak: drag the specimen across unglazed porcelain. Hematite leaves a red-brown streak, magnetite a grey-black one, most meteorites no streak or a faint grey one. Magnet: nearly all meteorites (irons, stony-irons and ordinary chondrites) attract a magnet because of their nickel-iron; a piece that does not is almost certainly not one, though a few rare types are exceptions. Density: irons run 7–8 g/cm³, chondrites 3.3–3.7, far heavier than most terrestrial rocks of the same size; weigh it and measure its volume by water displacement.

A cut and polished face is decisive for the common types: ordinary chondrites show round chondrules and bright metal flecks; irons show the Widmanstätten pattern when etched with dilute nitric acid; pallasites show olivine in metal. Nickel is the chemical signature — terrestrial iron has none — and a nickel spot test or an XRF reading settles most disputes.

Names: a meteorite is named for the place it fell or was found and classified by the Meteoritical Society's Nomenclature Committee; the Meteoritical Bulletin Database is the register. Finds from the Sahara without a precise locality are “Northwest Africa” plus a number (NWA 869, NWA 7034); the number is assigned when a type specimen is deposited and the classification is published. A meteorite offered without a Bulletin entry, or with a name that does not match its description in the database, needs a reason.

Storage is the pyrite problem again: nickel-iron rusts, and chlorides from the desert or a fall site drive it. Keep irons and chondrites dry, in a sealed box with silica gel, and never handle them with damp hands. Ownership: in the US a find belongs to the landowner (on federal land, casual collection of small amounts is allowed under BLM policy but commercial collection is not); many countries — Australia, Argentina, Morocco with export permits, and others — claim or regulate meteorites, so the same provenance rules as fossils apply.