Fusion Crust

Melted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of  4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. As the meteorite slows and the fireball is extinguished, the molten material cools and fuses to form a thin skin typically of 1 mm to 2 mm that covers the whole meteorite. Freshly recovered chondrites will most often exhibit a dull or matte finish crust that is black or bluish-black if the iron content of the meteorite if high enough. Freshly recovered achondrite meteorites can exhibit fusion crusts of other colors as well, like brownish-green, brown, and light tan, and often have shiny and glossy crusts. Eucrites are often referred to having very glassy and shiny fusion crusts.  The distinct coloring can help make the meteorite stand out against the background of terrestrial rocks, sand or ice. Primary crust is the material that forms from the beginning of incandescent flight until dark flight. Secondary crust form if a piece breaks off the main mass during incandescent flight and a new crust forms on the broken surfaces.

Terrestrial weathering will begin to impact the meteorite the moment it lands on Earth. If the meteorite is recovered long after its arrival on Earth, the fusion crust that remains will often be rust-brown colored as the iron in the meteorite oxidizes.

Lafayette meteorite. Image Source: Chip Clark, courtesy of Smithsonian Institution