Anthropogenic (Man-made industrial byproduct)
Slag
Ferrous Slag (Siliceous industrial waste)
Hardness: 5-7 (Mohs); Color: Dark grey, brown, or black with rusty oxidation; Luster: Dull to glassy; Structure: Amorphous/Vesicular (holes formed by gas bubbles); Cleavage: None/Conchoidal.
- Hardness
- 5-7 (Mohs)
- Color
- Dark grey, brown, or black with rusty oxidation
- Luster
- Dull to glassy
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Physical properties
Hardness: 5-7 (Mohs); Color: Dark grey, brown, or black with rusty oxidation; Luster: Dull to glassy; Structure: Amorphous/Vesicular (holes formed by gas bubbles); Cleavage: None/Conchoidal.
Formation & geological history
Formed as a byproduct of smelting ores (iron, steel, or copper) where impurities are separated from the molten metal. The 'vesicles' or holes are trapped gas bubbles that formed as the material cooled quickly at the surface.
Uses & applications
Used widely in road construction (as asphalt aggregate), railroad ballast, cement manufacturing, and abrasive blasting. Sometimes used in glass-making or as rock wool insulation.
Geological facts
Slag is frequently mistaken for meteorites due to its dark color, heavy weight, and 'burnt' appearance. However, the presence of spherical bubbles (vesicles) is a definitive indicator that it is slag, as meteorites almost never contain internal bubbles.
Field identification & locations
Identify in the field by looking for a 'cinder' or 'lava' like texture, often found near old railroad tracks, industrial sites, or urban fill. It is often denser than natural volcanic rock and may be slightly magnetic if iron content is high.
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