anthropogenic / industrial byproduct
Iron Slag
Iron smelting byproduct (Glassy/vesicular silicate)
Hardness: 6-7 (MoHs); Color: Dark grey, brownish, or black with rusty orange oxidation; Luster: Glassy (vitreous) to dull; Structure: Amorphous/vesicular (contains gas bubbles/holes); Cleavage: None/Conchoidal.
- Hardness
- 6-7 (MoHs)
- Color
- Dark grey, brownish, or black with rusty orange oxidation
- Luster
- Glassy (vitreous) to dull
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Physical properties
Hardness: 6-7 (MoHs); Color: Dark grey, brownish, or black with rusty orange oxidation; Luster: Glassy (vitreous) to dull; Structure: Amorphous/vesicular (contains gas bubbles/holes); Cleavage: None/Conchoidal.
Formation & geological history
Formed during the iron smelting process where impurities (gangue) are separated from the molten metal. This occurs in blast furnaces or historical bloomery sites when silica from ore or flux (limestone) melts and then cools rapidly in the open air.
Uses & applications
Historically discarded as waste. Modernly used as road ballast, aggregate in concrete, or sometimes used in landscaping and aquarium decor.
Geological facts
Slag is frequently mistaken for meteorites due to its dark color, heavy weight, and 'burnt' appearance. However, the presence of vesicles (gas bubbles) as seen in this specimen is a definitive indicator that it is terrestrial slag, as meteorites do not contain bubbles.
Field identification & locations
Identify in the field by looking for a 'pock-marked' or hole-filled surface (vesicles), high density, and glassy interior if chipped. Commonly found near old railroad tracks, historical foundries, or industrial sites. Does not show the 'fusion crust' found on meteorites.
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