Anthropogenic (Man-made industrial byproduct)
Iron Slag
Ferrous slag (primarily silicates and oxides of calcium, iron, and magnesium)
Hardness: 5-6 Mohs; Color: Dark grey, black, or reddish-brown (if rusted); Luster: Dull to vitreous/glassy; Structure: Amorphous or microcrystalline with vesicles (air bubbles); Specific Gravity: 2.8 - 3.9
- Hardness
- 5-6 Mohs
- Color
- Dark grey, black, or reddish-brown (if rusted)
- Luster
- Dull to vitreous/glassy
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Physical properties
Hardness: 5-6 Mohs; Color: Dark grey, black, or reddish-brown (if rusted); Luster: Dull to vitreous/glassy; Structure: Amorphous or microcrystalline with vesicles (air bubbles); Specific Gravity: 2.8 - 3.9
Formation & geological history
Formed as a byproduct of smelting iron ore in a blast furnace. The liquid slag is separated from the molten iron and cooled rapidly. It is not an 'age' based geological feature but represents industrial history from the late 19th century to the present.
Uses & applications
Used in construction as road aggregate, railroad ballast, raw material for cement, and occasionally as rock wool insulation. Small glassy pieces are sometimes used in amateur lapidary (jewelry).
Geological facts
Slag is often mistaken for meteorites due to its dark, heavy, and often magnetic nature. However, the presence of vesicles (bubbles) is a 'dead giveaway' that it is terrestrial slag, as true meteorites do not contain bubbles.
Field identification & locations
Identify in the field by looking for a pitted/bubbly texture and a glassy or metallic surface. Common near old railways, steel mills, and historic blast furnace sites. Use a magnet; most iron slag will be weakly to strongly magnetic.
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