Mineral/By-product
Iron Slag
Iron Slag (Industrial Vitreous Material)
Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey; Luster: Dull to sub-metallic/glassy; Structure: Amorphous (non-crystalline); Vesicular (contains air bubbles/pores); Specific Gravity: 2.5-4.5 (varies by iron content).
- Hardness
- 5-6 (Mohs)
- Color
- Dark brown, black, or grey
- Luster
- Dull to sub-metallic/glassy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey; Luster: Dull to sub-metallic/glassy; Structure: Amorphous (non-crystalline); Vesicular (contains air bubbles/pores); Specific Gravity: 2.5-4.5 (varies by iron content).
Formation & geological history
Formed as a byproduct of smelting iron ore or during coal/coke combustion in industrial furnaces. The gas bubbles seen in the specimen were trapped as the molten material cooled rapidly at the surface.
Uses & applications
Commonly used as road ballast, railroad track bedding, aggregate in concrete and asphalt, and occasionally for sandblasting media.
Geological facts
Slag is often mistaken for meteorites due to its dark color, heavy weight, and occasional magnetic properties. However, the presence of vesicles (holes) is a primary indicator of terrestrial slag, as true meteorites rarely have internal gas bubbles.
Field identification & locations
Identify in the field by looking for 'vesicular' (pitted) textures, a glassy/metallic look inside, and its presence near former industrial sites or railroad beds. Collectors often find it while searching for meteorites.
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