igneous
Iron Slag
Anthropogenic pyrometallurgical byproduct
Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey with possible metallic or glassy highlights; Luster: Sub-metallic to vitreous; Structure: Amorphous/vesicular (contains gas bubbles); Specific Gravity: 2.5-4.0
- Hardness
- 5-6 (Mohs)
- Luster
- Sub-metallic to vitreous
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark brown, black, or grey with possible metallic or glassy highlights; Luster: Sub-metallic to vitreous; Structure: Amorphous/vesicular (contains gas bubbles); Specific Gravity: 2.5-4.0
Formation & geological history
Formed as a byproduct of smelting iron ore in a blast furnace. Occurs when impurities like silica and alumina react with fluxing agents (limestone) and are separated from the molten metal. Generally dates from the 18th to early 20th centuries in historical industrial sites.
Uses & applications
Historically a waste product; modernly used as road aggregate, railroad ballast, and in cement manufacturing due to its durability.
Geological facts
Slag is often mistaken for meteorites due to its dark, melted appearance and occasional magnetism, but the presence of small gas bubbles (vesicles) is a definitive indicator of slag rather than meteorites.
Field identification & locations
Identify by looking for air bubbles, flow textures, and a dark, melted surface. It is commonly found near historic iron furnaces, railroads, and industrial riverfronts.
