igneous
Ferrocoke or Slag
Anthropogenic pyrometallurgical residue
Hardness: 6-7 (Mohs scale), Color: Dark grey to charcoal black, Luster: Metallic to dull, Structure: Vesicular (contains air pockets) and amorphous/glassy, Cleavage: None/Conchoidal fractures, Specific gravity: Variable (typically 2.5-4.0)
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Physical properties
Hardness: 6-7 (Mohs scale), Color: Dark grey to charcoal black, Luster: Metallic to dull, Structure: Vesicular (contains air pockets) and amorphous/glassy, Cleavage: None/Conchoidal fractures, Specific gravity: Variable (typically 2.5-4.0)
Formation & geological history
Formed as a byproduct of historical or modern industrial smelting (specifically iron production). It is created when fluxing agents melt together with impurities from iron ore and fuel (coke or charcoal) and then cool rapidly above ground.
Uses & applications
Historically used for road aggregate, railroad ballast, and in some forms of cement production. In modern times, it is often studied as an industrial archaeological artifact.
Geological facts
Slag can often be mistaken for meteorites due to its dark color, metallic sheen, and pitted surface; however, the presence of bubbles (vesicles) proves it was formed at Earth's surface pressure.
Field identification & locations
Identify by looking for air bubbles (vesicles), a glassy 'conchoidal' fracture pattern, and a high density. It is often found near old railways, industrial sites, and riverbanks near historical foundries.
