sedimentary

Banded Iron Formation (BIF)

Banded Iron Formation (Magnetite/Hematite and Chert)

Hardness: 5.5-7 (Mohs); Color: Alternating bands of silver-black, metallic gray, and red or dull yellow/tan; Luster: Metallic to dull; Crystal structure: Microcrystalline; Specific gravity: 3.5-5.0 (heavy for its size).

Hardness: 5
Luster: Metallic to dull

Identified June 19, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine environments during the Precambrian Eon (approx. 1.8 to 3.8 billion years ago) through the precipitation of iron and silica from sea water as oxygen levels rose due to photosynthesis by cyanobacteria.

Uses & applications

Primary global source of iron ore used in steel production. High-quality specimens with red jasper (Jaspilite) are used as decorative gemstones, cabochons, and ornamental carvings.

Geological facts

These rocks provide the primary evidence for the 'Great Oxygenation Event' when the Earth's atmosphere first began to accumulate free oxygen. They are essentially the fossilized remains of the ancient atmosphere's chemisttry.

Field identification & locations

Identify in the field by its distinct striped or banded layers and its unusual weight compared to surrounding rocks. Often magnetic due to magnetite content. Found in ancient shield regions like the Lake Superior area, Australia, and Brazil.

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