Banded Iron Formation (BIF)

Physical Properties

Hardness: 5.5-6.5 (iron oxides) and 7 (chert); Color: Alternating bands of grey/black and reddish-brown; Luster: Metallic to dull; Crystal structure: Microcrystalline; Cleavage: None, breaks along bedding planes.

Formation & Geological History

Formed in Precambrian oceans, mostly between 2.4 and 1.8 billion years ago. Created by the production of oxygen by cyanobacteria, which caused dissolved iron in the seawater to oxidize and precipitate out as layers on the ocean floor.

Uses & Applications

Primary source of iron ore globally for the production of steel. Occasionally used as ornamental stone (Tiger Iron) or for lapidary work.

Geological Facts

Banded Iron Formations provide key evidence for the Great Oxygenation Event, recording the transition of Earth's atmosphere from oxygen-poor to oxygen-rich.

Field Identification & Locations

Identify by distinct, rhythmic dark (iron-rich) and light (silica-rich) layering. Often magnetic if magnetite is present. Found in ancient 'cratons' like those in Western Australia, Brazil, and the Lake Superior region.

Identified on: 4/23/2026

Mode: Standard