Description: Magnetite is one of the most widespread iron oxide minerals and occurs in a variety of geologic environments. It is a common accessory mineral in igneous rocks, but seldom forms crystals large enough to be observed directly in samples (visible under a microscope). If a (mafic) magma cools slowly enough, the denser magnetite crystals can settle to form ore bodies. Magnetite is not common in modern sedimentary rocks, but 2.5 to 1.6 billion years ago (early Proterozoic), large iron-rich sediments were deposited as the earth was undergoing dramatic chemical transitions in the atmosphere and oceans.

Magnetite is the most strongly magnetic mineral found in nature. Pieces of magnetite (variety lodestone) formed man's earliest magnetic compasses when suspended from strings. Most of the iron ore mined today comes from PreCambrian Banded Iron Formations (Australia, north-eastern Minnesota) that consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe₃O₄) or hematite (Fe₂O₃), alternating with bands of iron-poor chert, often red in color, of similar thickness. A single banded iron formation can be several thousand feet in thickness and extend laterally for hundreds of miles.

Color: Greyish black or iron black.

Streak: Black

Luster: Metallic, sub-metallic

Fracture: Irregular/Uneven

Cleavage: None

Hardness: 5½ - 6½ on Moh’s scale

Density/Specific Gravity: 5.175 g/cm³

Distinguishing Physical Properties:

• Magnetism: naturally strong

• Color: Black

• Streak: Also black

• Crystals: Eight-sided crystals (octahedrons)

• Specific gravity: feels heavy