Magnetite
Iron(II,III) oxide (Fe3O4)
A naturally magnetic black iron oxide and a major iron ore; strongly magnetic specimens are known as lodestone.
- Mohs hardness
- 5.5-6.5
- Color
- black to brownish black, metallic
- Type
- mineral
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Overview
Magnetite is a black, metallic iron oxide and one of the most important ores of iron. Its defining property is that it is strongly magnetic — it is attracted to magnets and, in the variety called lodestone, is itself a natural permanent magnet.
It crystallizes in the cubic system, commonly forming octahedral (eight-sided) crystals, as well as massive and granular aggregates. Magnetite is hard, dense, and leaves a distinctive black streak.
Widespread in igneous, metamorphic, and sedimentary rocks, magnetite also concentrates as 'black sand' in stream and beach placers and records Earth's ancient magnetic field, making it vital to paleomagnetism.
Formation & geology
Magnetite crystallizes from cooling magma as an early accessory mineral in many igneous rocks, and can concentrate into massive magmatic ore bodies. It is also abundant in metamorphic rocks and in banded iron formations — ancient chemical sediments that are the world's largest iron-ore deposits.
Its hardness and density mean it resists weathering and accumulates as black magnetic sand in river and beach placer deposits.
Major sources include Sweden (Kiruna), the iron ranges of the USA, Brazil, Australia, and South Africa. Lodestone forms when magnetite is magnetized by lightning strikes or Earth's field.
How to identify it
The diagnostic test is magnetism — magnetite is strongly attracted to a magnet, and lodestone will pick up iron filings on its own. Add to that a black metallic-to-dull luster, black streak, hardness around 5.5-6.5, and high density.
Well-formed crystals are octahedral. Streak on unglazed porcelain is black.
Look-alikes: Hematite is similar but gives a red-brown streak and is generally non-magnetic (only weakly so). Chromite is dark and weakly magnetic with a brown streak. Ilmenite is similar and weakly magnetic. The strong magnetism plus black streak is the clearest identifier.
Uses & significance
Magnetite is a primary ore of iron and steel, mined in enormous quantities worldwide. Its high density also makes it useful as a heavy aggregate in concrete and as a dense medium for separating ores by density (heavy-media separation).
It is used in water treatment, magnetic recording research, and as a catalyst in some industrial processes. Magnetite nanoparticles have important uses in medicine and electronics.
Lodestone was the original material for compasses and remains a curiosity and metaphysical stone associated with grounding and attraction. Octahedral crystals are popular with mineral collectors.
Frequently asked questions
Is magnetite the same as lodestone?
Lodestone is a naturally magnetized variety of magnetite that acts as a permanent magnet; all lodestone is magnetite, but not all magnetite is magnetized.
How can I tell magnetite from hematite?
Magnetite is strongly magnetic and gives a black streak, while hematite is weakly or non-magnetic and gives a distinctive red-brown streak.
Why is some magnetite naturally magnetic?
Lodestone forms when magnetite is magnetized by powerful natural fields, especially lightning strikes, which align the magnetic domains within the mineral.
What is magnetite used for?
It is a major iron ore, a heavy aggregate, a dense medium for ore separation, and the source of magnetite nanoparticles used in medicine and technology.
Magnetite identified by the community
Real specimens identified with Rock Identifier.
