Magnetite and Lodestone

Magnetite is one of the most common oxide minerals and also one of the most common iron minerals. It is an important ore of iron and is found in igneous, metamorphic and sedimentary rocks. It can also be abundant in sediments.

Identification of Magnetite

Magnetite is easy to identify. It is a black, opaque, submetallic to metallic mineral with a hardness between 5.5 and 6.5. It is often found in the form of isometric crystals. However, its magnetic properties are distinctive. It is one of just a few minerals that are attracted to a magnet. It is the most magnetic mineral found in nature. Sometimes it is automagnetized and attracts metal objects.

Magnetite as "Lodestone"

Lodestone is a form of magnetite that acts as a natural magnet. Normal magnetite is attracted to a magnet but lodestone acts as a magnet, attracting iron particles (see photo).

Use of Magnetite As An Ore of Iron

Most of the magnetite mined is used as an ore of iron. Iron liberated from the ore is usually used to make steel.

Use of Magnetite as a Heavy Media

Powdered magnetite is often mixed with a liquid for used as a heavy media for specific gravity separations. Much of the high sulfur coal that is mined is floated across a slurry of magnetite. Clean coal particles float and those contaminated with pyrite (a sulfur mineral) sink into the high-density slurry.

Use of Magnetite as an Abrasive

The abrasive known as "emery" is a natural mixture of magnetite and corundum. Some synthetic emery is produced by mixing magnetite with aluminum oxides. Producing it synthetically allows control over the particle size and the relative abundance of aluminum oxide and magnetite. Some finely ground magnetite is also used as an abrasive in water jet cutting.

Other Uses of Magnetite

Other uses include: as a toner in electrophotography, as a micronutrient in fertilizers, as a pigment in paints, as an aggregate in high-density concrete.

Magnetite and Earth's Magnetic Field

Tiny crystals of magnetite are present in many rocks. In the crystallization of an igneous rock, tiny crystals of magnetite form in the melt, and because they are magnetic, they orient themselves with the direction and polarity of Earth's magnetic field. This preserves in the rock the orientation of Earth's magnetic field at the time of crystallization.

Today geologists can study the magnetic properties of rocks of various age and reconstruct the history of change in Earth's magnetic field. This information is available for multiple locations on multiple continents. It can also be used to learn about the movement of continents over time.

A similar orientation of tiny magnetite grains occurs in the settling of sediment particles, locking clues to Earth's magnetic history into some sedimentary rocks.