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Magnetite and Lodestone |
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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 MagnetiteMagnetite 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 IronMost 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 MediaPowdered 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 AbrasiveThe 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 MagnetiteOther 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 FieldTiny 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. |
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Magnetite - Mineral Properties and Uses |
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Uses of Magnetite |
Important ore of iron; heavy media for specific gravity separations; abrasive; toner in electrophotography; micronutrient in fertilizers; pigment in paints; an aggregate for high-density concrete. (More details provided above.) |
Color |
black |
Streak |
black |
Luster |
metallic to submetallic |
Diaphaneity |
opaque |
Cleavage |
none |
Hardness |
5.5 - 6.5 |
Specific Gravity |
5.2 |
Distinguishing |
strongly magnetic, color, streak, octahedral crystal habit |
Crystal System |
isometric |
Chemical Classification |
Oxide |
Chemical Composition |
Fe3O4 |
Typical magnetite. Specimen is approximately four inches (10 centimeters) across.
Lodestone: note now the specimen attracts small particles of iron. Specimen is approximately four inches (10 centimeters) across.
Magnetite sand obtained by dredging a stream and separating out the heavy minerals. Pile is approximately four inches (10 centimeters) across.
More Information About Minerals!
Mineral photographs that appear on this page with a Geology.com copyright mark are property of Geology.com and may not be used beyond our website. If you like these photos and would like to share them with others please link to this page or email its URL. The photos were taken and composed by Ann Bryant. |