Minerals » Magnetite
Magnetite and Lodestone
An ore of iron, a mineral used in heavy media separation and a recorder of Earth magnetism
What is Magnetite?
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 Mohs 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).
Physical Properties of Magnetite
||Metallic to submetallic
||5.5 to 6.5
||Strongly magnetic, color, streak, octahedral crystal habit
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 iron, steel and other alloys.
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 sulfide mineral with a high specific gravity) 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.
In the past few decades synthetic abrasives have filled many of applications where magnetite was previously used.
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.
Contributor: Hobart King
Find it on Geology.com
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|Minerals: Information about ore minerals, gem materials and rock-forming minerals.|
|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. Some beach and river sands contain high concentrations of magnetite. Magnetite sand obtained by dredging a stream and separating out the heavy minerals. Pile is approximately four inches (10 centimeters) across.|