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Hematite

earth science Maturity 7-9 Next article →

Hematite is a common iron oxide compound with the chemical formula Fe2O3. It is widely found in rocks and soils across the world. This mineral is a major ore used to produce iron. It is also electrically conductive and has a high density. Because of its weight, it is used in shipping as ballast. Its density also makes it an effective barrier against X-rays for radiation shielding.

Hematite structure
Hematite structure

The mineral forms through several different natural processes. In some environments, gray hematite precipitates in still, standing water. It can collect in layers at the bottom of lakes or mineral hot springs. For example, these deposits occur in Yellowstone National Park in North America. Hematite can also form without water due to volcanic activity. In soil, clay-sized crystals form through weathering. This process creates iron oxides that turn tropical soils red.

Hematite exists in many different varieties and forms. Kidney ore is a term for hematite that is botryoidal, mammillary, or reniform in shape. Martite is a variety that forms as pseudomorphs after magnetite. There is also specularite, which is a shiny form of specular hematite. Even though these varieties look different, they all leave a rust-red streak. Some hematite is found in massive deposits called banded iron formations. These formations contain very large amounts of the mineral.

Humans have interacted with hematite for hundreds of thousands of years. The name comes from the Greek word "haima," which means blood. This refers to the deep red color of some varieties. The earliest known use of the mineral was 164,000 years ago. Inhabitants of the Pinnacle Point caves in South Africa used the powder. Residues of the mineral have also been found in graves from 80,000 years ago. Later, red chalk mines in Poland and Hungary were used around 5000 BC.

Hematite is highly valued as a pigment for art and industry. It is a primary component of ochre, which is a colored clay. Red ochre contains unhydrated hematite, while yellow ochre contains hydrated hematite. The hematite content in ochre can range between 20% and 70%. India is currently the main producer of hematite for the pigment industry. Other pigments like sienna and umber also rely on iron oxides. In Sweden, tailings from the Falun Mine are used to make Falu red paint.

The magnetic properties of hematite are quite complex. It shows only a very feeble response to a magnetic field. It is not noticeably attracted to ordinary magnets like magnetite is. Below a specific temperature called the Morin transition, it is antiferromagnetic. Above this transition, it becomes a canted antiferromagnet or weakly ferromagnetic. At very high temperatures, it becomes paramagnetic. These magnetic behaviors change based on the size of the particles. For example, the Morin transition temperature decreases as particle size decreases.

Scientists have even discovered hematite on the planet Mars. NASA spacecraft detected its spectral signature using infrared spectrometers. It was found in large amounts at Terra Meridiani and Aram Chaos. The Opportunity rover found hematite in the form of small spherules. The science team nicknamed these small round shapes "blueberries." These spherules appear to be concretions formed from a water solution. Studying how hematite formed on Mars helps scientists understand if the planet once supported life.

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