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Forsterite

earth science Maturity 7-9 Next article →

Forsterite is a magnesium-rich mineral that belongs to the olivine solid solution series. It is often called white olivine. This mineral is a vital component of many rocks on Earth and in space. It serves as the magnesium-rich end-member of the olivine group. This means it represents one extreme end of a chemical spectrum.

Forsterite orange - Ochtendung, Eifel, Germany
Forsterite orange - Ochtendung, Eifel, Germany

The other extreme end is fayalite, which is rich in iron. Forsterite is also isomorphous with fayalite. This means they share the same crystal structure despite different chemical makeups. Understanding forsterite helps scientists study the composition of planetary bodies.

At the atomic level, forsterite has a very specific and dense structure. It is composed of magnesium, silicon, and oxygen. The chemical formula for forsterite is Mg2SiO4. The structure is built around the SiO44− anion. In this anion, a single silicon atom sits in the center. It is bonded to four oxygen atoms by single covalent bonds. Because of these bonds, the oxygen atoms carry a partial negative charge. These charges cause the oxygen atoms to repel each other. To minimize this repulsion, the atoms form a tetrahedral shape.

Atomic structure of olivine 1
Atomic structure of olivine 1

The magnesium cations, or Mg2+, occupy two different octahedral sites. These sites are labeled M1 and M2. The M2 site is larger and more regular than the M1 site. These cations form ionic bonds with the silicate anions to create a dense packing.

Forsterite belongs to the orthorhombic crystal system. This means its crystal shape follows specific geometric rules. Its space group is Pbnm. The unit cell has specific dimensions. The a-axis measures 4.75 Å. The b-axis measures 10.20 Å. The c-axis measures 5.98 Å.

Atomic structure of olivine 1
Atomic structure of olivine 1

This specific arrangement allows for chemical substitutions. For example, iron can replace magnesium in the structure. This happens because iron(II) ions have a similar charge and size to magnesium. However, the ratio of iron ions in magma can change the mineral's composition. If iron(II) oxidizes into iron(III), it can no longer form olivine. This is because iron(III) has a 3+ charge instead of 2+.

This mineral is found in many different geological environments. It is a major part of igneous and metamorphic rocks. It is also a primary component of meteorites. In 2005, the Stardust probe found forsterite in cometary dust. In 2011, scientists observed tiny crystals in the gas clouds of a forming star.

Peridot olivine on basalt
Peridot olivine on basalt

Within the Earth, forsterite-rich olivine is very common. It is the most abundant mineral in the mantle above a depth of about 410 km. It is often found in rocks called dunite and peridotite. Dunite typically contains olivine with at least 92% forsterite. Peridotite contains olivine that is also at least 92% magnesium-rich.

Peridot olivine on basalt
Peridot olivine on basalt

Forsterite can change into different forms under extreme pressure. This is known as a phase transition. In the Earth's upper mantle, forsterite transforms into wadsleyite. This transition occurs at pressures of approximately 14 to 15 GPa. Wadsleyite is also orthorhombic. Another polymorph is ringwoodite, which has an isometric or cubic crystal system.

Forsterite-pV
Forsterite-pV

High-pressure experiments show that forsterite can remain metastable. This means it can exist temporarily at pressures up to almost 50 GPa. These transitions are essential for understanding how the deep interior of our planet behaves.

History shows us how humans have identified this mineral. Forsterite was first described in 1824. It was found at Mount Somma near Vesuvius in Italy. A man named Armand Lévy named the mineral. He named it after the English naturalist Adolarius Jacob Forster.

Forsterite orange - Ochtendung, Eifel, Germany
Forsterite orange - Ochtendung, Eifel, Germany

Beyond geology, forsterite has interesting chemical properties. It can form through the metamorphism of high magnesium limestones. For example, dolomite and quartz can react to form forsterite. This reaction also produces calcite and carbon dioxide. Forsterite can also react with quartz to create a mineral called enstatite.

Today, forsterite is being studied for modern technology. Scientists are looking at it as a potential biomaterial for medical implants. This interest is due to its superior mechanical properties. It is a strong and stable mineral. Its presence in space also connects it to the study of astronomy. By studying forsterite, researchers learn about the building blocks of stars and planets. It bridges the gap between the deep Earth and the vastness of space.

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🖼️ Images & Media (4)
File:Forsterite orange - Ochtendung, Eifel, Germany.jpg
Forsterite orange - Ochtendung, Eifel, Germany.jpg
File:Atomic structure of olivine 1.png
Atomic structure of olivine 1.png
File:Forsterite-pV.svg
Forsterite-pV.svg
File:Peridot olivine on basalt.JPG
Peridot olivine on basalt.JPG
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