Olivine

Olivine is a common mineral found deep inside the Earth. It is made mostly of magnesium and iron mixed with silicon and oxygen, and its chemical formula is (Mg,Fe)₂[Si][O]₄. Olivine is a type of mineral called a nesosilicate or orthosilicate. It is a major part of the Earth's upper mantle, which is the layer just below the outer crust.

Olivine is important because it can be used as a gemstone called peridot, also known as chrysolite. It is also used in industrial processes, such as metalworking. The amount of magnesium compared to iron in olivine can change, leading to different types of olivine. One type, called forsterite, has more magnesium and melts at very high temperatures, while another type, called fayalite, has more iron and melts at lower temperatures.

Olivine belongs to a group of minerals known as the olivine group. These minerals share a similar crystal structure. Olivine's structure is based on an orthorhombic Bravais lattice, where each piece of silica (SiO₄) is connected to metal ions, creating a pattern that is similar to a structure found in magnetite.

Identification and paragenesis

Olivine is named for its olive-green color, which comes from small amounts of nickel. It can also turn reddish when iron in it changes. When translucent, olivine can be used as a gemstone called peridot, which is the French word for olivine. It is also known as chrysolite, a name that is rarely used today.

Olivine is found in many types of rocks, both those formed from melted rock material and changed rocks. It forms when melted rock that is rich in magnesium and low in silica cools. This melted rock becomes rocks like gabbro and basalt. Some rocks contain a lot of olivine, and those with over 40% olivine are called peridotites. Dunite, a rock with over 90% olivine, is thought to form when olivine crystallizes and settles from melted rock. Olivine makes up more than half of the Earth’s upper mantle and is one of the most common minerals on Earth by volume. It has also been found in meteorites, on the Moon, and on Mars.

Crystal structure

Olivine minerals form in a special pattern called the orthorhombic system. They have shapes with isolated groups of oxygen and silicon atoms. This makes olivine a type of mineral called a nesosilicate.

In olivine's structure, oxygen atoms are arranged in a close-packed pattern. Half of the spaces where metal atoms fit are filled with magnesium or iron atoms, and a smaller number of spaces are filled with silicon atoms. There are different places for oxygen, metal, and silicon atoms in this pattern.

High-pressure polymorphs

Deep inside the Earth, where it is very hot and squeezed tightly, olivine changes into other minerals. Around 410 kilometers below the surface, olivine turns into wadsleyite. At about 520 kilometers deep, wadsleyite changes into ringwoodite. Finally, at around 660 kilometers deep, ringwoodite splits into two other minerals. These changes make parts of the Earth denser and affect how the layers move.

The exact depth where these changes happen depends on how hot it is and how much iron is in the olivine. Higher temperatures and more iron can change when these transformations occur.

Weathering

Olivine does not stay stable for long on Earth's surface. When water is present, it changes quickly into a mix of clay minerals, iron oxides, and ferrihydrite, called iddingsite. Because it weathers so fast, olivine is not often found in sedimentary rock.

Scientists think that if iddingsite is found on Mars, it might mean that liquid water was there in the past. This could help them learn when water last existed on the planet.

Mining

Norway

Norway is the main place in Europe where we find olivine. It is especially found in areas from Åheim to Tafjord, and from Hornindal to Flemsøy in the Sunnmøre district. About half of the olivine used around the world for jobs comes from Norway. In a place called Svarthammaren in Norddal Municipality, which is now part of Fjord Municipality, people dug up olivine from around 1920 until 1979. They could get up to 600 metric tons every day. In Robbervika in Norddal municipality, an open mine has been working since 1984. The olivine there has a special red color, which can be seen in local names like Raudbergvik (Red rock bay) and Raudnakken (Red ridge).

A person named Hans Strøm wrote in 1766 about how olivine looks red on the outside but blue inside. He said that in the Norddal area, people broke olivine from rocks and used it to make sharpening stones.

Kallskaret near Tafjord is a nature reserve that contains olivine.

Applications

Olivine can be used instead of dolomite in steel factories. In making aluminium, olivine sand is used because it needs less water than other sands but still keeps the shape of the mold when metal is poured.

In Finland, olivine is used in sauna stoves because it is very dense and can handle being heated and cooled many times. Some very beautiful olivine is cut and polished to make a gemstone called peridot.

Main article: peridot

Experimental uses

Scientists have looked at using a mineral called olivine to help remove carbon dioxide from the air. When olivine is crushed and left on beaches, waves can help it react slowly with the air. This process creates silicon dioxide, magnesium carbonate, and iron oxides.

Olivine is also being tested as a material to make cement that does not add extra carbon to the air or even takes some away. A group called Project Vesta is studying how waves can help this process on beaches.