Olivine

Olivine is a common mineral found deep inside the Earth. It is made mostly of magnesium and iron mixed with silicon and oxygen. Olivine is a major part of the Earth's upper mantle, the layer just below the outer crust.

Olivine can be used as a gemstone called peridot. It is also used in industrial processes, such as metalworking. Olivine has different types depending on the amount of magnesium and iron it contains.

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.

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 clear, olivine can be used as a gemstone called peridot. It is also known as chrysolite, but this name is rarely used today.

Olivine is found in many types of 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. Rocks with over 40% olivine are called peridotites. Dunite, a rock with over 90% olivine, forms 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. 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 groups of oxygen and silicon atoms. This makes olivine a type of mineral called a nesosilicate.

In olivine's structure, oxygen atoms are arranged closely together. Half of the spaces for metal atoms 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.

The exact depth where these changes happen depends on how hot it is and how much iron is in the olivine.

Weathering

Olivine does not stay the same for long on Earth's surface. When water is there, it changes fast into a mix of clay minerals, iron oxides, and ferrihydrite, called iddingsite. Because it changes 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 long ago. 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 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 comes from Norway.

In a place called Svarthammaren in Norddal Municipality, now part of Fjord Municipality, people dug up olivine from around 1920 until 1979.

In Robbervika in Norddal municipality, an open mine has been working since 1984. The olivine there has a special red color.

A person named Hans Strøm wrote in 1766 about olivine. 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 replace dolomite in steel factories. When making aluminium, olivine sand is used because it needs less water but still holds its shape when metal is poured.

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

Main article: peridot

Experimental uses

Scientists are testing a mineral called olivine to help clean the air. When olivine is crushed and placed on beaches, waves help it react with the air. This makes silicon dioxide, magnesium carbonate, and iron oxides.

Olivine is also being studied to make a special kind of cement. This cement does not add extra carbon to the air and might even take some away. A group called Project Vesta is looking at how waves can help this process on beaches.