Lamproite

Lamproite is a special kind of volcanic rock that comes from deep inside the Earth. It is formed from the mantle, which is the layer beneath the Earth's crust. This rock has very low amounts of certain minerals like calcium and aluminum, but it has a lot of potassium and magnesium.

Lamproites can be found in many places around the world, even though they do not make up a large amount of rock. Unlike another type of rock called kimberlite, which is only found in very old parts of the Earth, lamproites are found in rocks of different ages. They exist in places from very old to relatively recent times, with the youngest known example being about 56,000 years old in Antarctica.

The way lamproite erupts as a volcano can also be different. Some eruptions create a type of crater called a diatreme, while others form small cone-shaped hills made of ash and rock.

Petrology

Lamproites come from melted rock deep inside the Earth, more than 150 kilometers below the surface. This melted rock pushes up through volcanic pipes, sometimes carrying pieces of rock and even diamonds from deep within the Earth.

Studies of lamproites, like those found at Gaussberg in Antarctica, suggest they may come from melted rock that has been trapped at the bottom of the Earth’s outer layer. This helps explain both where they melt and their special chemical makeup.

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Mineralogy

Lamproites have special minerals because of their unique chemical makeup. They usually contain rare minerals that don't have much silica. Common minerals in lamproites include a special type of olivine, iron-rich leucite, titanium-rich phlogopite, potassium- and titanium-rich richterite, low aluminium diopside, and iron-rich sanidine. These rocks also have a lot of potassium, with 6 to 8% potassium oxide. They often contain high amounts of chromium and nickel. Over time, lamproites can change into minerals like talc, carbonate, serpentine, chlorite, and magnetite. Zeolites and quartz can also be found in them.

Lamproites can have different amounts of certain main minerals, such as titanium-rich phlogopite, titanium-rich "tetraferriphlogopite," titanium- and potassium-rich richterite, special olivine, low aluminium diopside, iron-rich leucite, and iron-rich sanidine. You don't need all these minerals to call a rock a lamproite. Having just one main mineral and a few others is enough to identify it as a lamproite. However, if a rock contains certain minerals like plagioclase, melilite, monticellite, kalsilite, nepheline, sodium-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite, or kimzeyite, it cannot be classified as a lamproite.

Geochemistry

Lamproites have special chemical traits. They have a lot of potassium compared to sodium, more than three times as much. They also have plenty of potassium compared to aluminum, usually more than eight-tenths but often more than one full unit.

These rocks also have more potassium and sodium together than aluminum, making them special in that way. They contain small amounts of certain elements like barium, zirconium, strontium, and lanthanum.

Economic importance

Lamproite became important for mining when the Argyle diamond pipe was found in Western Australia in 1979. Before this, only kimberlite was thought to be a good source of diamonds. Today, the Argyle diamond mine is the main place where diamonds come from lamproite. Many of these diamonds are pink and not of the highest quality.

Diamonds can sometimes be found in olivine lamproite rocks and narrow paths of rock called dikes. These diamonds are carried to the surface by the lamproite. Another place where diamonds have been found in lamproite is Crater of Diamonds State Park near Murfreesboro, Arkansas.

Nomenclature

Lamproites were known by different names in different places because they have varied minerals and are rare. Today, all these rocks are called lamproites, with extra details added based on their minerals, following rules set by the IUGS.

Related rock types

Here are some rocks that are related to lamproite:

• Kimberlite – An igneous rock that sometimes contains diamonds.
• Lamprophyre – A type of igneous rock that is very rich in potassium.
• Ultrapotassic igneous rocks – A group of rare rocks that are also very rich in potassium.