Dunite: Formation, Composition, Properties

Dunite is an intrusive igneous rock of ultramafic composition and with a phaneritic texture. Dunite, also known as olivinite, is known for its coarse-grained texture and almost exclusive composition of the mineral olivine. In fact, dunite is typically 90% or more olivine, with minor amounts of other minerals like pyroxene, chromite, magnetite, and pyrope.

Dunite is an ultramafic rock, meaning it has a very low silica content (less than 45%). This makes it a tough and durable rock, but also one that is easily weathered by water and other atmospheric factors. Dunite is found in a variety of geological settings, including the Earth's mantle, ophiolites, and kimberlites.

Key features of dunite:

Group: Plutonic.
Composition: More than 90% olivine.
Texture: Coarse-grained and phaneritic.
Color: Greenish-brown.
Occurrence: Found in the Earth's mantle, ophiolites, and kimberlites.
Uses: Dunite is used as a building stone, an abrasive, and a source of chromium and magnesium.

Dunite composed of forsterite olivine with black specks crystals of chromite

Dunite Formation

Dunite can form in a few different ways.

Crystal Settling: As magma cools, different minerals crystallize at different temperatures. Olivine is one of the first minerals to crystallize because it has a high melting point. These dense olivine crystals sink to the bottom of the magma chamber due to gravity. Over time, they can accumulate and form a layer of dunite. This process is called crystal settling or gravitational accumulation.

Partial Melting: When mantle rock melts, the liquid melt is usually richer in silica and other elements that melt easily. The solid part that remains behind, called the residue, is enriched in minerals like olivine that are more resistant to melting. As more melt is removed through volcanic activity or other processes, the residue becomes increasingly enriched in olivine, eventually reaching the point where it becomes dunite. This process is called fractional melting or residue enrichment.

Dunite Composition

Dunite's composition is truly unique! It's an ultramafic rock, meaning it's incredibly low in silica (less than 45%) and rich in magnesium and iron. Here's a breakdown:

Olivine: olivine is a magnesium iron silicate mineral with the chemical formula (Mg,Fe)₂SiO₄. This green, yellow-green, or brown mineral makes up a whopping 90% or more of dunite. Olivine is a silicate mineral with a high melting point, making it a crucial component of the Earth's mantle.

Minor Minerals:

Pyroxenes: These silicate minerals, including clino- and orthopyroxene, can be present in small amounts (less than 10%).
Chromite: This black spinel mineral is often found in dunite due to its association with the mantle. It's a valuable source of chromium.
Magnetite: This black iron oxide mineral can also be present in trace amounts.
Garnet: This red silicate mineral is occasionally found in certain dunite varieties.

Dunite Mantle Xenoliths

Dunite Properties

Color: Olive green to yellowish-green, often with a speckled appearance due to minor minerals.

Texture: Coarse-grained, with interlocking crystals visible to the naked eye.

Hardness: 6.5-7 on the Mohs scale, comparable to steel.

Density: High, around 3.3 g/cm³, due to the heavy magnesium and iron content.

Magnetism: Slightly magnetic due to the presence of magnetite.

Refractory, Melting point: High melting point (around 1,550°C) due to its olivine composition.

Roughness: Abrasive texture due to the hard olivine crystals.

Weathering resistance: Resists chemical and physical weathering well due to its strong mineral composition.

Where is Dunite Found

1. Earth's Mantle: The true home of dunite lies deep within the Earth's mantle, where it makes up a significant portion of the upper mantle. However, accessing this hidden treasure trove is no easy feat, as it requires venturing miles underground.

2. Ophiolites: These remnants of ancient oceanic crust, often thrust onto land through plate tectonics, offer a glimpse into the Earth's interior. Dunite can be found at the base of ophiolite sequences, where it reveals the composition of the underlying mantle.

3. Alpine Peridotite Massifs: These massive rock bodies, exposed during mountain building processes, also offer a window into the Earth's mantle. Dunite can be found within these massifs, providing valuable insights into the Earth's internal structure and history.

4. Xenoliths: These are "foreign rocks" found within other igneous rocks. Dunite xenoliths are often found in basaltic rocks, hinting at the composition of the deeper mantle regions from which the molten material originated.

Dun Mountain, New Zealand

Dunite Occurrence and Localities

While not as widespread, dunite can also be found in specific geological settings on the Earth's surface. Some notable examples include:

Dun Mountain, New Zealand: This mountain gave dunite its name and showcases a large dunite body formed by an ancient ophiolite.

Bushveld Igneous Complex, South Africa: This massive layered intrusion contains significant dunite deposits, along with other peridotite rocks.

Tulameen, British Columbia, Canada: This region boasts a dunite complex enriched in platinum group metals and other valuable minerals.

Dunite Uses

Construction: Crushed dunite builds sturdy roads and forms durable foundations.

Industry: Its high melting point makes it ideal for furnace linings and its hardness tackles tough jobs in sandblasting and grinding.

Environment: Dunite's potential to capture carbon dioxide makes it a promising tool in the fight against climate change.

Beyond the basics: Dunite can be polished into unique jewelry, studied by scientists, and even improve soil quality.

For scientists, dunite serves as a window into the Earth's interior, holding valuable clues about the planet's formation and composition. And surprisingly, even crushed dunite finds a role in agriculture, improving soil drainage and aeration, setting the stage for healthy plant growth.

Facts about Dunite

Dunite was named after Dun Mountain near Nelson, New Zealand, where the rock is exposed and was recognized for its unusual composition.

Due to its low silica content, dunite weathers easily, often forming red soils rich in iron oxide.

Despite its abundance in the Earth's mantle, dunite is a rare rock on the surface, making it a fascinating glimpse into the composition of our planet's interior.

It's sometimes called "olivinite," but don't confuse it with the copper arsenate mineral olivenite, which is different.