Bowen's Reaction Series #2: Fascinating Facts About Olivine

August 19, 2024

Deep green and resilient, olivine is a mineral forged in the heart of our planet. A cornerstone of geology, it offers a glimpse into Earth's fiery past. From the depths of the mantle to the sparkle of a gemstone, olivine has fascinating stories to tell!

What is olivine?

Olivine is a silicate mineral with the chemical formula (Mg,Fe)₂SiO₄. Its name is derived from its olive-green color, a hue that can vary depending on the iron content. This mineral is a key component of the Earth's mantle, the layer between the crust and the core.

The Olivine Family

Olivine is not a single mineral but a group of minerals with similar structures. Two primary end members define the olivine series:

Forsterite: The magnesium-rich end member with the formula Mg₂SiO₄.
Fayalite: The iron-rich end member with the formula Fe₂SiO₄.

Most olivine found in nature is a mix of these two end members, creating a range of compositions and colors.

In general, geologists recognize olivine by the following characteristics:

Color: Typically olive-green, but can range from yellow-green to brown depending on the iron content.
Hardness: Ranges from 6.5 to 7 on the Mohs scale, making it relatively hard, but at the lower-limit of what can be used for durable jewelry.
Cleavage: Poorly developed, meaning it breaks irregularly rather than along smooth planes.
Fracture: Conchoidal, similar to glass.
Luster: Vitreous (glassy).

Crystal Structure

Olivine belongs to the orthorhombic crystal system, meaning its crystals have three unequal axes at right angles to each other. Its structure is characterized by isolated silicate tetrahedra (SiO₄) linked together by magnesium and iron ions. This unique arrangement contributes to olivine's stability at high temperatures and pressures, making it a dominant mineral in the Earth's mantle.

Olivine Stability

Olivine, a high-temperature mineral formed under the intense pressure and temperature conditions of the Earth's mantle, is inherently unstable at the surface. This instability arises from thermodynamic principles, specifically related to Gibbs free energy.

Gibbs free energy is a thermodynamic property that predicts the spontaneity of a process. A system will spontaneously move towards a lower Gibbs free energy state. In the case of olivine, its formation is favored under high-pressure, high-temperature conditions, representing a low Gibbs free energy state within the mantle.

Upon exposure to the Earth's surface, olivine finds itself in a thermodynamically unfavorable environment. The lower pressure and temperature conditions at the surface correspond to a higher Gibbs free energy state for olivine. Consequently, the mineral undergoes transformation to reach a more stable, lower energy configuration. This transformation often involves the formation of new minerals, such as clay minerals, iron oxides, and silica.

Imagine Gibbs free energy as a measure of a system's stability. Things naturally tend to move towards a more stable state, just like a ball rolling downhill. Olivine is very stable deep in the Earth where it's hot and under a lot of pressure. It's like the ball is at the bottom of a deep hole.

When olivine comes to the Earth's surface, it's like suddenly lifting the ball up a tall hill. It's no longer in a stable position and wants to roll back down to a lower energy state. This is why olivine breaks down into other minerals at the surface.

In a magma body, although olivine crystals are the first to form, they are also some of the first to breakdown and reform into new minerals - specifically pyroxene, the next mineral in Bowen's Reaction Series!

As magma cools, the temperature and pressure decrease, altering the stability of minerals. Olivine, which is one of the first minerals to crystallize due to its high melting point, becomes less stable in the evolving magma environment.

The process of olivine breakdown involves several factors:

Temperature Reduction: As the magma cools, the energy available to maintain the olivine structure decreases.
Chemical Changes: The magma composition evolves as minerals crystallize, altering the chemical environment for olivine.
Pressure Decrease: In some cases, magma ascent and eruption can lead to a decrease in pressure, affecting mineral stability.

The specific reaction often involves the addition of silica to the olivine structure. This process results in the formation of pyroxene, a mineral with a higher silica content than olivine. The exact type of pyroxene formed (orthopyroxene or clinopyroxene) depends on the specific magma composition and cooling conditions.

Peridot: Olivine's Gemmy Alter Ego

Peridot is name we use for the gem-quality variety of olivine. Its vibrant green color, often described as "evening emerald," has captivated humans for millennia.

Color: Peridot's color is influenced by its iron content. Higher iron content results in a deeper, darker green.
Clarity: High-quality peridot is typically transparent with few inclusions.
Size: Peridot crystals can vary significantly in size, from small faceted stones to larger cabochons.
Location: Although not as rare as some other precious stones, gem-quality peridot free of inclusions is uncommon, but deposits of varying quality have been found in the USA, China, Pakistan, Myanmar, and Finland!

A Brief History of Olivine and Peridot

Human fascination with olivine dates back to ancient civilizations! Check out the references at the bottom of this post for further reading.

Ancient Egyptians were particularly enamored with peridot, referring to it as the "gem of the sun."They believed it possessed mystical powers, capable of warding off evil spirits and bringing good fortune. As early as 1500 BC, Egyptian papyri documented the mining of peridot on the island of Topazos* (now called Zabargad) in the Red Sea.
Other Ancient Civilizations: Peridot was also valued by the Greeks, Romans, and other cultures.
Modern Times: Peridot's popularity has endured, with modern jewelry featuring this vibrant gemstone in various styles.

* Note the similarity to the modern word 'topaz' here! There have been several gemstones used throughout history which were misnamed or miscategorized for many years. A subject for another blog post...

When was olivine discovered and by whom?

As discussed above, people have been fascinated by the gemmy beauty of olivine since antiquity! In fact, throughout history it has often been confused and misnamed with other minerals, including emerald and topaz. The mineral as we define it today was first formally described in 1789 by the German mineralogist Abraham Gottlob Werner.

What are the uses of olivine in industry and fashion?

From the fiery depths of our planet to the elegant adornment of human civilization, olivine has played a multifaceted role. Its journey from a high-temperature mantle mineral to a cherished gemstone is a testament to the Earth's dynamic processes and humanity's enduring fascination with nature's beauty.

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