A December 2025 study showing that a specific type of crystal defect in olivine- the main mineral in Earth’s upper mantle- is more common than geologists previously assumed.

Olivine makes up most of the rock in the upper mantle (down to ~400 km depth). Under extreme heat and pressure, it deforms extremely slowly through dislocation creep—tiny defects (dislocations) in the crystal lattice let the solid rock flow like ultra-viscous putty over millions of years. This flow drives mantle convection, which powers plate tectonics and reshapes Earth’s surface.

Olivine is a common rock-forming mineral (actually a group of minerals) and one of the most abundant in Earth’s interior. It is a magnesium-iron silicate with the general chemical formula (Mg,Fe)₂SiO₄.

Olivine forms a solid solution series between two end-members:

• Forsterite (Mg₂SiO₄) — magnesium-rich, typically lighter green or yellowish.
• Fayalite (Fe₂SiO₄) — iron-rich, darker green to brownish or almost black.

Most natural olivine is intermediate in composition (a mix of Mg and Fe), with magnesium-rich varieties being far more common in the mantle and mafic rocks. Minor elements like nickel, manganese, or calcium can substitute in small amounts.

In the upper mantle, olivine dominates the rock and deforms slowly under heat and pressure through mechanisms like dislocation creep. Its behavior controls how the mantle flows, driving plate tectonics.

Olivine has an orthorhombic crystal structure, so deformation requires multiple slip systems (combinations of slip plane + slip direction) to accommodate arbitrary strain without fracturing (following the von Mises criterion, which needs at least 5 independent systems).The dominant slip directions (Burgers vectors) are traditionally:

• a-slip: Burgers vector parallel to [100] (most common at many conditions).
• c-slip: parallel to [001].

These produce characteristic fabrics (e.g., A-type fabric with [100] axes aligned in the flow direction). A third type, b-slip (Burger’s vector component along [010])- was long considered rare or insignificant and was mostly ignored in mantle flow models.

Scientists long focused on two main slip directions for dislocations in olivine:

• “a” slip ([100] direction)
• “c” slip ([001] direction)

A third type, “b slip” with a Burgers vector component along [010]- was considered rare or insignificant under typical mantle conditions and was mostly ignored in flow models.

A team led by Professor John Wheeler (University of Liverpool) studied an experimentally deformed olivine sample (deformed under conditions mimicking subduction zones: 2 GPa pressure and ~850°C). Using a clever two-step imaging method, they found that **17%** of the crystals showed clear evidence of active b dislocations- far more than expected. These b dislocations form mobile subgrain walls (linear features) via glide and climb, just like the better-known a and c types.

How they did it:

• Electron Backscatter Diffraction (EBSD): Mapped tiny orientation changes across thousands of crystals to spot strain gradients hinting at b slip (color-coded maps show smooth gradients where dislocations are concentrated).
• Transmission Electron Microscopy (TEM): Zoomed in to atomic scale on the flagged spots, directly imaging the actual b dislocations (including partial screw dislocations with characteristic plane offsets).

This correlative approach (EBSD for screening + TEM for proof) is a breakthrough for spotting these subtle defects.

Why It Matters:

• Mantle flow models: b slip may help olivine satisfy the von Mises criterion (needing at least 5 independent slip systems for general deformation). Including it could refine predictions of how the mantle convects and how seismic waves travel through it.
• Depth “thermometer”: Because b dislocations appear sensitive to pressure, temperature, and stress, finding them in natural mantle rocks (from xenoliths or ophiolites) could reveal exactly where and under what conditions deformation happened. scitechdaily.com
• Broader applications: Olivine’s crystal structure is similar to industrial perovskites; understanding dislocations helps materials scientists too (e.g., in semiconductors where defects hurt performance). dailygalaxy.com

Wheeler summarized it well:

“Our findings suggest that these dislocations may be more widespread than previously thought, improving our understanding of how the Earth’s mantle deforms. Their presence may be influenced by pressure, temperature, and stress levels. Measuring ‘b’ dislocations in natural samples could therefore help scientists determine the depth of deformation and the conditions experienced during it.”

_____________________________________________________________________________________

Scientists Uncover Unexpected Behavior Deep Inside Earth’s Mantle

Tiny defects in common minerals may reveal unexpected patterns in how Earth’s interior moves.

Minerals quietly shape the world around us, from the rocks beneath our feet to the deep interior of the planet. At their core are crystals, orderly arrangements of atoms that repeat in precise, three-dimensional patterns. SciTech Daily has the story.

While these structures may seem rigid, they are far more dynamic than they appear.

Under intense pressure and heat, such as those found deep inside Earth, crystals can bend and flow rather than break. This ability comes from tiny defects known as dislocations, small irregularities in the atomic structure that act like microscopic slip zones. They allow solid rock to slowly deform over time, a process that ultimately helps drive the movement of tectonic plates.

In some crystals, dislocations are abundant, while in others they are rare and difficult to detect. Finding them can be as challenging as searching for a needle in a haystack.

Olivine, the most common mineral in the upper 400 km (about 250 miles) of Earth’s interior, has long been studied for how it deforms. Scientists have identified two primary directions of dislocation movement, labeled “a” and “c.” A third direction, known as “b,” has traditionally been considered uncommon and less important.

A recent study led by a University of Liverpool Earth scientist revisited this assumption. The research aimed to better understand how olivine deforms, a key process that drives plate tectonics, and to identify the types of dislocations involved.

Advanced Microscopy Techniques

The team used Electron Backscatter Diffraction (EBSD), an advanced microscopy method that maps subtle differences in crystal orientation at a microscopic scale.

Their analysis showed that about 17% of the crystals examined displayed deformation linked to the previously underestimated “b” dislocations.

To confirm this result, the researchers turned to Transmission Electron Microscopy (TEM), which can directly image dislocations. By focusing on regions flagged by EBSD as showing “b” slip, they captured detailed images that verified these structures were indeed present.

Read the full story here.

_____________________________________________________________________________________

Researchers Just Detected Unexpected Deformation in Earth’s Mantle That They Once Thought Was Rare

Something unusual is happening deep inside Earth, hidden within one of its most common minerals. Scientists have picked up signals that do not quite match what decades of research predicted.

Deep inside Earth’s mantle, scientists have spotted something unexpected in a very common mineral. Tiny defects once thought to be rare may actually play a much bigger role in how the planet slowly shifts and moves. The Daily Galaxy has the story.

Minerals might look solid and unchanging, but under extreme heat and pressure, they can bend and flow over time. This slow deformation is one of the forces behind plate tectonics, shaping continents and oceans over millions of years.

One mineral in particular, olivine, dominates the upper mantle. It has been studied for decades, yet researchers are still uncovering new details about how it behaves when pushed to its limits.

A “Rare” Mechanism That Isn’t So Rare After All

Scientists long believed olivine mainly deformed along two directions, called “a” and “c.” A third one, “b,” was mostly ignored, considered too uncommon to matter.

Read the full story here.