What are Dendrites?

Dendrites in Rocks and Minerals

A dendrite is a branching, tree-like growth pattern that can form in crystals, rocks, minerals, metals, and even ice.

Dendrites form through different processes depending on the material and environment. In rocks, they can occur:

• within the rock, where mineral-rich fluids seep into tiny cracks or cavities and precipitate minerals
• on the surface of the rock, where the same process produces flat, stain-like patterns.

When on the surface, they appear visually as patterns rather than raised growths.

Dendritic patterns, which are relatively common, are secondary mineral growths that form along fractures or different surfaces within the rock.

The word 'dendrite' comes from the Greek 'dendron', meaning 'tree'.

Dendritic patterns can resemble organic matter, and in the past, stones have been mistaken for fossils.

Dendrites are often composed of manganese oxides or iron oxides.

This thread on Mindat includes many amazing photos of stones with dendritic patterns. 

Why Dendritic Agate is Different

In dendritic agate, the branching patterns formed in existing cavities or fine fractures in the host rock, before the agate existed.

These spaces are relatively common in volcanic rocks, where gas bubbles, cooling cracks, or structural weaknesses create voids.

As mineral-rich fluids carrying manganese or iron oxides moved through these spaces, thin, branching mineral deposits formed on internal surfaces.

At a later stage, silica-rich fluids passed through, and, as conditions changed, silica accumulated and hardened into chalcedony and agate, gradually filling the spaces.

As the agate formed, the dendritic patterns became trapped inside.

A very similar process produces the same effect in dendritic opal, although unlike agate, opal is non-crystalline.

Dendrite Means Branching, Tree-Like Form

The term dendrite is used in more than one scientific field, so can be misunderstood.

In physics and other sciences, dendrites are described as crystals that grow in a branching, tree-like form, such as snowflakes or frost on windows. In those contexts, the patterns are formed through crystal growth.

In geology, the word is used differently and refers to the pattern rather than the crystal structure.

Because they can look similar, geological dendrites are sometimes mistaken for crystals, even though the processes through which they formed are different.

Article Pictures

The picture at the top of our article shows dendritic inclusions in the mineral talc. Photo: Courtesy of Stan Celestian.

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Dendritic mineral growth photographed at high magnification. Photo: Courtesy of Captain Tenneal.