Mysterious tunnels in garnet..

Title: Mysterious tunnels in garnet may be caused by munching microbes

Minerals can feature unique structures inside them, as a result of biological or physical processes that occured after their formation. Soft minerals can hold tunnel networks left over from microbes which ate away at the mineral. Uniquely, Magnus Ivarsson and his team found tunnel networks like these in garnet, which was unique and surprizing, since biological structures are seldom found in minerals as hard as garnet. Microbial creatures normally would tunnel through softer minerals, though Dr. Ivarsson has an idea on how this could have occured in garnet.

Below: Three-dimensional reconstruction produced by synchrotron-based X-ray tomography. In this reconstruction the complexity of the tunnels are seen including frequent branches and anastomosis between branches forming a coherent network of tunnels. © 2018 Magnus Ivarsson

Q & A - Magnus Ivarsson

How do the tunnels in your studied garnet samples appear to resemble biological structures. Why may certain microbes such as fungi have helped sculpt these?

What first struck me was that the overall morphology and the complexity of the network of the tunnels was similar to fungal mycelia.

The frequent branching and especially the anastomosis between branches, which connects the tunnels into a coherent network are features resembling fungal mycelia. However, other features of the tunnels, like the hexagonal cross

sections, looks more like abiotic (non-biological) structures.

So, what we did was to test the tunnels against biogenecity criteria, and thus test it against all known abiotic processes that can form tunnels like this. We were able to exclude most abiotic explanations, and at the end the morphology and features of the tunnels were in favour of a biogenic interpretation rather than an abiotic interpretation.

Usually, microbial tunnelling is due to trophic reasons or acquiring of habitable space. In the current garnets we`re not sure of the reason

to bore but an educated guess would be that the microbes used the iron for their metabolism.

In oxidized environments as the garnets are found in they represent a rare source of reduced iron that microbes can oxidize to gain energy from.

Would microbial tunnel structures such as these be particularly surprizing to find in a resistant and hard mineral like garnet?

Response: Yes. Most microbial borings are found in softer minerals or substrates that are easier to dissolve chemically or penetrate by mechanical force, like carbonates, bones, wood or volcanic glass.

The harder a mineral is the more energy has to be used to break the chemical bonds when boring. Garnets are up to today, the hardest mineral on the so called Mohs scale of mineral hardness in which microbial borings have been shown.

Garnets is seven out of ten on the scale. Diamond is a ten. But at the same time this is in favour of a biological interpretation because the most probable non-biological process would be so called ambient inclusion trails (AITs), which are produced by a

mineral grain of a greater hardness (higher on the Mohs scale) propelled through a mineral with a lower hardness.

In the case of the garnets we would need a mineral grain of a ruby or sapphire to be propelled through the garnets, and they are just not common enough to produce these abundant features.

What other gemstones and minerals can also feature microbially bored tunnels?

Response: Many minerals like silicates, oxides carbonates are bored by various microorganisms like cyanobacteria, algae or fungi but gemstones have not been investigated with respect to microbial dissolution before. So, this is a question for the future. We have to look for it in other type of gemstones and see if the structures are there.

Going forward, what are your next steps for investigating your theory, and these mysterious tunnel structures further?

Response: Next step is to go back to Thailand and sample for DNA/RNA, and isolation of microorganisms living inside the garnets. By

doing that we can first of all see what type of microbes living in the garnets, and if we manage to isolate a specific boring microorganism, do controlled laboratory experiments and document the actual production of such tunnel structures.

I also would like to look in ancient geologic material for these type of structures. Garnets are relative stable over geologic time scales so this could be a useful trace fossil to be used in old rocks and paleo-soils.

Final thoughts

While the traces of microorganisms can be found in various soft minerals, the reason this could have been present in garnet is due to the consumption of a form of iron, which they may have used for their metabolism. At this stage, that is a guess, but an interesting thought. Hopefully, further investigation may shed some more light on what past physical, or biological processes caused these mysterious tunnels to form.