Riddle about Garnet Gears

Could microbes have dug the tiny tunnels into the gems?

Garnet crystal with countless small passages. How did they come about? © Ivarsson et al, 2018
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Enigmatic Phenomenon: Tiny passages and drilled holes in garnet minerals make for guesswork. Because how these micro-tunnels have come into the gems is not yet known. Now researchers have analyzed some of these tunnel grenades in more detail. They discovered evidence that microbes could have drilled these corridors. Given the great hardness of the grenade this would be very unusual.

The often shimmering red shells were coveted as gemstones in ancient times, in the Middle Ages they were called Karfunkelstein. Behind these collective headings hides a whole group of silicate minerals, which get their color shades through different foreign atoms. But grenades alone but their transparency, their cubic crystal lattice and their particular hardness.

Palisades and branched networks

But it is precisely this hardness of the grenade that makes a phenomenon particularly puzzling: some of these gems are criss-crossed by a network of tiny gears. Although these minerals are so tough and chemical resistant, something must have drilled through these crystals. But what? Magnus Ivarsson from the University of Southern Denmark in Odense and his colleagues have investigated this question and have examined the mysterious granate tunnels using state-of-the-art microscopy and analysis methods.

They discovered that these microtunnels always leave the garnet surface and have a round to hexagonal cross-section. Towards the inside, the tunnels, which are between five and 100 micrometers thick, become narrower and end in a thin tip. The structure of these gait systems, however, is surprisingly diverse: "The range extends from strictly regular palisades from parallel tunnels to irregularly branched and rejoining networks, " the researchers report.

Microscope image of a complex tunnel network in a garnet © Ivarsson et al, 2018

Who was the author?

But the key question was: are there any clues to their origin inside these garnet tunnels? So far, these and similar defects in minerals have mostly been considered abiotic - as geochemically generated structures. But there are also living things that can drill themselves into hard rock, as Ivarsson and his colleagues explain. display

"A number of microorganisms, including bacteria, fungi and algae, are known to produce organic acids or chelates that can chemically corrode and protect minerals, " the researchers said. However, hard silicate minerals in particular hardly respond to such chemical attacks and are therefore rarely colonized by such endolithic organisms. Also, the garnet was previously considered more immune to such biting attacks.

Organic filaments

But that could be a mistake, as the analyzes revealed. Because inside the microtunnel, the researchers discovered fumigated structures that contain both carbon and nitrogen and fat acids - components that are typical of living things. "The organic content and the complex nature of these organic molecules indicate a microbial presence in this tunneling system of the garnet, " Ivarsson and his colleagues report.

The fibrous structures could therefore be remnants of the bacteria or fungi that once inhabited these tunnels. "This is the first evidence of grenades as a habitat for endolitic microorganisms, " say the researchers.

"Teamwork" of chemistry and biology?

However, whether these microbes have bored the tunnels themselves or have only benefited from existing ones is still unclear. For example, it would be possible for the mostly larger, often hexagonal, tunnel junctions to be created by geochemical processes. Later, the courses were colonized by bacteria or fungi that expanded and expanded these ductwork.

"There are morphological structures that speak for a combination of abiotic and biological processes, " say the researchers. On the other hand, the anastomoses in many of these tunnel networks are more in favor of a biological origin: "The only natural processes that can lead to such fusion of gene are biological in nature, " explains the scientists. It seems clear: the mystery of the garnet tunnels has not yet been completely solved. (PLOS ONE, 2018; doi: 10.1371 / journal.pone.0200351)

(PLOS, 09.08.2018 - NPO)