Something Digs Intricate Tunnels in Garnets. Is It Alive? [nytimes.com] (archive [archive.fo])
[Sometimes] garnets are marred with intricate traceries of microscopic tunnels. When Magnus Ivarsson, a geobiologist at the Swedish Museum of Natural History, first saw these tunnels, he wondered what could be making them. After Dr. Ivarsson and his colleagues traveled to Thailand, they found that an assortment of evidence contradicted standard geological explanations for how the cavities might be formed. In a paper in PLOS One [plos.org] [open, DOI: 10.1371/journal.pone.0200351] [DX [doi.org]], the researchers are floating a new hypothesis: Perhaps what's making the tunnels is alive.
From the beginning, the researchers looked for alternative explanations. One of the most promising was that grains of another stone wore their way through the garnet. However, the mineral doing the tunneling must be harder than the surrounding substance, and garnets happen to be very, very hard. About the only things that could do that to garnet are diamonds or sapphires. But those aren't present in significant quantities where these garnets were found, said Dr. Ivarsson. In that area, "there is basically no mineral grain that can be propelled through a garnet like that," he said.
Furthermore, the tunnels branch and connect with each other in a very unusual pattern, looking a bit like the structures made by some kinds of single-celled fungus colonies. When the researchers cracked the garnets open, they tested the insides of the tunnels and found signs of fatty acids and other lipids, potential indicators of life. [...] At the moment, the researchers' best guess for the origins of the tunnels goes like this: At first, normal wear-and-tear on the surface of a garnet creates divots. Microorganisms, probably fungi, can colonize these hollows. Then, if the stone is the best nearby source for certain nutrients, such as iron, perhaps they use an as-yet mysterious chemical reaction to burrow deeper, harvesting sustenance as they go.
"Garnets [wikipedia.org] are nesosilicates having the general formula X3Y2(SiO4)3. The X site is usually occupied by divalent cations (Ca, Mg, Fe, Mn)2+ and the Y site by trivalent cations (Al, Fe, Cr)3+ in an octahedral/tetrahedral framework with [SiO4]4− occupying the tetrahedra."