Intricate tunnels in garnets from soils and river sediments in Thailand – Possible endolithic microborings

Abstract

Garnets from disparate geographical environments and origins such as oxidized soils and river sediments in Thailand host intricate systems of microsized tunnels that significantly decrease the quality and value of the garnets as gems. The origin of such tunneling has previously been attributed to abiotic processes. Here we present physical and chemical remains of endolithic microorganisms within the tunnels and discuss a probable biological origin of the tunnels. Extensive investigations with synchrotron-radiation X-ray tomographic microscopy (SRXTM) reveal morphological indications of biogenicity that further support a euendolithic interpretation. We suggest that the production of the tunnels was initiated by a combination of abiotic and biological processes, and that at later stages biological processes came to dominate. In environments such as river sediments and oxidized soils garnets are among the few remaining sources of bio-available Fe2+, thus it is likely that microbially mediated boring of the garnets has trophic reasons. Whatever the reason for garnet boring, the tunnel system represents a new endolithic habitat in a hard silicate mineral otherwise known to be resistant to abrasion and chemical attack.

On the formation of arrays of micro-tunnels in pyrope and almandine garnets

“…Moreover, the emergent point where a dislocation intersects a free surface is known to be more reactive, and when etched, has geometrical features that reflect the underlying crystal symmetry. The observation that most tunnels in the images of the garnets published in Ivarsson et al (2018) have hexagonal cross-sectional symmetry is fully consistent with the presence of screw This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change.…”

“…The intricate tunnels imaged in pyrope and almandine garnets found in soils and river sediments (Ivarsson et al 2018) can be fully explained, without straining credulity, by abiogenic etching of dislocation microstructures contained within the minerals. There are striking geometric similarities between these tunnels and dislocation networks that have been observed and published in numerous natural and synthetic garnets, and further studies involving local crystallographic orientation mapping via electron backscatter diffraction (EBSD) or similar techniques would be highly informative.…”

“…Here, we briefly review and discuss dislocations in garnets, tunnel formation due to abiogenic etching of dislocations in minerals, and the similarities between the geometry of dislocation substructures and the intricate tunnels and networks observed in these pyrope and almandine garnets (Ivarsson et al 2018).…”

“…These tunnels form highly regular miniature palisades in some regions; in others, they exist as more chaotic branched networks with kinks and junctions with uniquely prescribed angles. The networks of curvilinear, branching and anastomosing (interconnected) tunnels were interpretated as evidence that these tunnels are independent of crystallography, thus providing an indirect foundation for the authors' hypothesis of biological tunneling (Ivarsson et al 2018).…”

“…The intricate and beautiful X-ray computed tomographic images recently published by Ivarsson et al (2018) contain clear evidence of highly aligned parallel tunnels that originate at the mineral surface and extend into the interior, see for example Figure 1. These tunnels form highly regular miniature palisades in some regions; in others, they exist as more chaotic branched networks with kinks and junctions with uniquely prescribed angles.…”