Agate and fossil from watery habitat (2)   ---  Eerie glow
deutsche Version
level stackAs discussed in Part (1), the combination of agate and fossil may be surprising but is not really so, although the aspect of the agate itself may be. The "banded" structure usually seen in sections of former cavities is due to silica deposition in layers, mostly at the cavity walls, often at differential rates or with interruptions.
The present sample is different, not only because of its eerie glow under incident (!) light but rather because of the evenly stacked horizontal layers.

Image: Former gas bubble in swamp water, now filled with solid chalzedony of various aspect, with branching fungus hyphae. Image width 2.8mm.

This agate began with a swamp gas bubble. After the surrounding silica-rich water had become silica gel, thus stabilizing the shape, diffusion made the gas escape and silica solution enter. Branching hyphae of some aquatic fungus grew there. Next, cavity walls and hyphae got a thick coating of silica gel. This must have occurred in one process stage since the coating is not layered.
In the remaining water pocket, SiO2-clusters grew until they sank to the bottom, forming a heavy liquid suspension there, which separated itself from water with a horizontal boundary. Apparently this had been going on repeatedly, governed by unknown means with surprising accuracy, thus creating the wondrous structure.
The black debris in the above compartments, possibly decayed microbes, must have come later since no such is seen among the layers below. Finally, the remaining water and everything else silicified into chalzedony.
The yellow-green streak below marks a crack slightly inclined to the cut face, with a gap narrower than the light wavelength
since it does not reflect the incident light, and with traces of some staining mineral. For comparison, see the wider crack above right, which makes a shadow.
The conspicuous apparent glow is brought about by reflection of the incident light at silica grains once formed in the water but now fused into solid chalzedony.

Sample: Rh13/14.1 (2012)

H.-J. Weiss     2019

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