Agates and fossils from watery habitat (1)
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agate and fossil
This image combined with the above headline may be confusing in more than one way: The horizontal line could be thought to be a former water level, which is not. Agates are often thought to be formed at high temperatures but they are not. Hence it will be interesting to consider what had been going on here 400 million years ago in the watery habitat part of which had turned into the famous Rhynie chert.

Image: Banded agate, coated fungus hyphae, early land plant with fungus globules. Image width 4.3mm.

This sample shows one of the early land plants, Aglaophyton,  lying flat. Some fungus had formed globular objects seen here below right, also known as chlamydospores, in the inundated and probably decaying plant. It is not known whether the same fungus or another species, a water-dwelling one, had grown thin hyphae throughout the silica-rich water surrounding the plant. The hyphae got coatings of silica gel, now seen as bluish chalzedony.
Then, for reasons unknown, a shallow trough had formed, with an upward bend seen here on the left. The space above the trough, except for the coated hyphae, was still watery. Silica clusters formed in the slightly supersaturated liquid. When they became so big that thermal motion could not keep them afloat any more, they settled into a heavy liquid suspension which separated itself from the lighter liquid by a level surface. In the present case, something dark must have got stuck to the settling clusters but not to the coatings, which must remain unexplained here. Eventually the dark sediment solidified into gel and got a thin layer of bright-coloured sediment on top. Next came a layer of pale silica gel covering the level face and the other gel already present in the water.
After these deposition and coating processes, a watery cavity had remained, whose walls became coated with pale brown silica gel irrespective of position. This means that the silica clusters sticking to the wall were so small that they had been evenly distributed in the liquid by thermal motion. Other than the even distribution in space, the process did not go on smoothly with time, judging from the "banding" usualy seen with agates. The causes of the variations are not known.
It seems that the process of forming a dark suspension with a level surface had repeated itself in the final small water pocket inside the agate.
It may well be that the pale brown of the agate bands
and the dark brown of the sediment are caused by the same staining substance.
Sample: Rh2/164.3 (2011) 0.61kg, obtained from Shanks.

H.-J. Weiss     2019

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