Horneophyton tubers – a preferred site for level bands in chalcedony
deutsche Version
Level bands in chalcedony, also known as onyx agate, Uruguay bands, or geopetal fills, are frequently found in the Rhynie chert (Lower Devonian), usually in former cavities, as swamp gas bubbles, hollow straws, sporangia, and arthropod moults. They are generated when hydrated silica clusters suspended in water make an emulsion which settles, thus forming a perfectly level boundary with clear solution above, subsequently fixed by gel formation and further silicification. Stacks of parallel bands result from repetition, modification, and even occasional reversion of this process.
The level bands are seen unexpectedly often in the tubers of the small plant Horneophyton (Fig.1). (The tubers can persist in the ground while the upper parts wilt and dry, much like the recent onions. Therefore they are often found separately, as in these pictures.) The bands are there although the tubers have never been hollow. They go right through the distinctly seen tissue as if it were not there, which is very peculiar (Figs. 2-4).

Horneophyton tubers with level bands in Rhynie chert2 Horneophyton tubersHorneophyton tuber with level band stack
Figs.2,3: Horneophyton tubers with level band stacks: Slight tilting during silicification is evident.

Fig.1 (above left): Horneophyton tubers with level bands in Rhynie chert.
Note also the very thin level line in the tuber below right. Width of the picture 20mm.

Horneophyton tuber tissue traversed by level band
Fig.4 (left):  Horneophyton tuber in Rhynie chert with level bands formed independently within the tissue and outside. The deformation on the left indicates small-scale disturbances during silicification of the inundated habitat. Width of the tuber 4mm. (The tubers are usually pictured much too big by factors up to 5.)

Although many a detail of the phenomenon will probably remain obscure, a few facts seem to be certain: Dissolved silica in the form of siliceous acid tends to polymerize into clusters, a process which is sensitive to the pH and the presence of other substances in the water. Apparently the decaying Horneophyton "onions", with the cell content mostly turning into a watery solution, had a special "smell" favouring the formation of siliceous clusters of the right size to form a suspension which settles, thereby forming a horizontal boundary between suspension and clear solution.
Also it is thinkable that the decay products of the cell plasma formed suspensions with boundaries, which influenced the deposition of SiO2 .This must have occurred while the tissue was still coherent but the cell walls were slightly perforated so that the clusters could move through by diffusion and form horizontal bands as if the cell walls were not there.
It is also possible that the horizontal bands in Rhynie chert hint at hitherto neglected peculiarities of the silicification process.

H.-J. Weiss     2008   
completed 2011
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