Level fills in hollow Aglaophyton

level fills
It is well known from small agates in volcanic rock that their aspect can be quite different even though they may be only a few millimeters apart, and that occasionaly the banding or part of it is horizontal. Similar formations, called here level fills for short, are not rare in the Lower Devonan Rhynie chert, which is silicified water with plants, debris, and mud, where former cavities due to swamp gas bubbles or hollow plant parts became gradually filled with silica clusters of various size which fused into solid chalcedony later. Also it is known that the horizontal bands sometimes thought to be a relic of a water level in the cavity are no such.

The present small chert sample of 60g shows level fills in hollow Aglaophyton shoots lying prostrate, 5 of them with different aspect on a rectangular area of less than 3cm2, which is a not quite common sight: Fig.1. Obviously the fills vary in colour and layer thickness. The same applies to the linings on the cavity walls. This suggests that silica deposition and gel formation had been highly sensitive to slight variations of the concentration of substances, perhaps decay products, in the watery silica solution.

Fig.1: Rhynie chert with Aglaophyton cross-sections, 5 hollow ones, now partially filled with a series of silica deposits: linings and level layers of chalzedony, coarse quartz crystals as the final formation. Width of the picture 18mm. See enlarged details below.

In the present sample, the level layers are there in the hollow plant parts only but not in the former gas bubbles in the swamp. This suggests that the watery emulsion of tiny silica clusters, which settled down into a "lake" with a distinct plane boundary against the clearer water above, had formed preferably in the compartments where possibly some decay products of the plant tissue were confined.

level fill in hollow Aglaophyton level fill in hollow Aglaophytonlevel fill in hollow Aglaophyton
Figs.2-4: Aglaophyton sections, largely hollow before silicification owing to decayed tissue.
           (Fig.3 is not a detail of Fig.1 as it is 1cm outside that frame.) Height of the pictures 6mm.

What appears as a dark fill in Fig.2 is only a thin dark plate of 40Ám, slightly inclined and thus seen from below through the clear quartz. A similar but thinner plate, 20Ám, is there in Fig3, also inclined but not seen from below through the milky bluish chalcedony. Above the dark plates, clear quartz crystals grew more slowly, with diameters up to 0.3mm, and part of the cavities has remained empty until now.
Fig.4 is more complex since there had been compartments where silicification proceeded in different ways. What is seen as dark brown on the left is a compact fill with a plane light-coloured bottom in plain sight because the space below is empty. The latter phenomenon is unexpected but not very rare. What is now empty below a plane bottom of a deposit must have been a plane surface of another deposit which became dissolved much later.
Much earlier when there were no deposits but only water in the cavity of Fig.4, a fungus hypha had been there on the left. When the cavity got its thick bluish lining, the hypha got its thick bluish coating, which remained unaffected later while deposits accumulated and vanished around it. It is still seen with a tiny dark dot in the middle, which is the hypha cross-section. The bright bar is a layer consisting of conspicuously glittering quartz crystals in the depth. Below it there are smaller levels independent of those on the left.
Also the thin dark plate in Fig.2 must have been free-standing for a long time after the deposit below had dissolved, similar as in Fig.4, so that the space became free for the slowly growing clear quartz which is there now.
Aglaophyton sections, one hollow with level stacklevel fill in hollow Aglaophytonlevel fill in hollow AglaophytonFigs.5-7: Aglaophyton sections, largely hollow before silicification owing to decayed tissue. Height of the pictures 5mm, 4mm, 5mm.
Same scale for Figs. 2-7.

The dark ring of cells indicating the presence of the symbiotic fungus Glomites, which is often found with Aglaophyton, is faintly seen on the well preserved cross-section of 4mm in Fig.5.
What the fill in Fig.5 has in common with Figs.2,3 is the dark layer on top of the stack, here 90Ám. As a peculiarity in Fig.5, it becomes obvious that at least two layers of the stack grew simultaneously with the lining. This feature and the thick light-coloured layer are not found with the other fills in this sample.

Fig.8 is to demonstrate that the hollow plant parts with silica fill may offer rich detail concerning silicification and plant life as well. It suggests that later in its life the plant was able to live as a hollow straw, with a wall of well-preserved cells along the circumference, some degraded tissue inside, and the central strand, probably still functioning, leaning to one side.

hollow Aglaophyton with silica fill

Fig.8: Enlarged part of Fig.2, hollow Aglaophyton not quite filled by a series of silicification processes.

The fills in Figs.6,7 are not plane on top but not shaped such that they could have been a meniscus at a liquid surface. Hence, they had not been a liquid emulsion but rather a slurry of silica clusters which had accumulated by raining down from the upper parts of the cavity where they had been precipitated from silica solution with high supersaturation. All cavity fills which obviously followed the direction of gravity, be they plane or not, are often called geopetal fills.
In Figs.5-7, too, quartz crystals grew in the cavities left after the deposition of linings and layers. The cavity became completely filled in Figs.5,6 but is still partially empty in Fig.7.
So it appears that much of what is seen in this small sample of Rhynie chert with rich detail is compatible with the logic of silicification but the cause of the diversity remains enigmatic. Also the brown cloud in the upper part of the former cavity in Fig.6, above the clear coarse quartz, does not seem to fit in.  

H.-J. Weiss  2015    
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