Rhynie chert sample with peculiar details

An incidental combination of more or less uncommon features make this sample worth being commented on:
Rhynia 3D
 - no other plant than Rhynia seen on the surface and on cut faces,Rhynia upright
 - except for a 1.5cm bottom layer, Rhynia in upright growth position,
 - kinks indicating differential silicification rates,
 - mixed calcite and quartz in former cavities,    
 - first (?) evidence of violet calcite (?) in Rhynie chert,
 -
uncommon ellipsoidal chlamydospores of some fungus.

Several upright shoots of Rhynia and half pipes left by shoots that must be on the other side of the divide now are seen in Fig.1. They are not seen in full height in this chert layer of about 9cm thickness. Possibly their upper parts had been above the water level, thus did not become silicified but decayed and disappeared. (Sample: 0.5kg, found near Smithston in 2009.)
Often the shoots laid bare on the outside of the sample by fragmentation of the layer show the tissue structure of the epidermis, as seen in Fig.2 taken at another site on this sample. 

Fig.1: Sample surface with several upright Rhynia shoots and impressions. Image width 16mm.

Fig.2 (far right): Rhynia shoot, 1.5mm wide, with epidermis pattern and one of the typical warts (below the crack).

Fig.3 (below): Two Rhynia shoots on a cut face, surface deformed by stiff layer kinked under load from above: outward kink (below left), inward kink (above right); former voids with clear calcite, quartz, and bluish chalcedony. Image width 14mm.   

Rhynia with kinkskink indenting Rhynia

Fig.4 (left): Inward kink, detail of Fig.3.

The deformations on the surface of the shoots (Figs.3,4,5) indicate the transient presence of a stiff tube around the shoot, surrounded by soft matter, which is a hitherto unnoticed stage of the silicification process. 
There is abundant evidence in the Rhynie chert that most often the surrounding water had turned into silica gel before the plants rotted and shrunk away from the gel. Now it appears that, at least in this case, silica gel formed first as a layer on the surface of the plants. This layer became rather stiff while the surrounding substance remained still soft or even fluid. With this assumption, the kinks can be understood as the result of mechanical instability of the stiff tube under the load of sediment accumulated above in the meantime. This would be compatible with the established notion of changing currents loaded with mud during the formation of the Rhynie chert.
Resistance against lengthwise compression can also be expected from the xylem strand, which is known to retain its stiffness while the surrounding tisse decays so that eventually it reacts with kink formation (Fig.6).



Rhynia with deformationsRhynia xylem kinkviolet calciteviolet calcite
Fig.5 (far left): Rhynia, 2mm, indented by fragments
        of crushed stiff layer while still soft ?


Fig.6 (left): Rhynia, 2mm, with kinked xylem strand.



      Fig.7 (right): Ellipsoidal fungus chlamydospores
             inside Rhynia; violet crystal. Image width 3mm.

                                                Fig.8 (far right): Violet calcite (?), detail of Fig.7.



As another peculiarity, fungus chlamydospores or resting spores with ellipsoidal shape are seen in degraded Rhynia tissue in Fig.7. This is worth mentioning since the chlamydospores preserved in the Rhynie chert, which are often abundant and may be of largely differing sizes depending on the fungus species, are nearly always spherical.
Numerous former cavities in the chert, doubtless filled with water at an early stage of chert formation, are now seen on the cut faces as crystalline inclusions consisting of a random assemblage of quartz and calcite (Fig.3). At the sample surface, no calcite is left, obviously dissolved since this layer fragment had been exposed to weathering, possibly since glacial times. The cavities seen in Fig.1 (below and at the edge on the right) contain only quartz crystals but no calcite.
Most unusual are the violet spots (Fig.8) extending into the depth of the crystalline inclusions. The colour strongly resembles that of amethyst but the aspect of the crystalline substance suggests that it may be a rare violet variety of calcite.
Again it has appeared that various information may be derived from only one sample, in this case covering biology (plant growth and anatomic details, fungus organs), mechanics (kink formation), chemistry (differential silicification rates), mineralogy (clear and coloured crystals grown in water).

H.-J. Weiss    2017

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