Peculiar cracks in Rhynie chert
deutsche Version

crack with blunt tip
According to fracture mechanics, crack tips in solids are tiny, determined by fracture toughness and elasticity of the material. Hence the sudden end of the wide crack in Fig.1 does not seem to make sense at first sight. A closer look offers an explanation: The tip of the crack propagating through the more or less hardened silica gel must have disappeared at the boundary of the gel against the water in the water-filled cavity when the crack flanks snapped asunder and became displaced so that their spacing is 75Ám now.
(The gap width of this "through-crack" is not a characteristic feature of the material)
Dissolved silica, possibly entering by diffusion along the crack, gave rise to a quartz lining on top of the silica gel and in the crack.
As a conspicuous fact, the gel of the crack flanks has got a distinct dark stain. This stain slightly turns around the corners at the end of the crack. Obviously this stain shows some affinity to the gel surface. The question arises why not all gel surfaces are stained.
Dark stains or deposits in the Rhynie chert are often due to microbes. Black filaments and dots of microbial origin are seen
enclosed in gel in Fig.1 on the right.
Also seen in Fig.1 is a fungus hypha grown in the water-filled cavity before gel deposition, now enclosed in bluish gel below left, diameter 15Ám.

Fig.1: Detail of a cylindrical cavity left in silicified swamp water, now partially filled with bluish chalzedony, broken, with quartz cladding. Image width 1.4mm.

Fig.2 (below): Former cavity of an Aglaophyton "hollow straw" half filled with mineral debris, upper half with chalzedony and quartz cladding similar as in Fig.1. Image width 5.5mm.

crack across Aglaophyton hollow straw
The slightly curved thick layer of bluish silica gel (now chalzedony) on the left of Fig.1 is the lining of a cylindrical cavity, 3.3mm across, remaining in the silica gel of the silicified swamp water (far left, dark) after the complete decay of an Aglaophyton shoot.
A similar cavity is seen in Fig.2, here within a "hollow straw" of Aglaophyton, half filled with mineral debris so that the bluish lining is restricted to the upper half of the former cavity. Again a crack entering from the left ended at the inner surface of the gel, now chalzedony. Its later formed fill of quartz crystals merges with the cladding of the cavity of reduced size. Obviously this half cavity has got a quartz cladding on the chalzedony similar to the one in Fig.1.
What looks at first sight like the seemingly simple phenomenon of "a crack crossing a fossil" is  worth closer consideration. A crack, of course, cannot propagate through a cavity. It has to find its way around the cavity. Usually it goes either way round so that there are two crack fronts which need not meet behind the cavity. Nevertheless, eventually they connect by a step and move on as one again. The kink of the crack on the right in Fig.2 might be due to complications of such kind.

The lining in Fig.2 is not all bluish chalzedony. Part of it, seen here with irregular white reflections, is coarse crystalline quartz. A small part of this quartz lining is positioned in the picture plane on the left, where the presence of coarse grains is obvious from the rugged crack flanks.
The innermost layer of the lining before crack propagation is chalzedony partially turned white by Ám-size quartz crystals. This layer had got a black deposit before the crack had entered into the cavity, as if microbes living in the water had rained down.
Subsequently the crack was formed, then crystalline quartz was deposited along the cavity wall and within the gap. The latest quartz deposit is shaped like a curved plate, hollow below, which seems to preclude a simple explanation. Part of the cavity has remained empty.
The white dots inside (Fig.1) and outside (Fig.2) the hollow plant are spherulites.

Sample from the collection of Steffen Koehler, Meissen, collected decades ago by Brian Beveridge, Gloucester,
on the now protected site near Windyfield formerly owned by A.G. Lyon, Rhynie; documented here under the label Rh2/303, Parts 1,3.

  H.-J. Weiss

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