Real and illusory cracks in red Permian chert
deutsche Version

These cut faces of chert samples from Doehlen basin, Germany, are meant to draw attention to the unexplained formation of "illusory cracks". spooky shape in deformed layer stack

Fig.1: Fossiliferous Permian chert with a deformed stack of sheets of assumedly microbial origin: silicified, cracked, healed, stained.
Image width 11mm.

The background of the scene in Fig.1 is provided by a large stack of sheets of assumedly microbial origin: grown in water, originally nearly plane as seen below, then largely deformed by some plant part growing inside the stack on the left, then silicified into hard chalcedony, which broke and formed a long nearly straight crack at about 45° irrespective of the stack texture, indicating that the silicified matter had become hard and mechanically isotropic. The gap of the crack had been so narrow that
the sheets had not been displaced by the crack. It is seen as a pale line in the dark patches and as a dark line between.
Another crack in Fig.1 apparently had been able to somehow attract dissolved iron and cause it to precipitate alongside as a streak of bright red hematite, a rare phenomenon in these cherts. Less rare are red streaks as in Figs.2,3, called here "illusory cracks" for short.

Fig.2: Stack of assumedly microbial sheets, silicified into brittle Permian chert, cracked, healed with quartz fill; "illusory crack" made of red stain traversing both sheet stack and real crack. Image width 3.5mm.
real and illusory crack
As seen in Fig.2, the red streak is a late formation since it traverses structures which doubtless existed before: sheet stack and real crack. (The brittle quartz fill of the latter had broken and partially fallen out after cutting.)
The red streak is certainly not a former wide crack filled later since careful inspection shows that it did not interrupt the thin sheets. One might suspect that a real crack, very narrow and unseen here, could be hidden in the red streak. This cannot be excluded with certainty but is much less probable in view of similar red shapes in other chert samples, curved such that an interpretation as unseen cracks inside seems impossible.
The enigmatic red streaks in Fig.3 ("illusory cracks"), mostly sections of odd-shaped 3D- structures, had grown inside the stack of sheets, nearly unaffected by the apparent texture of the stack.

Fig.3 (below): Fancy structure of enigmatic origin, grown inside a silicified stack of sheets in Permian chert. Image width 8mm.

enigmatic red shapesThe "illusory cracks" in Fig.3 seem to have grown in aimless ways through the solid chert but did not leave the stack. Real cracks in Figs.2,3, too, kept within the stack as they grew when the stack was solid but the vicinity was still viscous.
More examples of wondrous red structures, probably formed last in solid siliceous matter, are presented in Permian Chert News 18, 27, 28, 36, 37, 38.
The thin sheets in the stacks have been assumed to be of microbial origin although the very microbes have not been seen in these cherts. Occasi
onally visible tiny granular or cell-like structures could possibly be some kind of pseudocells formed by microbes.

The red formations unrelated to fossil structures must have resulted from diffusional transport of dissolved iron with subsequent oxidation and precipitation as hematite, combined with the yet to be found reason behind the awkward shapes with often distinct boundaries.

Figs.1,2,3: Samples H/333.2, H/375.1, H3/102
found in 2001 at Hänichen, Doehlen basin, Saxony.

H.-J. Weiss    2022
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