What one piece of red chert can reveal
deutsche Version
Red cherts found at a few small areas in the Doehlen Basin near Dresden, Saxony, had formed by silicification of silica-rich water and swamp matter, including plant fragments. (Hence the statemet in [1] that cherts had formed only after deposition of the surrounding rocks is false.)
Scolecopteris pinnules
Fig.1: Tree fern pinnule cross-sections in stained chert. Image width 5mm.

Fig.2: Various mineralogical and biological phenomena
        preserved in an old fragment of a red chert layer thicker than 23cm,
        with microbial formations stained yellow and red.
       Same sample, same scale as Fig.1. Image width 11mm.

The pinnule cross-sections of the tree fern Scolecopteris in Fig.1 imply that the chert had formed in a Permian swamp with tree ferns fallen in.

Permian chert, details resulting from various processesWhen some of the Permian strata were eroded by the advancing Cretaceous sea, the hard chert layers broke into boulders and pebbles rolling about on the beach and getting buried in the sand which turned into Cretaceous sandstone. When
the sandstone eroded in the Tertiary the chert came out again. Judging from loose samples, the thickness of the chert layers was up to about 30cm.
Since there were no deep valleys in the Doehlen Basin at that time, the chert pieces were scattered over the area. Most of them were washed to the sea when the valleys formed in the Quaternary. A few can still be found in the remaining gravel deposits of Tertiary or Quaternary streams. Their changeful history through the ages is proven by small remains of sandstone adhering to them even now.

Still more interesting are the processes which created colourful aspects as seen in Fig.2. The large yellow and red parts on either side of the former water-filled cavity in the middle seem to be freshwater stromatolites, layered deposits of silica gel with cyanobacteria (?) on every layer, whose oxygen production led to the deposition of yellow and red iron oxides so that the thin layers became stained, less distinctly here than in Permian Chert News 43 .
Microbes floating in the water-filled cavity precipitated yellow iron oxide as tiny grains slowly sinking down. They did not form a deposit but a yellow fluid suspension heavier than water, forming a horizontal boundary against the water above and turning into yellow silica gel. Subsequently the cavity walls became coated with silica gel whose bluish hue is not brought about by pigments like the prevailing red and yellow stains but by light diffraction on µm-size clusters grown in the colourless silica gel. Much later, quartz crystals grew very slowly in the water, depending on the supply of silica by diffusion from outside.
Several details in Fig.2, although interesting, must remain unexplained here. Very thin sheets in clear chalcedony like those in Fig.2 below right, deformed into fancy shapes with shadows enhancing the 3D-aspect, and other enigmatic structures awaiting explanation are not rare in these red cherts.

H.-J. Weiss   2023

[1] R. Rößler, T. Zierold, F. Spindler, F. Rudolph : Strandsteine ... ;    Veröff. Mus. Naturkunde Chemnitz 30(2007), 5-24.
[2] P. Freytet, E. Verrecchia: Freshwater organisms that build stromatolites: A synopsis ...  Sedimentology (1998), 45, 535-563.
Scolecopteris pinnule cross-section, Sardinia Permian Chert News 44
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