Various diffusion effects in fossiliferous Permian chert
deutsche Version

The omnipresent phenomenon of diffusion, whose essential part in fossilisation is likely to be ignored by those who speak of circulating solutions, has been vividly explained in a separate text. This contribution is to draw the attention to a few of the various manifestations of diffusion in chert.
Cracks can influence diffusion in contrary ways: They provide easy diffusion paths along but obstruct diffusion across. Fig.1 shows a conspicuous case of a crack with an opening so narrow that it is seen on the image as a thin line only but nevertheless with a wide area of influence.  
crack as an easy diffusion path
Fig.1: Crack seen as a thin line with a wide halo resulting from diffusion, incidentally crossed by two unrelated cracks without halo. Width of the picture 4mm.

Apparently some agent coming along the crack, probably by surface diffusion along the crack faces, caused the small amout of hematite, which lends the red stain to the chalzedony, to turn into the yellow goethite. Also it seems to have reduced part of the goethite to some colourless soluble iron compound which probably escaped by diffusion.
The configuration of the three nearly straight cracks in Fig.1 is remarkable from a mechanical point of view: While propagating, they did not feel each other's presence, hence they must have propagated at much different times. The crack running from above left to below right is the oldest one. Its 20µm opening is completely filled with silica so that it does not act as a mechanical discontinuity. It must have been so when the horizontal crack came from the left and crossed it without the slightest deflection. 
The horizontal crack, too, must have been healed up when the third crack crossed it without getting deflected. (The third crack is still open, by which it has been recognized here as the third and last one.)  Obviously, the diffusion must have been going on while the second crack was open, which is much later than the instant of the first cracking and a long time before the instant of the third cracking. A very slight deformation of the yellow and discoloured parts of the halo indicate that the first crack slightly influenced the diffusion process although it did not influence the path of the second crack.
cracks as diffusion barriers
Fig.2: Cracks serving as diffusion barriers in chert with granular precipitates

From the even distribution of the precipitates of unknown nature on a small part of the sample (Fig.2) it can be concluded that 
precipitation came first. Next came the slanting crack, then healed up before the perpendicular crack ran across bcause the latter did not feel the former's presence.
It is not easy to reconstruct the sequence of staining and /or bleaching of the precipitates. At least part of it must have been going on when the second crack was there and already filled with crystalline quartz as it is now. This follows from the observation that the second crack did not serve as an easy diffusion path. Hence one can assume, as the least complicated version, that the diffusion barriers responsible for the different
colours did not consist in the cracks themselves but in some barrier function of the solid fill of the healed-up cracks.
Apparently one cannot find out here whether it was staining
or bleaching which came with diffusion of some agent through the chalzedony and was halted at the filled-up cracks. Again it appears that there can be particular faces of discontinuity in the chalzedony, as the faces of filled-up cracks seen as lines on the cross-sections in Figs.1,2, which do not make themselves felt mechanically but act as diffusion barriers only. The details seem to be hidden in the molecular structure.
Other small areas of the same sample section are suitable for contemplating diffusion effects without interference by cracks. Fig.3 shows the same type of precipitates as in Fig.2. The higher magnification apparently does not convincingly suggest an explanation.
enigmatc preciipitates in Permian chert
Fig.3: Enigmatic lumps of about 40µm in a milky patch of chalzedony. Width of the picture 1.2mm.

Judging from the evidence of bleaching in Fig.1 and possibly in Fig.2, the pale lumps in Fig.3 could well have been bleached from red to transparent. A conspicuous white spot in the middle of some bleached lumps might give a clue. Unfortunately, more than one possible ways could have led to what is seen here:
(1) A dissolving mineral grain in silica gel could have caused the formation of the little lump which became stained with hematite later.
(2) For reasons unknown, cyanobacteria could have grown preferably around the white spot, thereby producing organic gel which made the lump and oxygen which caused the
precipitation of hematite on the lump.

The latter interpretation is favoured by the dark coating seen on some lumps, especially on the red ones. It is possibly the latest layer of 
cyanobacteria, which overgrew the precipitate to get into the light, as it is known from cyanobacteria in the Banded Iron Ore [1].

The pictures have been taken from one chert sample found at the well-known site of fossiliferous pebbles and boulders at Kleinnaundorf (Kohlenstr.), Freital, Döhlen Basin, Lower Permian. This sample contains squeezed frond stalks and pinnules with forked venation and 4-fold synangia of the tree fern Scolecopteris, also moult parts of the crustacean Uronectes and fungus hyphae. Finally it is mentioned here that Scolecopteris, the "maggot fern", is found abundantly in these cherts, debris from Uronectes moults is less common, and fungus parts are rare.
Sample: found by
H. Ahlheim, polished and provided by H. Albrecht, Dresden, kept in the own collection under the label Bu7/207 .

H.-J. Weiss     2015

[1]  T.N. Taylor et al.: Paleobotany. Elsevier 2009
Scolecopteris pinnule cross-section, Sardinia Permian Chert News13

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