Silica gel boundaries in Rhynie chert
deutsche Version
coating on gel
The formation of silica gel in the process of silicification of silica-rich water with decaying plants into fossiliferous chert can be distinctly inhomogeneous. Most often the acidic decay products of the plants favour the deposition of silica gel first inside the plants, then outside. Occasionally this process stops so that a temporary interface between gel and water may establish itself and later become preserved in chert as a boundary with the aspect of a coating or lining, as in Fig.1 and
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. It is not an additional layer deposited on the gel but part of the gel itself, which deserves closer inspection.
Probably the boundary layer had been pale originally but mostly become dark later. As a peculiar fact, tiny dark dots, possibly microbes, are often seen on either side of the boundary, as in Figs.2,3.

Fig.1 (right): Rhynie chert with complex silicification history: dark boundary marking the end of a first stage of silicification with bulging growth of silica gel protruding into the surrounding water, later stage involving the formation of yellow quartz grains in the water.
Width of the image 11mm.

dots on silica gelsilica gel beset with microbial dos

Fig.2 (near left): Small cavity in silica gel with lining along the walls with sudden transition from pale yellow to dark. Width of the image 0.5mm.

Fig.3 (far left): Formerly water-filled space with silica gel protruding, with or without boundary layer beneath the gel surface, with  transitions from pale to dark, with tiny dots on or near the surface. Width of the image 1.4mm.
silica gel

Fig.4 (right): Silica gel, sub-surface layer and separate sphere (?) stained brown, detail of Fig.1 below right.
Width of the image 0.45mm.

Obviously the bulging gel parts in Fig.3 had not formed simultaneously. What looks like a black coating in Fig.1 appears like a not so well defined sub-surface layer at the left boundary of the big bulge in Fig.3.

The smaller bulge in the middle of Fig.3 with its light-brown appearance and a reflected light shining through from behind seems to indicate that the dark dots are so sparsely distributed that they cannot be the immediate cause of the brown stain. 
silica gel broken
Fig.5 (below left): Silica gel, beset with microbial dots, broken. Width of the image 0.9mm.

The crack in Fig.5 shows that silica gel is really a gel in the sense of solid state mechanics but not merely a sticky fluid. Since the crack is restricted to the gel, the vicinity must have been still watery when the gel broke. There are no microbial dots in the gap, hence their proliferation had ended before the gel broke.
A few more observations on gel boundaries will be presented in a forthcoming contribution. 

Samples: Rh7/10 (0.23kg), found by S. Weiss in 2003, Part 1: Fig.5, Part 2: Figs.1,2,4;
                    pictures taken by Gerd Schmahl, Dresden: Figs.2,4,5.
              Rh2/90 (1.66kg), obtained from Shanks in 2003, Part 1: Fig.3.

H.-J. Weiss   2020
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