Silica gel boundaries in Rhynie chert
deutsche Version
coating on gel
Fossiliferous cherts can reveal details of their formation from silica-rich water with decaying plants. The acidic decay products favour the formation of silica gel first inside the plants, then outside. The last deposited gel may appear as a conspicuous surface layer, as the black boundary on the bluish chalcedony in Fig.1.
This boundary with the aspect of a coating had been yellow or pale originally but mostly become dark later (see Rhynie Chert News 64 ). Tiny dark dots, possibly microbes, are often seen near the boundary, as in Figs.2-5.
The dark boundary in Fig.1 marks the end of a first stage of silicification with bulging growth of silica gel protruding into the surrounding water. In a later stage, yellow quartz grains grew in the water.

Fig.1: Once silica gel and water-filled cavity, now chalcedony and yellow quartz grains.
Image width 11mm. (Same sample at Rhynie Chert News 64 ).

dots on silica gelsilica gel

Fig.2 (near left): Small cavity in silica gel, lining along the walls with transition from pale yellow to dark. Image
width 0.45mm.

Fig.3 (far left): Formerly water-filled space with silica gel protruding, with transitions from pale to dark, with tiny dots near the surface. Image width 1.4mm.
silica gel

Fig.4 (right): Silica gel boundary (in Fig.1 below right). Image width 0.5mm.
Apparently the bulging gel parts in Fig.3 had formed separately.
The smaller bulge in the middle with a reflected light shining dimly through from behind seems to indicate that the gel lump is not filled with dark dots.
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. 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.
              Rh2/88 (3.3kg), obtained from Shanks in 2003, Part 1: Fig.3.

H.-J. Weiss   2020     modified 2021
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