"Snowing" siliceous flakes preserved in Rhynie chert
deutsche Version
Silica precipitation in cavityThis image provides the illusion of an intermediate stage of chert formation, with silica flakes seen as tiny white dots "snowing" down and accumulating at the bottom and on ledges of sloping walls of a cavity in silica gel, filled with water then but now with solid fill except for small remaining quartz-lined holes on the right.

Fig.1: Former water-filled cavity between silica gel, visualizing the precipitation of siliceous flakes. Width of Figs.1-3: 11mm.

The sequence of events leading to the aspect in Figs.1,3 can be reconstructed: It started with plants upset, submerged, and decaying. Apparently the decay products had caused the silica-rich water to form silica gel around the decaying plants, with water-filled pockets left in between. Apparently the growth of gel ended suddenly when the dissolved silica had been used up. The gel surface stayed put, which enabled microbes to settle there, leaving black coatings. (See also
Rhynie Chert News 83, )

Remarkably, additional silica entering into the water-filled cavity did not get stuck to the silica gel walls but formed white flakes or grains which sank to the bottom or settled on the ledges of the cavity walls, seen as white deposits on the left. Apparently the yellow stain is a later addition independent of the precipitation.
Eventually the water in the cavity became more viscous so that the grains did not sink any more but are seen now as tiny white dots seemingly suspended in space.

Fig.2: Former water-filled crack in silica gel with siliceous precipitate settled mainly on the lower crack face; deformed Aglaophyton section above left.
yellow deposits in cavities
The precipitation of siliceous flakes or grains was not restricted to the water-filled cavities left during early gel formation but also was going on inside wide cracks. The variable crack opening in Fig.2 indicates that the whole was still soft. Part of the crack faces had got a thin black microbial coating, less conspicuous than in Fig.1. 
As a peculiarity in Fig.2, there are two small parallel black stripes without yellow "snow". They must have formed earlier, similar as described in
Rhynie Chert News 75. They indicate the horizontal direction during silicification, thus being compatible with the perpendicular direction of "snowfall" in Figs.1-3.

caves in silica gel
Fig.3 (right): Poorly preserved Aglaophyton covered with pale silica gel; former water-filled cavities with quartz grains or empty now.

It is readily concluded from Fig.3 that the sequence of events started with plants upset, submerged, and decaying, causing the siliceous water to make pale silica gel around them. The well-defined gel surface got a thin coating of microbes, now seen as a thin black line on top of the gel. Apparently the concentration of dissolved silica had become so low then that no additional gel was formed but very slowly growing crystalline quartz instead, also in closed cavities, where SiO2 came in via diffusion. Likewise the water disappeared.

Sample: Rh2/10 (5.48kg) obtained from M. Shanks in 2000, here Part3 (3.31kg).

H.-J. Weiss      2022
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