boundaries in Rhynie chert
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
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
aspect of a coating or lining, as in Fig.1 and Rhynie
Chert News 64
. 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.
(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.
Fig.4 (right): Silica gel, sub-surface layer and
separate sphere (?) stained brown,
detail of Fig.1 below right.
of the image 0.45mm.
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.
Fig.5 (below left): Silica gel,
beset with microbial dots, broken.
Width of the image 0.9mm.
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
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:
pictures taken by Gerd Schmahl,
Rh2/90 (1.66kg), obtained
in 2003, Part 1: Fig.3.