Silicification steps in chert deduced
from fossil fungus hyphae
Chalzedony differing locally in hue and brightness
allows a complex sequence of silicification stages to be reconstructed
from Fig.1. This is made easier and more convincing by the presence of
fungus hyphae which obviously became covered with a whitish coating
contrasts against a dark brown matrix. Hyphae
found outside plant
matter must have grown and become silicified
in water. As another conspicuous feature, the
stack of levels resembling agates with horizontal banding indicates
discontinuous silicification, which will be explained below. The
sequence of events which materialized on the small area on
the cut face
of the Rhynie chert sample shown here begins with
a former swamp gas bubble trapped among plant parts.
Complex-structured fill within a former
swamp gas bubble in Rhynie chert.
(Lengthwise section of
the early land plant Aglaophyton
above left.) Note the level deposits,
slightly tilted towards each other.
Width of the image 10mm.
Fig.2: Detail of Fig.1. Note the hyphae of some aquatic
fungus coated with
whitish chalzedony whose thickness varies with height above bottom.
Width of the image 4mm.
Fig.3 (on the right): Detail of
Figs.1,2. Note the two small cross-sections
of whitish coated fungus
hyphae, diameter about 40Ám, in the lower dark stratum, one of them at
the bottom. Width of the
The apparent succession of events which brought about what
is seen in Fig.1 and details thereof is listed here:
(1) A patch of Lower Devonian vegetation became
upset and inundated in silica-rich water.
(2) A swamp gas bubble got trapped among plant parts.
(3) The silica solution became supersaturated and
turned into silica
gel rapidly ere the plants could
(4) With silica gel formation, the gas bubble became stabilized.
(5) After this early silica deposition had essentially ended, the gas
slowly escaped by diffusion
through the surrounding gel while water
entered in the same way.
(6) Other than the surrounding gel, the now
water-filled cavity provided a habitat for some aquatic
fungus to thrive. A
loose tangle of fungus hyphae spread throughout the water.
(7) Much later than the light-weight and hence quickly moving water
molecules, the heavier silica molecules or
complexes entered by diffusion and slowly increased the
silica concentration there.
(8) Apparently the
solution became supersaturated again, only slightly since the
water did not turn into gel as it did before, see (3).
(9) The silica complexes or clusters were still
so small that they got pushed about by thermal motion such that their
concentration remained homogeneous throughout
the water. Silica deposition began with the formation of a thin white
coating on available substrates, that is the cavity walls and
fungus hyphae. The result of this first silicification step inside the
water-filled cavity is seen as a thin white layer and as coatings of
about 20Ám in the lowermost part. (The hypha is so thin that the
of the coated hypha equals twice the coating thickness.) Two
coated hyphae are seen in Fig.3 as small white spots.
Apparently, as the silica clusters became larger, they settled to form
a comparatively heavy colloid solution, or colloid for short,
with a well-defined boundary against the lighter solution above. This
must have been a fast process since the
diameter of the upper coated hypha is not
larger than that of the one at the bottom of the fill: If the colloid
level had risen more slowly, the upper hypha would
have acquired a thicker coating while it was not yet immersed in the
colloid. The dark stain of the colloid could possibly be due to the
decay products of microbes living in the water.
(11) The production of
dark colloid stopped. The colloid solidified and became the lowest
stratum of dark silica gel, now chalcedony in Fig.1.
(12) Again with slight supersaturation, the process described in (9)
repeated itself in the cavity of reduced size. The white lining and
coatings are obviously much thicker this time.
This means that their deposition went on for some time without being
interfered by new colloid.
(13) Eventually, colloid
formation as described in (10) repeated itself at a slower pace. The
rising colloid level stopped the growth of the white lining and
hypha coatings as soon as it reached them.
(14) The colloid level rose so slowly this time that there was
more time available for the growth of coatings
on the hyphae higher up. This is evident in Fig.2 where the diameter of
coated hyphae is 130Ám below and 200Ám at the upper colloid level of
the second dark stratum, which is bounded by the next white line whose
origin is explained below.
(15) Meanwhile the solidifying swamp had got a slight tilt so that the
new colloid surface level, which has been choosen as the horizontal
direction in these pictures, is slightly tilted against the old
(16) Again, the production
of dark colloid stopped. Same as before with the lowermost stratum,
the colloid solidified and became the second stratum of dark silica
(17) In the silica solution above, a third lining
of white silica gel was deposited around the new cavity walls, and
since three coated hyphae, seen in Fig.2 as cross-sections, partially
stuck out of the wall, they, too, became covered by the third white
lining. This is a peculiar type of coating: lower half
thin, upper half thick.
(18) A third cycle of colloid formation ends the deposition of white
(19) Same as described in (16), a third dark stratum was
All the time while the lower strata were formed and solidified, the
hyphae higher up in the water-filled cavity surrounded themselves with
ever thicker coatings which fused into a coherent mass of white gel
with a few pockets of water in between.
(21) Eventually, colloid
formation and solidification ended further deposition of white gel
there, too. By that time, the thickest coating had reached a diameter
of 270Ám, hence a diameter of the coated hypha of 540Ám, as seen in Fig.1 on top.
The succession of processes which led to the agate-like fill and coated
fungus hyphae in this former
cavity may be confusing so that a summary is appropriate: Watery
colloids of silica particles heavier than water form a horizontal
interface against the water above. Any horizontal
plane faces in cherts, often thought to be former
water levels, are former colloid levels.
They could not have been former water levels since mm-size
water surfaces against air are
formed colloids may vary in colour but in the present case it is
always the same dark brown, the cause of which can be guessed and may be foumd out with higher magnification.
A tentative interpretation of this cavity fill as
alternating dark and white colloids with the same particle size is excluded by the fact
that the white
deposit is not only there as plane layers but as hypha coatings, too.
The hyphae could have got their whitish coating only when they were not
immersed in the brown colloid. Since plane white layers could
have been deposited only with the colloid below being solidified, and
no new colloid present, the production of dark colloid
must have stopped for a while, then started anew later.
Similar discontinuities in
silicification are well known from banded agates, where the
"bands" seem to have been deposited separately. No
explanation is proposed here for the underlying cause.
From the sorting of the coated hyphae according to diameter,
with thicker coatings positioned higher up than thinner ones, it can be
concluded that hypha growth was restricted to an early time span when
the silica concentration in the water-filled cavity was so low
that no coating was formed. Otherwise, if hyphae had grown in
later stages, too, no ordering with respect to coating
diameter would be observed.
it can be stated that the Rhynie chert sample shown here, with whitish
coatings and dark horizontal strata, offers evidence of different
ways of silicification, with deposition via colloid being highly discontinuous.
2016: In this text the term "emulsion" had been used for the
heavy suspension of silica clusters which separates itself from the
water above. As this term is restricted to the narrower meaning of
suspended droplets, it has been replaced by "colloid" throughout.