Phenomenological approach to the problem of chert formation
deutsche Version
Vague ideas concerning chert formation are still widespread, also in several textbooks on geology and petrology. According to [1], cherts are formed by subsequent silicification of pre-existing rocks. This is certainly not true for most fossiliferous cherts, including the Lower Devonian cherts from Rhynie, Scotland, and the Permian cherts from Saxony, Germany.
Doubtless there is valuable work on various aspects of the problem in the scientific literature but it would be virtually impossible for the fossil collector or palaeontologist to dig himself through, separate the valuable from the questionable, and arrive at a consistent view. The concise compilation of facts combined into a chain of arguments below is meant as a help in this situation.

quartz crystals grown in wood tissuePicture: Quartz crystals growing in wood during an early stage of silicification while everything else is soft. Apparently there is no strength left in the wood as it is easily pushed aside by the growing crystals. Possibly the wood is permeated by silica gel which prevents the decay and collaps of the tissue. This stage has been preserved by the gel turning into chalcedony. Since the formation of silicified wood is essentially similar to the formation of chert, this sample serves as evidence contradicting the widespread view that quartz comes last in the sequence of silicification stages.
Hence it seems always advisable to question established opinions.
Sample: Upper Carboniferous, Borxleben gravel pit, Kyffhäuser mountains, Germany.
Thanks are due to
W.+G. Etzrodt, Borxleben, for providing the sample.

1.  There is ample evidence that some (possibly most or all ?) fossiliferous cherts
     went through a transient state with the mechanical properties of gel:
     very low Young’s modulus and fracture toughness,
        as concluded from the aspect of early short cracks with wide opening and sharp tip,
           later filled with silica and now seen in the solid chert.
2.  It is known that gel is made up of a network with (liquid-filled) meshes
     formed by weak bonds, providing easy diffusion paths.
3.  The transient state mentioned in (1) could be realized as
         - homogeneous silica gel,   
         - gel with disperse inclusions of opal, chalcedony, quartz,
         - aggregate of silica clusters behaving mechanically as a gel.
     It is simply called gel henceforth.
4.  There is evidence that the formation of most fossiliferous cherts via gel
     took place at or near the surface but not in the depth.
5.  There is evidence that coarse quartz crystals can grow within a gel matrix
     which is strong enough to support the weight of the crystal.
6.  The quartz crystal growing within the gel gets its SiO2 not by dissolving the adjacent gel
     since otherwise it would slowly sink as it grows.
7.  The growing quartz crystal can give rise to crack formation in the gel.

8.  From (2), (6) and (7) one may conclude that the growing crystal obtains SiO2
     by diffusion of low-molecular dissolved silica through the gel network.
9.  The source of silica wandering through the gel could be the same as for gel formation.
10. There are ranges of parameter sets (silica concentration, pH, salinity, temperature etc.)
     where silica solutions are supersaturated, tending to gel formation.
11. Supersaturation is usually obtained via decreasing temperature, which takes minutes to hours.
12. Supersaturation can be obtained within seconds
     by mixing of saturated or even undersaturated solutions with suitable parameter combinations.
     Such unexpected effect is due to the non-linear dependence of solubility on parameters.

13. Quick mixing of fluids occurs in surface flows, hence (11, 12) are compatible with (4).
14. Given (8) and (9), it can be assumed that opal and chalcedony, too, can grow within silica gel
     in essentially the same way as quartz crystals do.
15. Finally the growth of precipitates results in a stage of tightly packed (micro-) crystals
     with virtually no gel left so that the solid chert is obtained.

Aglaophyton sperm released from antheridium, after KERPThe scenario outlined above can help to explain observations which seem enigmatic. For example, sperm of the gametophyte of Aglaophyton is seen in the Lower Devonian Rhynie chert at the instant of release from the antheridium. When presented by H. Kerp
at the Rhynie Chert Conference at Aberdeen in 2003 [2], the question of how the sperm wriggling through the water could have been caught and preserved near the outlet remained unanswered. Now it appears that quick supersaturation by mixing could have done the trick. (The picture on the left is also given in [3] Fig.8.29 but with erroneous size data.)
rotifer attack on spherical alga colony, Rhynie
A similar case of quick gel formation, possibly within seconds, seems to have spoilt the attack of a rotifer and preserved the scene for eternity, as seen in this picture of the one and only rotifer known from the Rhynie chert, first described in 2008, Rhynie Chert News 23.
Photograph by H. Eschrich
, Jena.

Some chert samples provide details of the sequence of events during silicification, as explained in
Rhynie Chert News 20, 27, 31, 59, 60, 64, 66,  Permian Chert News 6,  Fossil Wood News  12It appears that silicification often involves processes known from agate formation: deposition of silica linings along the walls of water-filled cavities, precipitation of larger silica clusters which settle into  emulsions and make horizontal boundaries.
silicification process can be highly discontinuous, with repeatedly alternating periods of isotropic and geopetal deposition, as explained in Rhynie Chert News 77.

H.-J. Weiss     2005,  emended 2010, 2015, 2016

[1] R. Rößler et al.: Strandsteine ...
      Veröff. Mus. Naturkunde Chemnitz 30(2007), 5-24.
[2] H. Kerp, N.H. Trewin, H. Hass : New gametophytes from the Early Devonian Rhynie chert,
     Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 411-428.
[3] T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany, Elsevier 2009. (Warning: numerous erroneous size data.)

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