Fossil fungus-related spheres with fancy fills
  spheres
spheres
Fig.1 (far left): Spherical former water-filled cavities of dubious origin, some pairwise fused, partially still hollow but  mostly filled with chalzedony from siliceous precipitates forming clouds or deposits with level boundaries.
Picture width 3.5mm.

Fig.2: Similar as Fig.1, chalzedony grown as level layers or as clouds, governed by changing silicification regimes.
Picture width 1.3mm.


Fig.3 (below): Similar as Figs.1,2, complex and
distinctly different silicification products inside spheres, with silica grain sizes covering the range of light wavelength (bluish), reflecting grains (white), and coarse quartz crystals (glittering).
Picture width 2.8mm
.
  
spheres                                                                                                                  Fossil spheres in chert, incidentally cut right through the middle, are often the most conspicuous objects on the cut face owing to a thin black circular line. Hence, a thin surface layer of the sphere must have persisted while the innards, like the tissue of nearby plants, had largely decayed before the sphere became filled with various deposits.
So one is tempted to assume that the spheres are fungus organs known as chlamydospores or resting spores from numerous fungus species
, and that after the decay of the fungus substance inside, silicification processes driven under varying conditions by silica gradually enterig by diffusion produced the fills. However, this seemingly straightforward logic is subjected to doubt by the rather large size of the spheres and by observations hinting at a quite different possible explanation as hypertrophied alga cells, as considered before. This makes the fascination of dealing with the phenomenon. Regardless of what might be revealed by further investigations, let us consider the fills here.
Most former cavities in cherts formed by silicification of swamp matter or water had been swamp gas bubbles trapped among plant debris or microbial slime. They differ from the spheres in these figures by their irregular shape. After the gas had escaped and water had come in by diffusion through the silica gel, they were just water-filled cavities ready for subsequent processes of mineralisation and deposition, not much different from the spheres considered here after the decay of their organic content.
In the water-filled cavities of either type, precipitates or suspensions may form, which most often settle at the bottom. Less often they settle at the top of the cavity, which means they are lighter than water, which can be expected from dead microbes or alga unicells, for example. Some of the spheres in these images have got dark deposits at both top and bottom. The boundaries of the deposits formed from suspensions, of course, indicate the horizontal direction during silicification.
It can be deduced from the extremely different aspect of fills in any of these figures that even minor differences in the chemical composition of neighbouring cavities or even of compartments inside the same cavity can strongly influence the result of silicification.
In Fig.2, upper sphere, a fine-grained suspension had settled at the bottom, then silica gel of cloud-like aspect had formed in the water above.
In Fig.3, big sphere, a highly complex silicification regime must have been at work. Certainly the dark deposit of light suspension at the top and the gray horizontal plate must have been there when the bluish fill  formed in between. The structure below the gray plate is rather confusing, apparently a result of dissolution of gel after the plate had formed, then governed by gel formation again. (Alternating formation and partial dissolution of silica gel is not uncommon during chert formation.)
On the right in Fig.3, the result of silicification differs much between the two spheres even though they are connected by a wide neck. Apparently, the formation of gel and chalzedony in the upper compartment had used up nearly all silica so that in the water below, quartz crystals grew slowly at a pace restricted by the influx of silica from outside via slow
diffusion.
Same sample as in Rhynie Chert News 116.

H.-J. Weiss    2017
119

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