The Problem of Agates

Among the topics discussed in geology, that of agate geodes is one where numerous explanations have been offered, without any official concensus being possible.  The rounded agate bodies, which occur above all in the upper zones of older volcanic extrusive complexes,  are generally thought to be quartz fillings of former gas bubbles of the extrusive rock, which have gone through complex formation- and transformation-processes.  Some earlier theories took their point of departure from rhythmically-hardening quartz droplets within the hot rock, others spoke of layering of gaseous material in hollows created by bubbles.  Hydrothermal explanations were considered (mineral solutions in hot water), and some researchers concluded that a colloidal condition of quartz must have played a role.

Left-hand and right-hand photos:  Following the hardening of various-colored silica layers, there often follows in the center a colorless, coarse-grained crystallization of the remaining silica.  In the right-hand picture, spherical formations (spherulites) are visible.

kopie von 09 das achatproblem 01 kopie_von_09_das_achatproblem_02.jpg

Two agates from the well-known location, Idar Oberstein, Germany
kopie_von_09_das_achatproblem_03.jpg kopie_von_09_das_achatproblem_04.jpg

Strongly spherulitic formation                                           Fragments of separated membranes
kopie_von_09_das_achatproblem_05.jpg kopie_von_09_das_achatproblem_06.jpg

Left: After a first phase of crystallization (white band, outside) there followed a period of banding-formation, again with coarse crystallization at the end (center of geode).
Right: cell-like formation, each cell containing its own spherulitic banding.
kopie_von_09_das_achatproblem_07.jpg kopie_von_09_das_achatproblem_08.jpg

 Moss agate formation in the lower part of the geode, detail on right
kopie_von_09_das_achatproblem_09.jpg kopie_von_09_das_achatproblem_10.jpg

Left: Inlet canals are not necessary for agate formation, but here the material has pressed through the banding formations due to the finest fluid movements.  In the geode on the right, disturbances are visible which are probably also caused by flow-movement.
kopie_von_09_das_achatproblem_11.jpg kopie_von_09_das_achatproblem_12.jpg

Agate-almonds:  Transformation of gas bubbles through the flowing of volcanic-basaltic material.
kopie_von_09_das_achatproblem_13.jpg kopie_von_09_das_achatproblem_14.jpg

Left: Almond form through flow-deformation of basalt lava, Steinkaulenberg, Idar Oberstein, Germany 
Right: Polygon form, inside a hollow created by tablet-like crystals
kopie_von_09_das_achatproblem_15.jpg kopie_von_09_das_achatproblem_16.jpg

Due to decades of research by specialists such as Michael Landmesser, the research into agates has reached an advanced level, in that earlier theories have been corrected and extended through the inclusion of colloidal processes.  According to this research, the agates formed at temperatures from 150°C to 200°, possibly 300°→, from watery-silicatic gels which formed in gas bubbles or other hollow spaces through diffused ("atomically-wandered") silica, and formed layers and hardened over time.   The delicate dissusion process would have been able to proceed through the outer shell of the geode.  The occasionally-observable "inlet-channels", according to this, are not actually flow channels, but rather zones of disturbance where stronger movement of the fluids hindered the formation of structure.  Quartz, when not crystalline, hardened in the form of amorphous (or sometimes microcrystalline) chalcedony.

The idea of a colloidal condition for agate formation is supported especially in reference to the occasionally-occurring horizontal layering, the so-called Uruguay-layers: Left: Aftr a period of spherical layering, there occurred a re-fluidification of the silica gel, with layered deposition in larger colloidal particles through gravity.  In the geodes in the right-hand picture, a shorter spherical layering was followed by a longer layered sedimentary deposition, and at the end again a spherical (from all sides) deposition, followed by crystallization of the remaining material.

kopie_von_09_das_achatproblem_17.jpg kopie_von_09_das_achatproblem_18.jpg


The agate clusterings pictured below show further features which are explainable by colloidal and self-organizing processes.  The outer zone of the forms consists of a greenish quartz material (rhyolite or rhyolitic volcanic tuff), which appears to consist of fibers.  In the central zone, this "fibrous substance" is torn apart and partly dissolved; the resulting space is filled with chalcedony, whereby one can suspect that this chalcedony consists in part of the dissolved "fiber substance".


Rhyolitic clusters: Lierbachtal, Black Forest, Germany
kopie_von_09_das_achatproblem_19.jpg kopie_von_09_das_achatproblem_20.jpg

Gourd- or cauliflower-like outer forms of rhyolite clusters
kopie_von_09_das_achatproblem_21.jpg kopie_von_09_das_achatproblem_22.jpg


With these rhyolite clusters, one can ask whether it is actually a matter of gas bubbles being filled later, or whether these were silica clusters which formed earlier by collecting-together of colloidal material within the rhyolitic groundmass.

The agates display a distinctive form of self-organization, which is responsible to a great degree for the differentiation and beauty they display.   Perhaps this self-organizing power is responsible for drawing the color-creating material from the surrounding silicatic mass through diffusion and binding it.  It can be assumed that the silica-gel material could become thinner through heating or movement, without fundamentally losing its self-organizing ability.  Ultimately the colloidal geode material would harden into the present-day agate or chalcedony, whereby the final portion of the silica frequently crystallizes out as transparent quartz.  Such crystallizations have, in some cases, actually alternated with the phases of agate-formation (compare pictures 1 and 7).

The following pictures show further inner spaces, which are broken or fractured through "de-watering", and subsequently filled with chalcedony or crystal structures.  The final picture makes visible, how chalcedony (here jasper) can dry out and split, similar to clay or dough.  It is clear that prior to such shrinkage cracks, thre must have been a higher content of water or silicatic fluid.  One should note that, despite the earlier soft stage, the rocks have become quite hard and can no longer be brought back to the colloidal condition (irreversability).

Left: Rhyolite nodule with Agate      Right: Shrinkage cracks found also in a Septarian Nodule (not related to agate), the inner zone filled with calcite.

kopie_von_09_das_achatproblem_25.jpg kopie_von_09_das_achatproblem_26.jpg

Left: Rhyolite nodule, Chalzedony fragments, white Uruguay layers
Right: Chalzedony- or Jasper nodule mit shrinkage cracks
kopie_von_09_das_achatproblem_27.jpg kopie_von_09_das_achatproblem_28.jpg


Agate fillings have also been found in fossil root-cells, lime snail-fossils and a dinosaur egg.  The colloidal silica filling is able to effect its self-organizing ability in the most varied hollow spaces.

What the agates make clear, as geological phenomena, is the presence of colloidal processes within rock complexes which have a connection with magmatic-volcanic processes with warm or hot water.  If one extends this connection hypothetically to the entire earth, and assumes that there had been a heavy, water-rich atmosphere over the warm magmatic early earth, then the following question could arise:  Could silica processes have taken place in the lower layers of this almost fluid atmosphere, which led to the precipitation and deposition of entire silicatic rock complexes, similarly to the deposition of the colloidal material in the Uruguay-layers of agates?  Only in distinction to the agates, the silica would have come, not from volcanic processes, but rather directly from the heavy atmosphere of the early earth which condensed around the arising planet out of the original "sun-cloud".