Two small agates from Dunure, Ayrshire, Scotland (Dun017 and Dun014) provide strong evidence for a pause, or hiatus, between the formation of the first chalcedony layer and the formation of the main agate.

Dunure 017

Dunure, Ayrshire, Scotland. 33mm. Dun017

This agate has a very clear and sharp boundary between the first chalcedony layer, coloured yellow by goethite particles, and the remainder of the agate, coloured red by haematite particles. The boundary is also marked by the development of hemiagates, a feature often seen at the base of the main agate. They probably represent agate formation when relatively little silica was available, with what silica that was available restricted to small areas. The change from all goethite to all haematite probably reflects a significant change in the geochemistry of the fluids present, and the sharpness of the boundary suggests that there was a pause, or hiatus, after the first chalcedony layer was deposited. During this pause, the geochemistry changed and when agate formation recommenced, conditions were significantly different. The first chalcedony layer displays ‘banding’ within the goethite, indicating somewhat complex deposition.

Another feature of this agate is the banding in the main agate after the initial hemiagates. It demonstrates a progressive increase in band width that is well known in some agates. I will comment on this in later posts. Note also that the haematite particles in the main agate seem ‘randomly’ distributed. They are not associated with individual bands and are mostly circular (spherical in 3D), with no flattening that might suggest growth limitation within bands. The most obvious interpretation of this is that the haematite precipitated when the agate bands were in place but were still in a gel with the bands hving similar properties, allowing the haematite to precipitate more or less anywhere.

The haematite particles in the main are not truly random. If they were, a different pattern would be seen, with some particles close to each other and variably sized gaps. The actual distribution is more typical of particles that developed by precipitation of the iron available in the immediately adjacent area, using it up and preventing other particles from developing nearby.

Detail of agate Dun017 showing a yellow, goethite rich first chalcedony layer and a red, haematite rich, main agate. The first chalcedony layer is subdivided into bands with different goethite concentrations and the boundary with the main agate is very sharp and marked by the development of hemiagates. Haematite distribution in the main agate is not controlled by banding and the bands show a progressive increase in width towards the agate centre.

Dunure 014

Dunure, Ayrshire, Scotland. 32mm. Dun014

Agate Dun014 shows the same sharp separation between a yellow, goethite rich, first chalcedony layer and a red, haematite rich main agate. However, the right hand side of the agate has broad areas of red, haematite rich agaten within the first chalcedony layer. Close examination reveals not only that these are associated with fracture zones, but that the fractures are limited to the first chalkcedony layer and do not continue into the main agate.

What does this signify? Well it strongly suggests that the first chalcedony layer was formed, crystallised and thrn fractured before deposition of the main agate. A possible alternative explanation might be that the fracturing occured after the main agate was formed, but before it crystallised and that it was unaffected because it was in a gel form. I favour the first interpretation because the hemi agates at the start of the main agate are completely unaffected by the fractures, being complete and undistorted.

Detail of agate Dun014 showing fractures within the first chalcedony layer coated with net-like haematite. The fractures are restricted to the first chalcedony layer. They do not penetrate the main agate and the hemiagates at the start of the main agate are entire and undeformed. The conclusion is that the first chalcedony layer was deposited, crystallised and fractured before the main agate formation took place.

Conclusion

These two agates demonstrate features that strongly support the idea that agates often develop by the initial deposition of a first chalcedony layer that crystallises and hardens before the main agate is formed. This concept was initally based on the frequent observation of a dilation in escape structures as they pass through gel like layers but meet resistance from, and sometimes fracture, the first chalcedony layer. Dun014 in partiular demonstrates that the first chalcedony layer had crystallised, and could be fractured, before the main agate was formed.