Formation mechanism of micrometric helices in agate and chalcedony

Determine the abiotic physicochemical processes responsible for the formation of micrometric three-dimensional silica helices observed in agate and chalcedony, and distinguish these structures from biogenic spirals and silica–carbonate biomorph analogues.

Background

Spiral and helical forms are common biosignatures (e.g., ammonites, gastropods, microbial stalks), but similar helices have been reported in agate and chalcedony that appear abiotic.

The review notes their composition (silica plus traces of iron) and morphological resemblance to biomorph nanorod assemblies, but highlights that the physical-chemical pathway generating these helices remains unknown.