Physico-Chemical Mechanisms of Formation of Concentrically Banded Agates in Basalts: Hypotheses and Experiments

Based on literature data, the main properties of agates and the conditions of their formation are summarized, and a critical analysis of the hypotheses of agate formation is performed. The hypothesis of layerwise precipitation and crystallization of silica extracted from the host rock (in particular, basalt) turned out to be the most adequate. However, difficulties remained in explaining the SiO₂ movement from the host basalt to the agate cavities, the causes of SiO₂ precipitation, the role of phase transformations, and the mechanism of banding formation. To clarify these questions, experiments were performed on the dissolution of basalt samples in water for 4 months at 300°C. The formation of silica was noticeable only in experiments where the basalt sample was half-submerged in water. In particular, amorphous silica (opal-A) was precipitated in the pores and on the sample surface above the water level, which formed agate-like layered textures and was replaced in places by chalcedony. The experimental results are explained within the framework of the distillation hypothesis, which may be suitable for the formation of agates. The hypothesis combines the possibilities of SiO₂ transfer in low-concentration regions and its precipitation in high-concentration regions, and also explains the agate banding by fluctuations in supply and consumption of dissolved silica in precipitation and phase transformation reactions.