Physico-chemical mechanisms of formation of concentrically banded agates in basalts: hypotheses and experiments
- Authors: Alekseyev V.A.1, Korost D.V.2, Stepanov N.V.3, Mokhov A.V.1, Gromyak I.N.1
-
Affiliations:
- Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences
- Lomonosov Moscow State University
- Moscow Automobile and Road Building State University
- Issue: Vol 70, No 3 (2025)
- Pages: 247-263
- Section: Articles
- URL: https://journals.rcsi.science/0016-7525/article/view/305583
- DOI: https://doi.org/10.31857/S0016752525030069
- EDN: https://elibrary.ru/fxswww
- ID: 305583
Cite item
Abstract
Based on the 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 layered deposition 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 movement of SiO2 from the host basalt to the agate cavities, the causes of SiO2 deposition, the role of phase transformations and the mechanism of banding formation. To clarify these issues, 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 deposited in the pores and on the surface of the sample 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 concentrations and SiO2 deposition in high concentrations, and also explains the banding of agate by fluctuations in the balance of supply and consumption of dissolved silica in precipitation and phase transformation reactions.
Keywords
About the authors
V. A. Alekseyev
Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences
Email: alekseyev-v@geokhi.ru
Kosygina str., 19, Moscow, 119991 Russia
D. V. Korost
Lomonosov Moscow State University
Email: dkorost@mail.ru
Vorobyovy Gory, 1, Moscow, 119234 Russia
N. V. Stepanov
Moscow Automobile and Road Building State University
Email: alekseyev-v@geokhi.ru
Leningradsky Prospekt, 64, Moscow, 125319 Russia
A. V. Mokhov
Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences
Email: alekseyev-v@geokhi.ru
Kosygina str., 19, Moscow, 119991 Russia
I. N. Gromyak
Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences
Author for correspondence.
Email: alekseyev-v@geokhi.ru
Kosygina str., 19, Moscow, 119991 Russia
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