Agates are unable to be reproduced in a laboratory environment, and hence the mechanism of their formation remains unknown. Present hypotheses regarding genesis are thus, inevitably, speculative. We describe novel experiments, at room temperature and pressure, producing banded amorphous silica gel (via a sol–gel transition) whose morphology is compared to that of agates. In addition, macro- and microscopic examination of agates from several sites revealed some distinctive physical features: surface pitting, indentations of the agate bands, the variation in morphologies of the bands, thinning and narrowing of the bands around the indentations and the escape tubes, shapes of the coloured impurities and persistence of the sharp angle of the bands at sector boundaries. These observations and experimental data lead us to conclude that agates can be formed in four stages: sol–gel transition with concomitant band formation, formation of escape tubes, rapid crystallization from multiple nucleation sites and late dissolution/recrystallization of the inner layers.
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Research Article|
January 01, 2018
A new model of agate geode formation based on a combination of morphological features and silica sol–gel experiments
Charles Brian Howard;
1
Macabee Health Fund, Orthopedic Department
, Beer-Sheva8489312, Israel
*
Corresponding author, e-mail: Charles@zahav.net.il
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Avinoam Rabinovitch
Avinoam Rabinovitch
2
Department of Physics, Ben Gurion University of the Negev
, Beer-Sheva84105, Israel
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European Journal of Mineralogy (2018) 30 (1): 97–106.
Article history
received:
24 Jan 2017
rev-recd:
28 Mar 2017
accepted:
03 Aug 2017
first online:
12 Jun 2018
Citation
Charles Brian Howard, Avinoam Rabinovitch; A new model of agate geode formation based on a combination of morphological features and silica sol–gel experiments. European Journal of Mineralogy 2018;; 30 (1): 97–106. doi: https://doi.org/10.1127/ejm/2017/0029-2685
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Index Terms/Descriptors
- agate
- amorphous materials
- banded materials
- colloidal materials
- EDS spectra
- experimental studies
- framework silicates
- gels
- geodes
- impurities
- laboratory studies
- models
- morphology
- nucleation
- recrystallization
- secondary structures
- sedimentary structures
- silica
- silica minerals
- silicates
- spectra
- X-ray spectra