Quartz Transformation into Opal at the Water–Vapor Interface

• Published in: Geochemistry international 2021-04, Vol.59 (4), p.377-387
• Main Authors: Alekseyev, V. A., Burmistrov, A. A., Gromiak, I. N.
• Format: Article
• Language: English
• Subjects: Analytical methods; Atomic emission spectroscopy; Cristobalite; Crystallization; Crystals; Data acquisition; Deposition; Dissolution; Dissolving; Distillation; Distilling; Earth and Environmental Science; Earth Sciences; Emission analysis; Evaporation; Evaporation rate; Geochemistry; Hydrothermal systems; Inductively coupled plasma; Kinetics; Light microscopy; Minerals; Opal; Optical microscopy; Quartz; Quartz crystals; Recrystallization; Silica; Silicon dioxide; Temperature gradients; Tridymite; Vapor phases; Water vapor; Water vapour
• ISSN: 0016-7029; 1556-1968
• DOI: 10.1134/S0016702921040029

— The kinetics of reactions between minerals and water vapor or two water phases simultaneously (liquid + vapor) is still understood inadequately poorly, and the absence of these data disable realistic and comprehensive quantitative description of the evolution of hydrothermal system after its fluid heterogenizes. To bridge this gap, we have conducted quenching experiments with quartz crystals and water at 300°C and P sat = 86 bar. In addition to usually applied analytical techniques (ICP-AES, SEM, XRD, and optical microscopy), we used 3d scanning of the crystals, measured their surface areas, and plotted the changes in the crystal sizes because of dissolution and deposition reactions. In the experimental runs with the crystal occurring in the vapor phase, the first ever data were acquired on the rate constant of quartz dissolution for saturated water vapor ( P sat = 86 bar) at 300°C (2.7 nmol m –2 s –1 ). The constant turned out to be 630 times lower than that for pure water. Calculations indicate that equilibrium between quartz and water and vapor is established during comparable time spans, but quartz recrystallization in vapor due to the temperature gradient proceeds two orders of magnitude more slowly than in water. In the runs with the crystal occurring in both water and vapor, not only the stable quartz dissolved, but also metastable cristobalite-tridymite opal was formed. The opal was deposited on the autoclave walls and even on the quartz itself above the water surface, and the silica concentration in the water remained remarkably lower that the quartz solubility. The rate of opal formation (10 –7.5 mol m –2 s –1 ) was 3.5 orders of magnitude higher than the quartz recrystallization rate (which is the only process possible in this system according to the traditional geochemical approach). This disagreement is explained within the framework of the distillation hypothesis, which is based on the preferential evaporation of the thin (