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Resolving the conundrum of equilibrium solubility of smectites

  • Stephen U. Aja
Published/Copyright: March 28, 2022
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American Mineralogist
From the journal American Mineralogist Volume 107 Issue 4

Abstract

Smectites are common clay minerals in surface and near surface terrestrial environments and have recently been shown to be ubiquitous on Mars. Because these minerals are products of water-rock interaction, the thermodynamics of their interaction with fluids constitutes a vital part of resolving the utility of smectite mineralizations as petrogenetic and paleoenvironmental indicators on both Mars and Earth’s near surface environments. Smectites, and other clay minerals of comparable compositional complexity, have been purported to be disequilibrium solids whose complexity derives from steep chemical gradients in their environments of formation rather than from crystal-chemical constraints. Solubility investigations of several natural smectites wherein none exhibited the predicted inverse correlation between pH-1⁄3pAl and pSi(OH)4 were adduced by May et al. (1986) as empirical proof of the disequilibrium solid concept, and hence they asserted unequivocally that “it is obviously impossible to obtain valid ion activity quotients for smectite solubilities in these systems.” However, the unattainability of equilibrium smectite solubility in those experimental systems was probably an artifact of the extremely high fluid-to-solid ratios employed therein. In subsequent experimental studies using significantly lower fluid-to-solid ratios, smectite-fluid interactions (Kittrick and Peryea 1989; Gaboreau et al. 2020), and chlorite-fluid interactions (Aja and Dyar 2002) yielded solubility data amenable to laws of chemical thermodynamics and thus invalidated the disequilibrium solid model. Therefore, the notion of smectite metastability and/or instability anchored on the disequilibrium solid model is false and warrants a more constrained definition of smectite metastability.

Keywords: Smectite; metastability; disequilibrium solid; fluid-to-solid ratio; reversibility; solubility equilibrium; colloidal electrolyte

Acknowledgments

Constructive and helpful review comments by Jim Wilson are acknowledged. This work was not directly funded by any external funding agencies.

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Received: 2021-07-11
Accepted: 2021-09-15
Published Online: 2022-03-28
Published in Print: 2022-04-26

© 2022 Mineralogical Society of America

Articles in the same Issue

  1. Perspectives
  2. Resolving the conundrum of equilibrium solubility of smectites
  3. Manjiroite or hydrous hollandite?
  4. Petrologic evolution of boninite lavas from the IBM Fore-arc, IODP Expedition 352: Evidence for open-system processes during early subduction zone magmatism
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  7. The electrical conductivity of albite feldspar: Implications for oceanic lower crustal sequences and subduction zones
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  9. Water in the crystal structure of CaSiO3 perovskite
  10. Release of chromite nanoparticles and their alteration in the presence of Mn-oxides
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  12. Atomistic mechanism of cadmium incorporation into hydroxyapatite
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https://doi.org/10.2138/am-2022-8213
Keywords for this article
Smectite; metastability; disequilibrium solid; fluid-to-solid ratio; reversibility; solubility equilibrium; colloidal electrolyte

Articles in the same Issue

  1. Perspectives
  2. Resolving the conundrum of equilibrium solubility of smectites
  3. Manjiroite or hydrous hollandite?
  4. Petrologic evolution of boninite lavas from the IBM Fore-arc, IODP Expedition 352: Evidence for open-system processes during early subduction zone magmatism
  5. Coupled hydrogen and fluorine incorporation in garnet: New constraints from FTIR, ERDA, SIMS, and EPMA
  6. Incorporation mechanism of structurally bound gold in pyrite: Insights from an integrated chemical and atomic-scale microstructural study
  7. The electrical conductivity of albite feldspar: Implications for oceanic lower crustal sequences and subduction zones
  8. A high-pressure, clinopyroxene-structured polymorph of albite in highly shocked terrestrial and meteoritic rocks
  9. Water in the crystal structure of CaSiO3 perovskite
  10. Release of chromite nanoparticles and their alteration in the presence of Mn-oxides
  11. The absorption indicatrix as an empirical model to describe anisotropy in X-ray absorption spectra of pyroxenes
  12. Atomistic mechanism of cadmium incorporation into hydroxyapatite
  13. Copper isotope evidence for a Cu-rich mantle source of the world-class Jinchuan magmatic Ni-Cu deposit
  14. Gamma radiation effects on quartz Al and Ti center electron spin resonance signal intensity: Implications for quartz provenance discrimination
  15. A new high-pressure experimental apparatus to study magmatic processes at precisely controlled redox conditions
  16. Effect of structural water on the elasticity of orthopyroxene
  17. Cryogenic heat capacity measurements and thermodynamic analysis of lithium aluminum layered double hydroxides (LDHs) with intercalated chloride
  18. A theoretical and experimental investigation of hetero- vs. homo-connectivity in barium silicates
  19. Radiation-induced changes in vanadium speciation in basaltic glasses: Implications for oxybarometry measurements using vanadium K-edge X-ray absorption spectroscopy
  20. The crystal structure of Fe2S at 90 GPa based on single-crystal X-ray diffraction techniques
  21. Hydration-driven stabilization and volume collapse of grain boundaries in Mg2SiO4 forsterite predicted by first-principles simulations
  22. Kinetics of dehydrogenation of riebeckite Na2Fe23+Fe32+Si8O22(OH)2: An HT-FTIR study
  23. Ferro-tschermakite with polysomatic chain-width disorder identified in silician magnetite from Wirrda Well, South Australia: A HAADF STEM study
  24. New Mineral Names: High-Pressure and Precious Minerals
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