Are Considerations of Mineralogical Equilibrium Relevant to Pedology? Evidence From a Weathered Granite in Central France

Metastability in Near-Surface Rocks of Minerals in The System Al2O3-SiO2-H2O

“…In other words, equilibrium concepts may have little relevance to pedology (contra Chesworth, 1980a), Indeed, structurally complex and/or disordered min, erals, such as certain clays and zeolites, may always be metastable with respect to denser, lower entropy minerals in the environments in which they form. Careful testing will be required to evaluate this hypothesis, both because of large uncertainties in equilibrium and kinetic aspects of ordering in many of these phases, and because of grave experimental difficulties in obtaining equilibrium at T < 200~…”

“…It is important to evaluate these arguments, because they imply the existence of large errors in current experimental and thermodynamic data for the aluminum hydroxides. Chesworth (1972Chesworth ( , 1975Chesworth ( , 1980aChesworth ( , 1980b presented several phase diagrams showing boehmite and gibbsite to be stable at STP without considering the possibility that diaspore is more stable. Thermodynamic and experimental data, however, show that gibbsite, boehmite, and bayerite are always metastable relative to diaspore and water at STP and that boehmite is metastable relative to diaspore at STP even if the uncertainties in the thermodynamic data are considered (Table 3).…”

Reactions Controlling Heavy Metal Solubility in Soils

“…Second, the assumption that equilibrium has been reestablished after the soils are moved from the field into the laboratory may not be justified. There is serious doubt whether soils in the field ever attain equilibrium beyond the microscopic scale (Chesworth and Dejou, 1980). Although one might expect that vigorous mixing of soils suspended in aqueous solution in the laboratory would overcome this difficulty, the soil solids are likely to undergo a sluggish response to the changed O2 and CO2 concentrations, pH, and electrolyte concentration.…”

Mineral Metastability In the System Al2O3-SiO2-H2O: A Discussion

“…Several phase diagrams in the system AlzO3-SiO2-H20 (e.g., Chesworth, 1972Chesworth, , 1975Chesworth, , 1980aChesworth, , 1980b imply that the phases nordstrandite, bayerite, boehmite, and gibbsite are stable phases in soil (p. 225).…”

“…The gibbsite-absent (G) and pyrophyllite-absent (P) invariant points oriented with respect to temperature and pressure axes. Anovitz et al (1991) further state: " Chesworth (1972Chesworth ( , 1975Chesworth ( , 1980aChesworth ( , 1980b presented several phase diagrams showing boehmite and gibbsite to be stable a STP without considering that diaspore is more stable" (p. 229). Chesworth (1972) examines "the relative stabilities of gibbsite and boehmite" (p. 369) since this was a question under dispute at that time.…”