Solid-solute phase equilibria in aqueous solution: Fundamentals and applications

Abstract

Carbonates 1 Phase Transitions 1 Solutions 1 ThermodynamicsSolid binary carbonate phases often dissolve congruently in aqueous media, even when they are not stable under the prevailing conditions. From the thermodynamic point of view these dissolution reactions lead to a metastable equilibrium with fixed composition, when neither diffusion in the solid state nor recrystallization from the solution occurs. The solubility of natural huntite, CaMg3(C03)4, was studied potentiometrically at 50°C and I = 3.0 mol kg-' (Na)C104. Since it dissolved congruently the solubility constant log * . Amazingly enough phase equilibria of solid mixtures in aqueous media have seldom been studied in detail, although the corresponding reactions play an important role in geochemistry as well as in hydrometallurgy. Lippmann [S] described the aqueous solubility of binary mineral systems using the total solubility product and the mole fraction as master variables of phase diagrams. These diagrams are based on the solubility products of the pure end members and a simple electrostatic mixing model, but have in most cases not been verified experimentally. One of the reasons for this lack of scientific activity in an obviously interesting field is certainly the fact that the rates of heterogeneous ionic reactions are rather slow at temperatures between the freezing and the boiling point of water at ordinary pressure, and differing results have been obtained when equilibrium was approached from undersaturation and supersaturation respectively. A pertinent example are the contradictory results for the solubilities of magnesian calcites obtained by different research groups [6].In this paper the thermodynamic formalism for solid-solute ionic equilibria in the system CaC03 -MgC03 -H,O is presented with particular emphasis on phase transitions without diffusion in the solid state [7]. It is applied for the determination of the solubility constant, phase equilibria and free enthalpy of formation of huntite, CaMg3(CO&. This mineral occurs naturally although its decomposition into dolomite, CaMg(CO&, and magnesite, MgC03, is thermodynamically favoured.

Thermodynamics of the $\alpha$-FeOOH (goethite)-ScOOH solid solution

“…Indeed, the solid solution is not continuous, but the calculated equilibrium x AlOOH is in stark conflict with common natural occurrence or laboratory syntheses of Al-goethite well exceeding 10 mol.% AlOOH (e.g., Lewis and Schwertmann (1979)). The discrepancy was rationalized by a thermodynamic model where α-FeOOH is being mixed with a hypothetical AlOOH phase (not diaspore) whose solubility is much higher than that of diaspore (Königsberger, 2013). Bazilevskaya et al (2011) argued that Al preferentially clusters when inserted into the structure of goethite and the solid solution is inhomogeneous on microscopic level.…”

Experimental study and reaction path modeling of the carbonation of natural serpentinites

“…In many experiments, owing to the high Na activity due to the addition of NaHCO 3 and Na 2 C 2 O 4 , the carbonate eitelite Na 2 Mg(CO 3 ) 2 is detected. Unfortunately, this rare mineral is not included in the Thermoddem database and the only data in the literature is the Gibbs free energy of formation of the mineral from the experimental study of Königsberger et al (1992) who measured a value at 25℃, 1 bar of -2079.76±1.2 kJ/mol. With no other data to speak of, the only way to incorporate eitelite in the simulations was to reconstruct its thermodynamic data from the sum of its polyhedral contribution following the method of La Iglesia and Félix (1994).…”