MINERAL REPLACEMENT REACTIONS IN SOLID SOLUTION-AQUEOUS SOLUTION SYSTEMS: VOLUME CHANGES, REACTIONS PATHS AND END-POINTS USING THE EXAMPLE OF MODEL SALT SYSTEMS

The volume change of solid phases associated with dissolution and precipitation reactions during mineral replacement is a critical factor for the advancement of the reaction boundary. Contributing parameters to the overall volume change of a replacement reaction are the molar volume of parent and product and their solubility ratio within a given solution. Based on simple model salt systems, the contribution of solubility to volume change is quantitatively determined. For NaCl-KCl as an example of a binary salt system without solid solution, the relative volume changes can be calculated for various reaction paths using the slope of the solubility from a simple solubility diagram. For KBr-KCl as an example of a binary salt system with complete solid solution, the determination of the solubility curve is based on a modified Lippmann phase diagram called a solubility phase diagram. It allows a quantitative calculation of the relative volume change based on the solid solution-aqueous solution (SS-AS) relationships for variable solution compositions and reaction paths in the salt-water system. Reaction kinetics, textures and the compositional evolution of replacements in both salt systems can be conclusively explained by the relative volume change on the basis of experimentally constrained reaction paths. The analogy from simple model system to replacement reactions at the Earth's surface and crustal conditions (for example in apatites or feldspars) may offer insights to successfully describe volume changes and porosity generation in mineral reactions on the basis of solubility data towards a more quantitative modeling of interface-coupled dissolution-precipitation reactions.