With her ERC Consolidator Grant entitled ‘CRYSTAL CLEAR’, Prof. Dr Mariëtte Wolthers investigated how minerals are formed under natural conditions. Minerals that form in water, form from tiny positively and negatively charged building blocks – ions – that coagulate and stick to each other in atomic-scale three-dimensional patterns. Scientists have been experimenting with creating minerals for years, working in their laboratories with the ideal ratio of positive to negative building blocks or continuously varying ratios. However, natural minerals rarely develop under these ideal circumstances. The ratio of positively to negatively charged ions can sometimes differ by up to a million. This meant that it is not possible to make direct comparisons between experimental observations and natural mineral formation.
Objectives and conclusions
The CRYSTAL-CLEAR objectives were to determine if the birth of minerals (nucleation) depends on ionic ratio, if the crystals formed are charged and show different size and mineral structure evolution. These objectives were explored with experiments and simulations on three different mineral systems: calcium carbonates (lime scale), barium sulfates and iron sulfides. The final objective was to combine the insights into a new nucleation theory and apply the insights on natural and engineered mineral formation processes.
In short, we observed that mineral nucleation varies strongly with skewed ionic ratios. Even with very skewed ratios in solution, minerals still formed, but more slowly, less material was formed and size, charge and shape varied. When charged particles formed, they were less likely to aggregate into larger particles. With atomic scale simulations, we showed that the ionic ratio affects the energies of the very first steps towards mineral nucleation in all three systems. And with the new mechanistic model we developed, we can describe (and predict) this process for mineral formation.
Scientific and societal impact
Understanding how and how fast minerals form in natural aqueous solutions is important in the Earth Sciences for a wide range of reasons. For example, mineral formation affects the composition of water on Earth and the stability of the subsurface cementing materials together, including earthquake prone fracture zones. Certain minerals store CO2 and/or immobilize toxic elements. Also, the minerals that form are our geological archives, recording past (sea)water composition or temperature. In Materials Chemistry and Physics, understanding mineral formation is important for another multitude of purposes, including synthesis of minerals with specific properties for ingredients for pharmaceuticals, food, glass, ceramics etc. Our CRYSTAL CLEAR results show the skewed ratios play a significant role in how and how fast minerals form with what shape and surface charge. This provides new avenues for predicting (subsurface) mineral formation processes, for reading the geological mineral and rock record and for (additive-free) tailoring of mineral formation.
A direct application of the CRYSTAL-CLEAR results is in the softening step in drinking water production. In the final stage of the project, we have implemented our insights on the formation of lime scale (calcium carbonate) on the softening process used in many drinking water production plants across the globe. We tested the optimization of drinking water softening in experimental set-ups of various sizes, up to pilot plant scale. Then we derived a new model that plant operators can use to tweak their softening set-up to optimize specific key performance indicators.
