Formation Mechanisms of Carbonates via Amorphous Precursors

Our current understanding of the Earth`s climatic evolution in the geological past is mainly based on the isotopic and chemical composition of biogenic and inorganic carbonates. The biogenic deposition of carbonates is of great importance especially to the marine ecosystem; inorganic carbonates often occur in the form of speleothems, travertine, and alkaline lake deposits in terrestrial environments. The use of isotopic and chemical signatures of carbonates is essentially based on the assumption that carbonates record the isotopic and chemical signature of their environment (e.g. sea water or dripping water in caves) at the time of their formation. However, recent studies increasingly confirm that various carbonate minerals found in different natural environments are not only formed via the classical crystallization pathway, but are also formed through the formation of an intermediate amorphous phase. In this context, significant gaps of knowledge still exist regarding the reaction mechanisms that control the transformation of highly reactive, amorphous (water-containing) carbonate phases into mainly water-free and non- reactive (stable) carbonates. The primary aim of this project is to elucidate the mechanisms controlling the formation of carbonates by an experimental approach: amorphous carbonate phases will be synthesized and their temperature-dependent transformation into crystalline phases will be investigated in the presence of different aqueous solutions as a function of the reaction time. During the experimental runs, the evolution of the solid phase composition will be analyzed at high temporal resolution (30 sec) by in situ Raman spectroscopy. The isotopic and chemical evolution of the solution and of the solid phase will be followed by separate and modern sample analysis. The mineralogical, isotopic and chemical results of this study will be used to improve our current understanding about the environmental controls and the formation mechanisms of carbonate minerals in natural systems. Moreover, the fate of primary isotopic and chemical signatures after the transformation of the amorphous to the crystalline carbonate phase will be assessed. This circumstance is of great relevance for the interpretation of isotopic and chemical signatures of carbonates, which are formed by amorphous precursor and are subsequently used as climate indicators.

The formation of crystalline carbonate minerals via amorphous precursors is a poorly understood process, but is of high relevance for biogenic and abiogenic carbonate precipitation. This project aimed at investigating the (i) reaction mechanisms controlling the formation of distinct crystalline calcium magnesium carbonate minerals via amorphous precursors and the (ii) impact of the amorphous precursor on the chemical and isotopic composition of the crystalline phase. The latter question is of key relevance for the accurate interpretation of isotopic and chemical signatures of carbonates, which are formed by amorphous precursor and are subsequently used as climate indicators. In the context of this project, amorphous calcium magnesium carbonate (ACMC) precursors were synthesized and their transformation into crystalline calcium magnesium carbonates were studied under controlled temperature conditions (T = 10 - 80C). In selected experiments, the synthesized ACMC phase was transformed into Mg-containing calcite in a solution of known 18O depleted isotopic composition. Sub-samples of the solid and fluid phase were sampled over time in order to follow the chemical composition of the fluid, as well as changes in the mineralogy and chemical composition of the solid phase. The results of this study provided an advanced understanding of the complex interplay between the chemical composition of the amorphous precursor, the corresponding solution and the finally formed crystalline phase. The chemical data show that ACMC is more soluble at higher temperature and the relative increase of its solubility as a function of the Mg content is similar from 10 to 80 C. Moreover, the results suggest a highly dynamic exchange behavior of ions (e.g. Mg) between the nano-porous ACMC solid phase and the experimental solution. The chemical composition of the prevailing solution (changed by the ion exchange) and the reaction temperature of the solution significantly affect the ACMC transformation pathway to distinct crystalline hydrous and anhydrous Ca-Mg-carbonates. The isotope data revealed that the final oxygen (18O) isotopic composition of the crystalline phase is significantly affected by the ACMC transformation pathway, whereas the clumped (47) isotopes show fewer effects. The findings improve our current understanding on the formation mechanisms of carbonate minerals in natural settings and offer new guides for the interpretation of chemical and isotopic signatures of carbonates formed via amorphous precursors.