CA-MG CARBONATE BIOMINERAL CONTROLLED BY ORGANIC COMPOUND AND SURFACE EFFECTS

Calcium carbonate minerals, like aragonite and calcite, are commonly formed by organisms as biominerals such as skeletons, shells, and exoskeletons. However, the increasing levels of pCO₂ associated with human-induced climate change pose a threat to these carbonate minerals, particularly in an acidifying ocean. To address this issue, some organisms may potentially utilize alternative minerals, specifically Ca-Mg carbonates such as disordered dolomite. These Ca-Mg carbonate minerals offer the advantage of being more chemically and physically resistant than calcium carbonates, and they hold a higher potential for carbon sequestration due to the availability of both Ca and Mg cations for bonding with the carbonate group (CO₃^2-). However, Mg-bearing carbonates are relatively rare biominerals due to the high kinetic energy barrier for the dehydration of the Mg-water complex which severely restricts Mg incorporation in carbonates at Earth surface conditions. This work presents the first overview of the effects of the physiochemical properties of amino acids and chitins on the mineralogy, composition, and morphology of Ca-Mg carbonates in solutions and on solid surfaces. We discovered that acidic, negatively-charged, hydrophilic amino acids like aspartic and glutamic acid, as well as chitin, can trigger the formation of high magnesium calcite (HMC) and disordered dolomite through precipitation in solution and on solid surfaces when these biomolecules are present. By looking into the amino acid makeups of shells and skeletons with bimineralic properties, it is also clear that different ratios help to control the selective formation of different carbonate polymorphs. This fundamental research on the effect of organic compounds and solid surfaces controlling carbonate phase, composition, and morphology gives us new insights into biomediated mechanisms for Mg-rich calcium carbonates that may play increasing important roles in low temperature geochemistry, carbonate biomineralogy, and climate proxies.