Observations demonstrated that Mg in biogenic calcite ranged from near zero-level to 50 mol % while the abiotic calcite rarely contained Mg over 12 mol %. Studies indicated that proteins and macromolecules can regulate Mg contents in those biogenic calcites. However, most of these studies focused on Mg contents in homogeneous precipitates affected by proteins and macromolecules, yet, little is known about Mg contents in heterogenous precipitates nucleated on the surface of these proteins and macromolecules. In this study, in-situ grazing incidence small-angle X-ray scattering (GISAXS) and grazing incidence wide-angle-X-ray scattering (GIWAXS) were used to characterize (Cax,Mg1-x)CO3 precipitates on different organics and in different Mg/Ca ratio solutions. Little heterogeneous precipitation but bulk solution deposition of amorphous calcium carbonates (ACC) was observed on either -COOH or -SH coated substrate surface. However, on the mixed -COOH and -SH coated substrate surface, intense heterogeneous ACC precipitation occurred with particle sizes around 1 nm in all solution conditions. Interestingly, Mg-enriched ACC precipitated on the mixed -COOH and -SH coated substrate surface although the bulk solution was undersaturated with ACC. Moreover, the Mg-contents in these heterogeneous ACC precipitates increased by 50 % compared with those in homogenous precipitates from the bulk solutions. Ion adsorption experiments demonstrated that mixed -COOH and -SH enriched cations in the local surface, thus increasing the local supersaturation. The high local supersaturation promoted ACC nucleation on organics even the bulk is undersaturated with ACC. This study provided new explanations for the formation of high Mg biogenic carbonates and offered new insights into the significance of mineralization in the microenvironment.

Heterogeneous (Cax,Mg1-x)CO3 precipitation on organics

2019 Spring National Meeting

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