The aim of this study is to contribute to understanding the mechanisms underlying the formation of biologically relevant minerals by comparing the properties of solid phases formed in calcium phosphate (CaP) or calcium carbonate (CaCO 3 ) precipitation systems, at defined initial experimental conditions: supersaturation, constituent ions ratio, ionic strength, and/or presence of relevant inorganic ions. Thus, three systems of different chemical complexities were investigated: (a) system containing constituent ions, (b) system containing additional co-ions, and (c) system with higher ionic strength and addition of Mg 2+ . The respective precipitation diagrams were constructed, and supersaturation domains of different CaP and CaCO 3 solid phases formation were identified. The obtained results may have implications not only for biomineralization and geochemistry, but also for materials science in general.

Spontaneous precipitation of CaCO3 polymorphs in the presence of selected amino acids (AA) has been investigated. The L-aspartic acid (Asp), L-lysine (Lys), L-asparagine (Asn), L-tyrosine (Tyr), Lphenylalanine (Phe), L-serine (Ser) and L-alanine (Ala) were selected because of different charge and polarity of their side chains at applied experimental conditions. The investigated AA are building units of soluble macromolecules, putatively responsible for biomineralization of molluscs' exoskeletons. It was assumed that not only the acidic, but also the polar (hydrogen bonding) AA might contribute to macromolecules' interactions with the mineral surfaces. The mineralogical composition, structure and morphology of precipitates formed in the presence of wide range of concentrations of AA have been determined by XRD, FT-IR and EPR spectroscopy, HPLC-MS and SEM. In the reference system, without AA addition, a mixture of typical calcite rhombohedral crystals and vaterite spherulites has been observed, while AA with negatively charged or polar side chains (Asp, Tyr, Lys, Asn, Ser) significantly changed the morphology, phase composition and crystal structure of the precipitates. The effects of nonpolar AA (Phe, Ala) on the structural and morphological properties of precipitates are less pronounced. The stronger impact observed for polar AA and particularly negatively charged Asp, may be correlated with the additional electrostatic interactions of side-chain groups with mineral surfaces. Response to Reviewers: REVIEWER #1 This work investigates the effect of polar, charged and nonpolar amino acids, as well as their increasing concentrations, on the precipitation of calcium carbonate at high supersaturation. The authors explore a wide range of concentrations of soluble additives and also uses a systematic approach to evaluate the effects of several amino acids. However, this is not the first study to have used this exact same approach to we believe that the specific comments given in this report actually refer to the Langmuir version (indeed, in this report the Reviewer even used the previous title: "The Role of Amino Acids as Simple Models of Soluble Macromolecules Relevant for Calcium Carbonate Biomineralization"). Nevertheless, we addressed in this response all points, regardless they have been changed and accepted previously.

Precipitation domains of ACP and ACP increase with the complexity of the system, the ACP one being always larger.

Recently, the results of experimental and theoretical investigations have revealed that, in vaterite, two or even more crystalline structures coexist. In this communication we report evidence of diverse vaterite structures in biogenic samples of different origin. In addition, it is shown that the synthetic vaterite precipitated in the presence of poly-l-aspartate has structures similar to those of biogenic samples.

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