*3.2. Mineralogy and Crystal Morphology*

Back-scatter electron (BSE) images of the CaCO3 surfaces show distinct morphological differences between the six treatments. The Mg-free (control Mg treatment) 47:1 Ca:CO3 incubations formed homogeneous and heterogeneous nucleation in the form of crosshatched vaterite pre-spherical and laminated cubed calcite (Figure 4a). In the Mg-free 2.8:1 scenario, laminated cube calcite precipitated, along with amorphic calcium carbonate (ACC) or a stable prenucleation calcium carbonate cluster c.f. [47], and presumably unfinished calcite transforming from proto-vaterite precursors (Figure 4b). Despite the absence of a measurable weight increase from both the Mg addition scenarios with the 2.8:1 Ca:CO3, newly formed aragonite needles were visibly precipitated on top of the seeding material (Figure 4d,f). Well-defined and abundant acicular crystals were precipitated in random directions and from multiple centers of nucleation as well as from cemented CaCO3 (Figure 4f). In the scenario with high Mg concentration (equal to present day seawater, 53 mM) and at a 2.8:1 Ca:CO3, CaCO3, cements were also observed along with dissolution pits in a needle form (Figure 4f). In the scenario with lower Mg concentration (equal to half the present-day seawater, 26.5 mM) with a 47:1 Ca:CO3, both ACC and dissolution pits were observed, in addition to Mg-calcite (Figure 4c). Conversely, in the scenario with the high Mg concentration and 47:1 Ca:CO3, cements primarily formed along with dissolution pits, and low-relief aragonite needles within the seed material crevices (Figure 4e).

**Figure 4.** Representative SEM images of the precipitated CaCO3 from each experimental scenario. This shows the influence of Ca:CO3 stoichiometry, as well as the Mg concentration on the CaCO3 morphology. The Mg-free incubations with a 47:1 Ca:CO3 (**a**) resulted in crosshatched vaterite and layered rhombohedral calcite. Mg-free incubations (control Mg treamtent) with a 2.8:1 Ca:CO3 stoichiometry (**b**) resulted in an intermediate form of ACC together with rhombohedral calcite and some signs of dissolution. Where the Mg concentration was half that of the modern seawater values (26.5 mM) and with a 47:1 Ca:CO3 stoichiometry (**c**) resulted in a Mg-calcite structure c.f. [48]. Mg concentration of 26.5 mM with a 2.8:1 Ca:CO3 stoichiometry (**d**) resulted in the formation of aragonite needles. In the scenario where the Mg concentration was 53 mM (equivalent to modern seawater values) no matter if the Ca:CO3 stoichiometry was 2.8:1 or 47:1 (**e**,**f**) very little new CaCO3 material precipitated, instead we can see ACC with low relief aragonite needles or unconnected aragonite needles and dissolution pits.
