I have precipitated minerals using artificial seawater and attached a SEM image of the resulting minerals. Initially, I thought they were aragonite. However, based on the XRD analysis, they appear to be vaterite. I'm wondering if vaterite can form as shown in the attached image, particularly if magnesium is involved in the process.
I would trust the XRD results. If the solution was highly supersaturated and contained a high amount of Mg, vaterite precipitation is quite likely. The vaterite probably formed by recrystallization from an ACC precursor. There is a huge amount of literature on this.
I had similarly observed that magnesium (even in small concentrations) could significantly influence the shape of biogenic calcite precipitates. I see elongated epitaxial crystals in the SEM here; however, we need to rely on XRD and/or RAMAN analysis for the mineralogy of the precipitates. Several reports verify how the shape of calcium carbonate precipitates changes with "enzyme-induced calcite precipitation", which could be referred to investigate the scenario further.
I would also trust the XRD results. But as we know, vaterite is a metastable CaCO3 polymorph so may be you should monitor the crystal growth to see if there is any transformation into calcite or high-Mg calcite over time.
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