Magnesium is a key component used by many organisms in biomineralization. One role for magnesium is in stabilizing an otherwise unstable amorphous calcium carbonate (ACC) phase. The way in which this stabilization is achieved is unknown. Here, we address this question by studying the chemical environment around magnesium in biogenic and synthetic ACCs using Mg K-edge X-ray absorption spectroscopy (XAS). We show that although the short-range structure around the Mg ion is different in the various minerals studied, they all involve a shortening of the Mg-O bond length compared to crystalline anhydrous MgCO ₃ minerals. We propose that the compact structure around magnesium introduces distortion in the CaCO₃ host mineral, thus inhibiting its crystallization. This study also shows that despite technical challenges in the soft X-ray energy regime, Mg K-edge XAS is a valuable tool for structural analysis of Mg containing amorphous materials, in biology and materials science.