To confirm this interpretation, we assessed the impact of the Sr precursor on anisotropic growth, by reacting several Sr sources (strontium hydroxide, strontium chloride, and strontium nitrate) at constant (q(H2O)K), q(Sr), q(Cl)) values of (2.8, 4.0, 0), as in Figure . XRD (Figure ) shows that while Sr(OH)2 yields the strongest crystal anisotropy, the preferential {hk0} orientation is reduced with strontium chloride and disappears when using strontium nitrate, indicating that anisotropic growth is hindered. In these conditions, strontium chloride or nitrate readily convert to strontium hydroxide (see supplementary text 2), so that the nature of Sr species and the Sr 2+ concentration close to the onset of crystallization are similar to the one when starting directly from strontium hydroxide as the precursor. This confirms that the Sr 2+ concentration cannot account for the difference in anisotropic growth.