First, to evaluate the concentration of Sr 2+ available in the liquid phase close to the nucleation temperature, we made the rough assumption that at 100 °C, the reaction medium behaves as a saturated Sr(OH)2 solution (see supplementary text 1). Although this approximation does not capture the specificity of the solvating behavior of hydrofluxes, it is instructive in providing a hint at the possible evolution of the strontium concentration in the liquid media. The Sr 2+ concentration is doubled when q(H2O)K increases from 2.2 to 2.8 so this parameter could play a role in anisotropic growth.