Energy-efﬁcient synthesis of materials locked in compositional and structural states far from equilibrium remains a challenging goal. Biomineralizing organisms routinely assemble such materials with sophisticated design and advanced functional properties, often using amorphous precursors. However, it has proven extremely challenging to accurately describe pathways and determine mechanisms, even for extensively studied system such as amorphous calcium carbonate (ACC). I will discuss recent work in my laboratory in which we use liposomes and microfluidic droplets as nano-to-microscale reactors to study the impact of confinement, biomolecules, and inorganic additives on the kinetics of crystallization of ACC and its barium-substituted derivative (ACBC). In the course of this work, we discovered bulk syntheses leading to (a) balcite, which is isostructural to a high temperature modification of calcite, and (b) gortatowskite, a quasi-two dimensional barium carbonate mono-hydrate.