Time-resolved cryofixation. The fast-freezing samples were prepared using a stopped-flow apparatus (BioLogic SFM-4000) connected with a freeze-quench accessory. Solutions were loaded into separate syringes (10 mL each) in the unit and kept at room temperature for all experiments. BIO-KINE software (BioLogic) was used to program mixing, aging, and spraying sequence. For NMR experiments, per each spray, calcium and phosphate-containing solutions were mixed in equal quantities with a total volume of 100 L and aged for 10 ms, 100 ms, 1s, and 1 min, then directly sprayed in cold isopentane (at -140 0 C) with a flow rate of 10 mL/ s. Frozen solid was then packed in a 4 mm zirconia rotor using a home-built packing tools (Figure -SI5), maintaining the temperature < -140 0 C. Isopentane was mixed with liquid nitrogen to reach -140 0 C in a nitrogen dewar. The solution aged for 2h was frozen by immersion in liquid N2.