Crystallization at the solid-liquid interface is difficult to spectroscopically observe and therefore challenging to understand and ultimately control at the molecular level. The Ce70-torroid formulated [CeIV70(OH)36(O)64(SO4)60(H2O)10]4-, part of a larger emerging family of MIV70-materials (M = Zr, U, Ce), presents such an opportunity. We elucidated assembly mechanisms by the X-ray scattering (small-angle scattering and total scattering) of solutions and solids as well as crystallizing and identifying fragments of Ce70 by single-crystal X-ray diffraction. Fragments show evidence for templated growth (Ce5, [Ce5(O)3(SO4)12]10-) and modular assembly from hexamer (Ce6) building units (Ce13, [Ce13(OH)6(O)12(SO4)14(H2O)14]6- and Ce62, [Ce62(OH)30(O)58(SO4)58]14-). Ce62, an almost complete ring, precipitates instantaneously in the presence of ammonium cations as two torqued arcs that interlock by hydrogen boding through NH4+, a structural motif not observed before in inorganic systems. The room temperature rapid assemblies of both Ce70 and Ce62, respectively, by the addition of Li+ and NH4+, along with ion-exchange and redox behavior, invite exploitation of this emerging material family in environmental and energy applications.