The cooling of steel containers in radioactive-waste storage was simulated in a step-by-step experiment from 90 to 40°C. Among newly formed clay minerals observed in run products, cronstedtite was identified by a number of analytical techniques (powder X-ray diffraction, transmission electron microscopy, and scanning electron microscopy). Cronstedtite has not previously been recognized to be so abundant and so well crystallized in an iron–clay interaction experiment. The supersaturation of experimental solutions with respect to cronstedtite was due to the availability of Fe and Si in solution, as a result of the dissolution of iron metal powder, quartz, and minor amounts of other silicates. Cronstedtite crystals are characterized by various morphologies: pyramidal (truncated or not) with a triangular base and conical with a rounded or hexagonal cross-section. The pyramidal crystals occur more frequently and their polytypes (2M1, 1M, 3T) were identified by selected area electron diffraction patterns and by automated diffraction tomography. Cronstedtite is stable within the 90–60°C temperature range. At temperatures of ≤ 50°C, the cronstedite crystals showed evidence of alteration.