Yeelimite, Ca4[Al6O12]SO4, is outstanding as an aluminatesodalite, being the framework ofthese type of materials flexible and dependent on ion sizes and anionordering/disordering. On the other hand, yeelimite is also importantfrom an applied perspective as it is the most important phase in calciumsulfoaluminate cements. However, its crystal structure is not wellstudied. Here, we characterize the room temperature crystal structureof stoichiometric yeelimite through joint Rietveld refinement usingneutron and X-ray powder diffraction data coupled with chemical soft-constraints.Our structural study shows that yeelimite has a lower symmetry thanthat of the previously reported tetragonal system, which we establishto likely be the acentric orthorhombic space group Pcc2, with a √2a × √2a × a superstructure basedon the cubic sodalite structure. Final unit cell values were a= 13.0356(7) Å, b= 13.0350(7) Å,and c= 9.1677(2) Å. We determine several structuresusing density functional theory calculations, with the lowest energystructure being Pcc2 in agreement with our experimentalresult. Yeelimite undergoes a reversible phase transition to a higher-symmetryphase which has been characterized to occur at 470 °C by thermodiffractometry.The higher-symmetry phase is likely cubic or pseudocubic possessingan incommensurate superstructure, as suggested by our theoreticalcalculations which show a phase transition from an orthorhombic toa tetragonal structure. Our theoretical study also predicts a pressure-inducedphase transition to a cubic structure of space group I43m. Finally, we show thatour reported crystal structure of yeelimite enables better mineralogicalphase analysis of commercial calcium sulfoaluminate cements, as shownby RFvalues for this phase, 6.9% and 4.8% for the previouslypublished orthorhombic structure and for the one reported in thisstudy, respectively. [ABSTRACT FROM AUTHOR]