Orientite is orthorhombic, a = 9.044(10), b = 6.091(7), c = 19.031(20) Å, with ideal crystal chemical formula Ca8Mn3+10[(SiO4)3(Si3O10)3(OH)10].4H2O in the unit cell. A structural model for orientite was proposed on the basis of chemical data, X-ray and HRTEM (High Resolution Transmission Electron Microscopy) analyses. The model was refined to R = 0.069, using 650 independent reflections collected with MoK-alpha radiation. The structure is characterized by the regular alternation in the c direction of two distinct structural modules; an "ardennite" module is built up from (SiO4J, (Si3O10) and (MnO6) groups, and the other module is built up from (Si3O10) groups and water molecules; calcium cations fill the cavities and are linked to seven oxygen atoms. HRTEM not only demonstrated the occurrence of faulted sequences, and a new polytypic variant with 38 Å c periodicity, but also was extremely valuable in assessing the most likely structure model for orientite. Finally, a characteristic feature of orientile is the extensive occurrence of microvoids, elongated in the c direction, and located at the borders between misfitting crystal regions.