The Diversity of Crystals, Microstructures and Texture That Form Ostreoidea Shells Sancho Vaquer, Anna Griesshaber, Erika Salas, Carmen Harper, Elizabeth M. Checa González, Antonio G. Schmahl, Wolfgang W. Microstructure and texture of oyster shells High-resolution EBSD Misorientation between crystals Crystal twin formation Gradedness in crystallographic axes orientation W.W.S., E.G. and A.S.V. were funded by the German Research Council Programs GR 9/1234, SCHM 930/11-2. A.G.C. and C.S.C. were funded by projects PID2020-116660GB-I00 and PID2023-146394NB-I00 (Spanish Ministry of Science and Innovation: MCIN/AEI/10.13039/501100011033/). A.G.C. also acknowledges the Unidad Científica de Excelencia UCE-PP2016-05 (University of Granada) and the Research Group RNM363 (Junta de Andalucía). Supplementary Materials. The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst15030286/s1 The shells of bivalved molluscs comprise, in general, few microstructures and very few textures. In the case of ostreoid oysters, a high diversity has been observed. The shells consist of columnar-prismatic, foliated, granular calcite and myostracal-prismatic aragonite. Furthermore, voids are incorporated into the ostreoid shell: the pores of the vesicular shell segments and the blades/laths of the chalk lenses. These initiate formation of additional microstructures and textures. We investigated the shells of Magallana gigas, Ostrea stentina, Ostrea edulis (Ostreidae), Hyotissa hyotis, Hyotissa mcgintyi and Neopycnodonte cochlear (Gryphaeidae) with high-resolution, low-kV, electron backscatter diffraction (EBSD) measurements and scanning electron microscopy (FE-SEM) imaging and review the diversity of ostreoid Ca-carbonate microstructures and textures. From a crystallographic perspective, we (i) characterized the sub-micrometer crystal assembly pattern of ostreoid microstructures and textures, (ii) investigated crystal organization at the changeover from one microstructure into the other and (iii) examined how curved crystal surfaces are generated at inner shell surface as well as within the shell, in and at aggregations of folia and foliated units. We show that Ostreoidea are capable of secreting single crystalline, graded and dendritic calcite within the same shell and, hence, are able to vary strongly the degree of crystal co-alignment. We demonstrate that Ostreoidea myostracal aragonite is twinned, while shell calcite is not twinned, neither within different microstructures nor at the changeover between adjacent microstructures. We highlight the very specific microstructure of the foliated shell and demonstrate the strongly regulated gradedness of both the c- and a*-axes orientation of the foliated calcite crystallites. 2025-03-25T13:03:28Z 2025-03-25T13:03:28Z 2025-03-20 journal article Sancho Vaquer, A.; Griesshaber, E.; Salas, C.; Harper, E.M.; Checa, A.G.; Schmahl, W.W. The Diversity of Crystals, Microstructures and Texture That Form Ostreoidea Shells. Crystals 2025, 15, 286. https://doi.org/10.3390/cryst15030286 https://hdl.handle.net/10481/103290 10.3390/cryst15030286 eng http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional MDPI