From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones
Fig 2
Nacre preservation in the late campanian ammonite Baculites sp. from Potelych (Potylicz), western Ukraine.
(A) Internal mould covered with a white coating and (B, C) close-ups of the suture pattern and fragment of iridescent nacre (B, arrow). (D, E (enlarged)) Thin section of the specimen in transmitted light; note the layered structure of the shell (the green frame outlines the region shown in F). (F) Aragonite (green) mineralogy of remnants of the nacre layer and calcite of internal mould deposit (magenta) visualized in a micro-Raman map. (G, H (enlarged)) FESEM micrographs of the white coating of the mould formed of small (ca. 2–3 μm in diameter, ca. 300 nm thick) polygonal tablets. The rounded cavities (arrows in H, I) in the tablets are traces of central organic accumulations. These holes also penetrate underlying tablets of nacre, as it is particularly well seen in the top view (I, arrows). (I) SEM micrograph showing parallel layers of thin polygonal tablets that form the iridescent surface of the shell. (J) Oblique view of shell surface showing that some of the tablets have preserved their division into sectors (SEM image). (K) FESEM close-up of (H) showing granular texture of aragonite tablets (black arrows, see also J and L). The rounded granules (ca. 60–90 nm in diameter) visible on the surface of tablets are remnants of the primary nanostructure of aragonite tablets. (L) FESEM close-up revealing the presence of small vertical structures (ca. 70 nm in diameter) that connect superposed tablets of nacre and resemble mineral bridges observed in the nacre of modern mollusks; see [43]). (M-O) SEM micrographs of fractured fragments of nacreous layer formed of vertical stacks of aragonite tablets. Occasionally, within the same specimen, the original nacre tablets can be slightly dissolved (reduced tablet thickness), which emphasizes the columnar microstructural pattern (O) and vertical connections (bridges) between tablets (O, arrow; compare with L). (A-O) Specimen ZPAL Am. 12/1374.