From pristine aragonite to blocky calcite: Exceptional preservation and diagenesis of cephalopod nacre in porous Cretaceous limestones
Fig 6
Models of structural alterations of cephalopod nacreous layer based on SEM and thin sections observations described in this paper (left column) and previous studies [28,58,59].
Arrows indicate various diagenetic pathways of aragonite. (A) Originally preserved nacre: superimposed, flat aragonite tablets form columnar units that locally may overlap. Thin organic layers separate individual tablets of nacre. (B-E) Different stages of aragonite inversion to calcite. (B) Tablets slightly lenticular in shape: degradation of organics, dissolution of aragonite, and crystallization of secondary calcite occur mainly along the boundaries between nacre columns; minor alterations occur on the lower and upper faces of tablets; dissolution exposes vertical connections between superposed tablets–mineral bridges, originally surrounded by interlamellar organics (see Fig 2L and 2O). (C) Fragments of columnar stacks of nacre tablets separated by secondary calcite. (D) Parallel lath-shaped inclusions of aragonite enclosed by calcite; the outlines of original stacks of tablets are still recognizable (enlargement of E). (E) Linearly arranged impurities (thinned tablets of aragonite) cross crystal boundaries of secondary, blocky calcite (similar impurities were previously observed by Dullo and Bandel [28] Text Fig. 14 but interpreted as relics of growth lines, not nacre tablets). (F) Blocky calcite with no traces of aragonite: primary aragonite could be neomorphically transformed to calcite or completely dissolved with resulting space filled with calcite spar. (G) Fusion of adjacent aragonite tablets that occurs after dissolution of organic boundaries between tablets (described from Plio-Pleistocene aragonite mollusks; [58]; see also [60]). (H, I) “Permineralization”–secondary minerals filled spaces between tablets of nacre (previously occupied by organic matter); (H) aragonite nacre tablets preserved [58]; (I) aragonite tablets dissolved or replaced by other mineral (see [59]). (J) Porous “Swiss cheese” structure–the nacreous layer severely damaged by bioerosion [28]. (K) Coarse prism of calcite [28].