Aragonite relic preservation in Jurassic calcite‐replaced bivalves

Shells of the aragonite bivalve Neomiodon (Great Estuarine Group, Jurassic, Scotland) replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure. The presence of these relics in such old altered shells, as well as the high Sr content of the replacement calcite, indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events, but rather a rapid, one‐step process. Aragonite relics, once encased in neomorphic spar, will survive as unequivocal evidence of original aragonite mineralogy, barring total remobilization of the enclosing stable calcite, a generally unlikely event. The retention of this residual aragonite and high‐Sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration (‘recrystallization’) invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones. Retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or, as in the case of our Neomiodon material, deeper burial environments. Pseudopleochroism of the replaced Neomiodon shells appears to be due to organic, largely graphitic, relics, not to the aragonite relics.