Structural control and Ostwald ripening as essential mechanisms during the fossilization process of sea urchin (Balanocidaris? Lambert) spines from Lower Cretaceous of Southern Spain

Abstract

ABSTRACT A comparative study has been made between some cidaroid fossil spines from Lower Cretaceous of southern Spain and other contemporary ones. The modern spines are a composite material formed by calcite nanocrystals oriented with their optical axes parallel to the elongated direction, which behave as a single crystal. Transmission Petrographic Microscopy (TPM) observations of the fossil spines prove that it is a calcite single crystal, whose optical axis coincides with the direction of elongation (as in the current spines). This paper presents a crystallographic approach to propose a secondary crystallisation mechanism, specifically the Ostwald ripening process, through a structural analysis that compares the spines and the conditions observed during the fossilisation process.