Crystallographic variability and uniformity in Cretaceous heterococcoliths

Abstract

The crystallographic orientations of calcite in several major groups of Cretaceous heterococcoliths in chalk from Dover, England, were investigated using electron backscatter diffraction (EBSD) and field-emission scanning electron microscope (FE-SEM) stereo-photogrammetry. The previous crystallographic classification of coccoliths according to sub-vertical and sub-radial orientations of the c -axis (V-units and R-units, respectively) were further divided into several types according to the orientations and inclination angles of c - and a i -axes. As a common property, all units were likely aligned with the calcite direction along coccolith rings. Most units of heterococcoliths developed calcite rhombohedral surface {1 0 1 4} planes, while V-units of imbricating muroliths and R-units of a placolith developed calcite {2 1 1 0} planes. The results indicate the crystallographic variability corresponding with the morphological variations during the 230 million years of heterococcolith history; however, the morphologies and crystallographic properties are constrained by the alignment of the calcite direction along the coccolith ring.