Original paper

Crystallographic variability and uniformity in Cretaceous heterococcoliths

Saruwatari, Kazuko; Tanaka, Yuichiro; Nagasawa, Hiromichi; Kogure, Toshihiro

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 (FESEM) 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 ai-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.

Keywords

biomineralizationcretaceous heterococcolithcalcitecrystallographic propertiesebsdsemstereo-photogrammetry