Morphological and Crystallographic Transformation from Immature to Mature Coccoliths, Pleurochrysis carterae

Abstract

Morphology and crystallographic orientations of coccoliths, Pleurochrysis carterae, at the various growth stages were investigated using electron back-scattered diffraction analyses and scanning electron microscope (SEM) stereo-photogrammetry to understand the developments of two different coccolith units, namely V and R units. SEM observation indicates that the immature coccolith units at the earliest stage were not perfectly fixed on the organic base plates and several units were often lacked. The all units showed platy morphology and often lay parallel to the organic base plate. Their crystal orientations were close to that of the mature R units. With further growth, the platy morphology changes to a trapezoid to anvil-shape for both units, resulting in the interlocking structure of VR units. Morphological analyses present that the edges of the platy crystals parallel to the organic base plate were estimated as $<48\;\overline 1>$, and their inner/upper surfaces were estimated as $\{ 10\;\overline 1 \,4\}$. As they interlocked further, R units inclined more outward to develop the inner tube elements with $\{ 10\;\overline 1 \,4\}$ and then each unit develops differently distal and proximal shield elements, which are respectively estimated as $\{ 10\;\overline 1 \,4\}$ in the distal view and $\{ 2\,\overline 1 \;\overline 1 \,0\}$ planes in the proximal view. Based on the above results, the formation of different coccolith units and their growth were discussed.