The structure of scleractinian coral skeleton analyzed by neutron diffraction and neutron computed tomography

Tatiana I Ivankina,Octavian G Duliu,Mohamed M Sherif,Safa Y Abdo,Sergey E Kichanov

doi:10.1038/s41598-020-69859-2

Abstract

Two analytical methods based on the neutrons high penetrability, i.e. neutron diffraction (ND) and neutron computed tomography (NCT) were used to investigate the structure of the aragonitic skeleton of an exemplar/sample of Dipastraea pallida (Dana 1846), a modern hermatypic coral. ND was used to reconstruct the orientation distribution function (ODF) of the crystalline fibrils which compose the coral skeleton. Accordingly, 684 ND spectra were analyzed using the Rietveld method. The result confirmed the aragonite as the sole mineral component of coral skeleton, allowing to recalculate the ODF of aragonite fibrils and to represent it by means of (100), (010) and (001) crystallographic planes pole figures (PF). Experimental PF showed a remarkable similarity with PF recalculated by considering that all aragonite fibrils are oriented either along the growth axis of polyp cups or perpendicular to this direction. This result confirmed the previous observations based on optical microscopy, proving at the same time the availability of ND for such types of investigations. In turn, NCT evidenced the individual polyp cups, their interlocked 3D rigid porous structure as well as a periodic variation of density which could be attributed to a seasonal influence of the marine environment. Different from the classical X-ray computed tomography, the NCT, in view of neutron high cross-section for hydrogen, demonstrated the presence of a small amount of organic matter, otherwise transparent for X- and gamma rays.

Highlights

Two analytical methods based on the neutrons high penetrability, i.e. neutron diffraction (ND) and neutron computed tomography (NCT) were used to investigate the structure of the aragonitic skeleton of an exemplar/sample of Dipastraea pallida (Dana 1846), a modern hermatypic coral
The hydrogen pumps present on the cell walls pump hydrogen ions ( H+) out of this space to produce more carbonate ( CO32−) ions that bond with (Ca2+) ions existing in the seawater to produce and deposit solid calcium carbonate (CaCO3) for their exo-skeleton[4]
The use of TOF diffraction for texture measurements allows for recording of diffraction patterns and measuring several pole figures (PF) simultaneously

Summary

Introduction

Two analytical methods based on the neutrons high penetrability, i.e. neutron diffraction (ND) and neutron computed tomography (NCT) were used to investigate the structure of the aragonitic skeleton of an exemplar/sample of Dipastraea pallida (Dana 1846), a modern hermatypic coral. Experimental PF showed a remarkable similarity with PF recalculated by considering that all aragonite fibrils are oriented either along the growth axis of polyp cups or perpendicular to this direction This result confirmed the previous observations based on optical microscopy, proving at the same time the availability of ND for such types of investigations. The most representative members of the Hexacorallia subclass are stony corals (Order Scleractinia) that build themselves a hard exoskeleton consisting of thin spicule of aragonite (orthorhombic C aCO3) that linked together form a 3D porous network with a remarkable mechanical rigidity. In this case, the specialized calicoblast cells are responsible for deposition of aragonite outside the polyp. The size of such colonies can reach meters in height or d iameter[6]

Methods

Results

Conclusion