posted on 2022-04-30, 01:43authored byMarie Albéric, Emil Zolotoyabko, Oliver Spaeker, Chenghao Li, Maryam Tadayon, Clemens N.Z. Schmitt, Yael Politi, Luca Bertinetti, Peter Fratzl
Biomineralized structures
with intricate shapes and morphologies,
such as sea urchin skeletal elements, grow via the deposition of hydrated
amorphous calcium carbonate (ACC) particles that subsequently crystallizes
into single-crystalline calcite. This process is accompanied by volume
changes due to density differences between the initial and final mineral
state as well as variations in hydration levels. For this reason,
the presence of macroporosity in synthetic systems was shown to be
pivotal in the formation of large single crystals through ACC precursors.
However, the role of macroporosity down to nanoporosity in the formation
of biogenic minerals remains unknown. Here, we investigate the micro-
and nano-porosity as well as the evolution of internal interfaces
in the spines and test plates of Paracentrotus lividus sea urchins during the heat-mediated crystallization of remnant
ACC and the destruction of intracrystalline organic molecules, using
SEM, FIB-SEM, and in situ heating synchrotron SAXS
measurements. We show the presence of nanopores likely filled with
hydrated organics and visualize the evolution of nano- to micro-pores
induced by heating, which may serve to accommodate the volume changes
between amorphous and crystalline phases. The obtained results analyzed
using thermodynamical considerations suggest that the growth in size
of the nanopores is controlled by Ostwald ripening and is well described
in the framework of classical pore coarsening theories. The extracted
activation energies manifest that nanopore coarsening in the test
plates is governed by surface diffusion, whereas in the spines by
bulk diffusion. We suggest that such striking differences in diffusion
mechanisms are caused by dissimilar levels of macroporosity and distributions
of nano- and micro-internal interfaces in pristine biominerals.