  Structural characterization of biologically formed materials is essential for
understanding biological phenomena and their environment, and generating new
bio-inspired engineering concepts. For example, nacre -- formed by mollusks in
the ocean -- encodes local environmental conditions throughout its formation
and has exceptional strength due to its nanoscale brick-and-mortar structure.
This layered structure, comprising transparent aragonite tablets bonded with an
ultra-thin organic polymer, also results in stunning interference colors.
Existing methods of structural characterization of nacre rely on some form of
cross-sectional analysis, such as scanning electron microscopy or
polarization-dependent imaging contrast (PIC) mapping. However, these
techniques are destructive and too time- and resource-intensive to analyze
large sample areas. Here we present an all-optical, rapid, and non-destructive
imaging technique -- hyperspectral interference tomography (HIT) -- to
spatially map the structural parameters of nacre and other disordered layered
materials. We combined hyperspectral imaging with optical-interference modeling
to infer the mean tablet thickness and disordering of nacre layers across
entire mollusk shells at various stages of development, observing a previously
unknown relationship between the growth of the mollusk and tablet thickness.
Our rapid, inexpensive, and nondestructive method can be readily applied to
in-field studies.
