15 research outputs found
Narrowly distributed crystal orientation in biomineral vaterite
Biominerals formed by animals provide skeletal support, and many other
functions. They were previously shown to grow by aggregation of amorphous
nanoparticles, but never to grow ion-by-ion from solution, which is a common
growth mechanism for abiotic crystals. We analyze vaterite CaCO3 multi
crystalline spicules from the solitary tunicate Herdmania momus, with
Polarization dependent Imaging Contrast PIC mapping, scanning and aberration
corrected transmission electron microscopies. The first fully quantitative PIC
mapping data, presented here, measured 0{\deg} 30{\deg} angle spreads between
immediately adjacent crystals. Such narrowly distributed crystal orientations
demonstrate that crystallinity does not propagate from one crystal to another
0{\deg} angle spreads, nor that new crystals with random orientation 90{\deg}
nucleate. There are no organic layers at the interface between crystals, hence
a new, unknown growth mechanism must be invoked, with crystal nucleation
constrained within 30{\deg}. Two observations are consistent with crystal
growth from solution: vaterite microcrystals express crystal faces, and are
smooth at the nanoscale after cryo fracture. The observation of 30{\deg} angle
spreads, lack of interfacial organic layers, and smooth fracture figures
broadens the range of known biomineralization mechanisms and may inspire novel
synthetic crystal growth strategies. Spherulitic growth from solution is one
possible mechanism consistent with all these observations.Comment: Chemistry of Materials 201
Nacre tablet thickness records formation temperature in modern and fossil shells
Nacre, the iridescent outer lining of pearls and inner lining of many mollusk shells, is composed of periodic, parallel, organic sheets alternating with aragonite (CaCO_3) tablet layers. Nacre tablet thickness (TT) generates both nacre's iridescence and its remarkable resistance to fracture. Despite extensive studies on how nacre forms, the mechanisms controlling TT remain unknown, even though they determine the most conspicuous of nacre's characteristics, visible even to the naked eye.
Thermodynamics predicts that temperature (T) will affect both physical and chemical components of biomineralized skeletons. The chemical composition of biominerals is well-established to record environmental parameters, and has therefore been extensively used in paleoclimate studies. The physical structure, however, has been hypothesized but never directly demonstrated to depend on the environment. Here we observe that the physical TT in nacre from modern and fossil shallow-water shells of the bivalves Pinna and Atrina correlates with T as measured by the carbonate clumped isotope thermometer. Based on the observed TT vs. T correlation, we anticipate that TT will be used as a paleothermometer, useful to estimate paleotemperature in shallow-water paleoenvironments. Here we successfully test the proposed new nacre TT thermometer on two Jurassic Pinna shells. The increase of TT with T is consistent with greater aragonite growth rate at higher T, and with greater metabolic rate at higher T. Thus, it reveals a complex, T-dependent biophysical mechanism for nacre formation
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Narrowly Distributed Crystal Orientation in Biomineral Vaterite
Biominerals formed by animals provide skeletal support and many other functions. They were previously shown to grow by aggregation of amorphous nanoparticles but never to grow ion-by-ion from solution, which is a common growth mechanism for abiotic crystals. We analyze vaterite (CaCO3) multicrystalline spicules from the solitary tunicate Herdmania momus, with polarization-dependent imaging contrast (PIC)-mapping and scanning and aberration-corrected transmission electron microscopies. The first fully quantitative PIC-mapping data, presented here, measured 0-30°angle spreads between immediately adjacent crystals. Such narrowly distributed crystal orientations demonstrate that crystallinity does not propagate from one crystal to another (0°angle spreads), nor that new crystals with random orientation (90°) nucleate. There are no organic layers at the interface between crystals; hence, a new, unknown growth mechanism must be invoked, with crystal nucleation constrained within 30°. Two observations are consistent with crystal growth from solution: vaterite microcrystals express crystal faces and are smooth at the nanoscale after cryo-fracture. The observation of 30° angle spreads, lack of interfacial organic layers, and smooth fracture figures broadens the range of known biomineralization mechanisms and may inspire novel synthetic crystal growth strategies. Spherulitic growth from solution is one possible mechanism consistent with all these observations
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Nanoscale Transforming Mineral Phases in Fresh Nacre
Nacre,
or mother-of-pearl, the iridescent inner layer of many mollusk
shells, is a biomineral lamellar composite of aragonite (CaCO<sub>3</sub>) and organic sheets. Biomineralization frequently occurs
via transient amorphous precursor phases, crystallizing into the final
stable biomineral. In nacre, despite extensive attempts, amorphous
calcium carbonate (ACC) precursors have remained elusive. They were
inferred from non-nacre-forming larval shells, or from a residue of
amorphous material surrounding mature gastropod nacre tablets, and
have only once been observed in bivalve nacre. Here we present the
first direct observation of ACC precursors to nacre formation, obtained
from the growth front of nacre in gastropod shells from red abalone
(<i>Haliotis rufescens</i>), using synchrotron spectromicroscopy.
Surprisingly, the abalone nacre data show the same ACC phases that
are precursors to calcite (CaCO<sub>3</sub>) formation in sea urchin
spicules, and not proto-aragonite or poorly crystalline aragonite
(pAra), as expected for aragonitic nacre. In contrast, we find pAra
in coral
Oxygen Spectroscopy and Polarization-Dependent Imaging Contrast (PIC)-Mapping of Calcium Carbonate Minerals and Biominerals
X-ray
absorption near-edge structure (XANES) spectroscopy and spectromicroscopy
have been extensively used to characterize biominerals. Using either
Ca or C spectra, unique information has been obtained regarding amorphous
biominerals and nanocrystal orientations. Building on these results,
we demonstrate that recording XANES spectra of calcium carbonate at
the oxygen K-edge enables polarization-dependent imaging contrast
(PIC) mapping with unprecedented contrast, signal-to-noise ratio,
and magnification. O and Ca spectra are presented for six calcium
carbonate minerals: aragonite, calcite, vaterite, monohydrocalcite,
and both hydrated and anhydrous amorphous calcium carbonate. The crystalline
minerals reveal excellent agreement of the extent and direction of
polarization dependences in simulated and experimental XANES spectra due to X-ray linear dichroism. This effect is particularly strong
for aragonite, calcite, and vaterite. In natural biominerals, oxygen
PIC-mapping generated high-magnification maps of unprecedented clarity from nacre and prismatic
structures and their interface in Mytilus californianus shells. These maps revealed blocky aragonite
crystals at the nacre–prismatic boundary and the narrowest
calcite needle-prisms. In the tunic spicules of Herdmania
momus, O PIC-mapping revealed the size and arrangement
of some of the largest vaterite single crystals known. O spectroscopy
therefore enables the simultaneous measurement of chemical and orientational
information in CaCO<sub>3</sub> biominerals and is thus a powerful
means for analyzing these and other complex materials. As described
here, PIC-mapping and spectroscopy at the O K-edge are methods for
gathering valuable data that can be carried out using spectromicroscopy
beamlines at most synchrotrons without the expense of additional equipment