Assignment of Polarization-Dependent Peaks in Carbon K-Edge Spectra from Biogenic and Geologic Aragonite

Nanoanalytical electron microscopy of events predisposing to mineralisation of turkey tendon

The macro- and micro-structures of mineralised tissues hierarchy are well described and understood. However, investigation of their nanostructure is limited due to the intrinsic complexity of biological systems. Preceding transmission electron microscopy studies investigating mineralising tissues have not resolved fully the initial stages of mineral nucleation and growth within the collagen fibrils. In this study, analytical scanning transmission electron microscopy and electron energy-loss spectroscopy were employed to characterise the morphology, crystallinity and chemistry of the mineral at different stages of mineralization using a turkey tendon model. In the poorly mineralised regions, calcium ions associated with the collagen fibrils and ellipsoidal granules and larger clusters composed of amorphous calcium phosphate were detected. In the fully mineralised regions, the mineral had transformed into crystalline apatite with a plate-like morphology. A change in the nitrogen K-edge was observed and related to modifications of the functional groups associated with the mineralisation process. This transformation seen in the nitrogen K-edge might be an important step in maturation and mineralisation of collagen and lend fundamental insight into how tendon mineralises.

Transformation of ACC into aragonite and the origin of the nanogranular structure of nacre

Currently a basic tenet in biomineralization is that biominerals grow by accretion of amorphous particles, which are later transformed into the corresponding mineral phase. The globular nanostructure of most biominerals is taken as evidence of this. Nevertheless, little is known as to how the amorphous-to-crystalline transformation takes place. To gain insight into this process, we have made a high-resolution study (by means of transmission electron microscopy and other associated techniques) of immature tablets of nacre of the gastropod Phorcus turbinatus, where the proportion of amorphous calcium carbonate is high. Tablets displayed a characteristic nanoglobular structure, with the nanoglobules consisting of an aragonite core surrounded by amorphous calcium carbonate together with organic macromolecules. The changes in composition from the amorphous to the crystalline phase indicate that there was a higher content of organic molecules within the former phase. Within single tablets, the crystalline cores were largely co-oriented. According to their outlines, the internal transformation front of the tablets took on a complex digitiform shape, with the individual fingers constituting the crystalline cores of nanogranules. We propose that the final nanogranular structure observed is produced during the transformation of ACC into aragonite.

Shell Structure and Growth in the Base Plate of the Barnacle Amphibalanus amphitrite

The base plate of the acorn barnacle Amphibalanus amphitrite (equivalent to Balanus amphitrite) is composed of hierarchically scaled, mutually aligned calcite grains, adhered to the substratum via layered cuticular tissue and protein. Acorn barnacles grow by expanding and lengthening their side plates, under which the cuticle is stretched, and adhesive proteins are secreted. In barnacles with mineralized base plates, such as A. amphitrite, a mineralization front follows behind, radially expanding the base plate at the periphery. In this study, we show that the new mineralization develops above the adhesion layers in a unique trilayered structure. Calcite crystallites in each of the layers have distinct sizes, varying from coarse-grained (>1 μm across) in the upper layer, to fine-grained (∼1 μm) in the middle layer, to nanoparticulate (∼40 nm) in the basal layer. The fine-grained crystallites dominate the growth front, comprising the bulk of the shell at the periphery, with later coarse grain development on the top of the base plate (toward the barnacle interior) and nanocrystalline calcite templating underneath in contact with the cuticle/protein layer. While the coarse-grained calcite on the upper surface contains a range of crystal orientations, the underlying fine-grained and nanocrystalline calcite are mutually oriented to within a few degrees of each other. Electron diffraction and X-ray absorption spectroscopy confirm that all of the crystallites are calcite, and metastable aragonite or amorphous calcium carbonate (ACC) phases are not observed. The complex morphology of the leading edge of the base plate suggests that crystallization initiates with the emplacement of mutually aligned fine-grained calcite, followed by the accumulation of coarser grains above and nucleation of highly oriented nanocrystalline grains below.

Probing carbonate in bone forming minerals on the nanometre scale

To devise new strategies to treat bone disease in an ageing society, a more detailed characterisation of the process by which bone mineralises is needed. In vitro studies have suggested that carbonated mineral might be a precursor for deposition of bone apatite. Increased carbonate content in bone may also have significant implications in altering the mechanical properties, for example in diseased bone. However, information about the chemistry and coordination environment of bone mineral, and their spatial distribution within healthy and diseased tissues, is lacking. Spatially resolved analytical transmission electron microscopy is the only method available to probe this information at the length scale of the collagen fibrils in bone. In this study, scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS) was used to differentiate between calcium-containing biominerals (hydroxyapatite, carbonated hydroxyapatite, beta-tricalcium phosphate and calcite). A carbon K-edge peak at 290 eV is a direct marker of the presence of carbonate. We found that the oxygen K-edge structure changed most significantly between minerals allowing discrimination between calcium phosphates and calcium carbonates. The presence of carbonate in carbonated HA (CHA) was confirmed by the formation of peak at 533 eV in the oxygen K-edge. These observations were confirmed by simulations using density functional theory. Finally, we show that this method can be utilised to map carbonate from the crystallites in bone. We propose that our calibration library of EELS spectra could be extended to provide spatially resolved information about the coordination environment within bioceramic implants to stimulate the development of structural biomaterials.

Carbon K-edge spectra of carbonate minerals

Carbon K-edge X-ray spectroscopy has been applied to the study of a wide range of organic samples, from polymers and coals to interstellar dust particles. Identification of carbonaceous materials within these samples is accomplished by the pattern of resonances in the 280-320 eV energy region. Carbonate minerals are often encountered in the study of natural samples, and have been identified by a distinctive resonance at 290.3 eV. Here C K-edge and Ca L-edge spectra from a range of carbonate minerals are presented. Although all carbonates exhibit a sharp 290 eV resonance, both the precise position of this resonance and the positions of other resonances vary among minerals. The relative strengths of the different carbonate resonances also vary with crystal orientation to the linearly polarized X-ray beam. Intriguingly, several carbonate minerals also exhibit a strong 288.6 eV resonance, consistent with the position of a carbonyl resonance rather than carbonate. Calcite and aragonite, although indistinguishable spectrally at the C K-edge, exhibited significantly different spectra at the Ca L-edge. The distinctive spectral fingerprints of carbonates provide an identification tool, allowing for the examination of such processes as carbon sequestration in minerals, Mn substitution in marine calcium carbonates (dolomitization) and serpentinization of basalts.

Nanotextures of aragonite in stromatolites from the quasi-marine Satonda crater lake, Indonesia

Stromatolites have been extensively used as indicators of ancient life on Earth. Although much work has been done on modern stromatolites, the extent to which biological processes control their structure, and the respective contributions of biological and abiotic processes in their formation are, however, still poorly constrained. A better description of the mineralogical textures of these formations at the submicrometre scale may help improve our understanding of how carbonates nucleate and grow in stromatolites. Here, we used a combination of microscopy and microspectroscopy techniques to study the chemical composition and the texture of aragonite in lacustrine stromatolites from the alkaline crator lake in Satonda, Indonesia. Several textural features are described, including morphological variations of aragonite from nanosized grains to micrometre-sized fibres, the presence of striations in the aragonite laminae showing a striking similarity with growth bands in corals, and clusters of small aragonite crystals sharing a common crystallographic orientation. These nanotextural features are compared with those observed in scleractinian corals, and possible processes involved in their formation are discussed.

The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

The sea urchin tooth is a remarkable grinding tool. Even though the tooth is composed almost entirely of calcite, it is used to grind holes into a rocky substrate itself often composed of calcite. Here, we use 3 complementary high-resolution tools to probe aspects of the structure of the grinding tip: X-ray photoelectron emission spectromicroscopy (X-PEEM), X-ray microdiffraction, and NanoSIMS. We confirm that the needles and plates are aligned and show here that even the high Mg polycrystalline matrix constituents are aligned with the other 2 structural elements when imaged at 20-nm resolution. Furthermore, we show that the entire tooth is composed of 2 cooriented polycrystalline blocks that differ in their orientations by only a few degrees. A unique feature of the grinding tip is that the structural elements from each coaligned block interdigitate. This interdigitation may influence the fracture process by creating a corrugated grinding surface. We also show that the overall Mg content of the tooth structural elements increases toward the grinding tip. This probably contributes to the increasing hardness of the tooth from the periphery to the tip. Clearly the formation of the tooth, and the tooth tip in particular, is amazingly well controlled. The improved understanding of these structural features could lead to the design of better mechanical grinding and cutting tools.