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 (CaCO₃) 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 (CaCO₃) 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

Experiments and Modeling of Volumetric Properties and Phase Behavior for Condensate Gas under Ultra-High-Pressure Conditions

Four reservoir samples under ultra-high-pressure and high-temperature conditions were collected from condensate gas fields in China. Constant-composition expansion tests were performed to determine the phase behavior and volumetric properties of reservoir fluid using an ultra-high-pressure fluid <i>PVT</i> test system. The compressibility factor and dew-point pressure were obtained at four temperatures for four samples. The range of pressure was from 22.03 to 118.89 MPa. For the samples studied, the experimental results showed that the dew-point pressure decreased with increasing temperature and the compressibility factors increased with increasing pressure but decreased with increasing temperature at a given high reduced pressure. A thermodynamic model based on an equation of state was developed to describe the volumetric properties and phase behavior of the condensate gas under ultra-high-pressure conditions. The calculated results are in good accordance with the experimental data, which is important for the development of condensate gas reservoirs in ultra-high-pressure environments

Parrotfish Teeth: Stiff Biominerals Whose Microstructure Makes Them Tough and Abrasion-Resistant To Bite Stony Corals

Parrotfish (<i>Scaridae</i>) feed by biting stony corals. To investigate how their teeth endure the associated contact stresses, we examine the chemical composition, nano- and microscale structure, and the mechanical properties of the steephead parrotfish <i>Chlorurus microrhinos</i> tooth. Its enameloid is a fluorapatite (Ca₅(PO₄)₃F) biomineral with outstanding mechanical characteristics: the mean elastic modulus is 124 GPa, and the mean hardness near the biting surface is 7.3 GPa, making this one of the stiffest and hardest biominerals measured; the mean indentation yield strength is above 6 GPa, and the mean fracture toughness is ∼2.5 MPa·m^1/2, relatively high for a highly mineralized material. This combination of properties results in high abrasion resistance. Fluorapatite X-ray absorption spectroscopy exhibits linear dichroism at the Ca L-edge, an effect that makes peak intensities vary with crystal orientation, under linearly polarized X-ray illumination. This observation enables polarization-dependent imaging contrast mapping of apatite, a method to quantitatively measure and display nanocrystal orientations in large, pristine arrays of nano- and microcrystalline structures. Parrotfish enameloid consists of 100 nm-wide, microns long crystals co-oriented and assembled into bundles interwoven as the warp and the weave in fabric and therefore termed fibers here. These fibers gradually decrease in average diameter from 5 μm at the back to 2 μm at the tip of the tooth. Intriguingly, this size decrease is spatially correlated with an increase in hardness