Iodate in calcite, aragonite and vaterite CaCO<inf>3</inf>: Insights from first-principles calculations and implications for the I/Ca geochemical proxy

The incorporation of iodine into each of the three polymorphs of CaCO3 – calcite, aragonite and vaterite, is compared using first-principles computational simulation. In each case iodine is most easily accommodated as iodate (IO3-) onto the carbonate site. Local strain fields around the iodate solute atom are revealed in the pair distribution functions for the relaxed structures, which indicate that aragonite displays the greatest degree of local structural distortion while vaterite is relatively unaffected. The energy penalty for iodate incorporation is least significant in vaterite, and greatest in aragonite, with the implication that iodine will display significant partitioning between calcium carbonate polymorphs. in the order vaterite > calcite > aragonite. Furthermore, we find that trace iodine incorporation into vaterite confers improved mechanical strength to vaterite crystals. Our results support the supposition that iodine is incorporated as iodate within biogenic carbonates, important in the application of I/Ca data in palaeoproxy studies of ocean oxygenation. Our observation that iodate is most easily accommodated into vaterite implies that the presence of vaterite in any biocalcification process, be it as an end-product or a precursor, should be taken into account when applying the I/Ca geochemical proxy

Metamorphic temperature investigation of coexisting calcite and dolomite marble––examples from Nikani Ghar marble and Nowshera Formation, Peshawar Basin, Pakistan

Using marble samples from the Nikani Ghar marble and Nowshera Formation from Northern Pakistan the determination of the temperature of metamorphism was undertaken with the help of calcite-dolomite solvus geothermometer. Two types of marbles, that is, calcite-dolomite marble and quartz-bearing calcite-dolomite marble were selected. Petrographic and scanning electron microscope analysis of dolomite samples indicated different grain sizes. X-ray diffraction technique indicated the calcites MgCO₃ content up to 7.93 mol.%. Nikani Ghar marble samples have shown lower contents of MgCO₃ as compared to samples from Nowshera Formation. The calcite-dolomite-quartz marble has also showed relatively lower MgCO₃ content and hence rather low temperature (~500 °C). The temperature reached during peak metamorphism of the investigated marble occurrence, based on calcitedolomite solvus was 628 °C. Metamorphic temperatures derived from the present study were shown as a linear graph and values were in good agreement with the published literature.The authors acknowledge the financial support extended by the Higher Education Commission (HEC), Pakistan and National Academy of Sciences (USA), project ID 131, under the PAK-USA S & T Cooperation Program, Award (No. 0521315). The authors are grateful to the HEC, Pakistan for their support in the form of “International Research Support Initiative Program (IRSIP)” to conduct a part of research at Department of Earth Sciences, University of Cambridge, United 996 Muhammad Fahad, Yaseen Iqbal, Mohammad Riaz, Rick Ubic and Simon A. T. Redfern Kingdom. The financial support extended by the Directorate of S & T, KP regarding minerals upgradation is also acknowledged

Deep carbon cycle through five reactions

What are the key reactions driving the global carbon cycle in Earth, the only known habitable planet in the solar system? And how do chemical reactions govern the transformation and movement of carbon? The special collection “Earth in five reactions - A deep carbon perspective” features review articles synthesizing knowledge and findings on the role of carbon- related reactions in Earth's dynamics and evolution. These integrative studies identify gaps in our current understanding and establish new frontiers to motivate and guide future research in deep carbon science. The collection also includes original experimental and theoretical investigations of carbon-bearing phases and the impact of chemical and polymorphic reactions on Earth's deep carbon cycle.Sloan Foundatio

Synchrotron X-ray microscopy of marine calcifiers: How plankton record past climate change

© Published under licence by IOP Publishing Ltd. We have used STXM and PEEM to reveal the underpinning chemistry and nanoscale structure behind palaeo-climate geochemical signatures, such as trace Mg in shells- proposed proxies for palaeo-ocean temperature. This has allowed us to test the chemical assumptions and mechanisms underpinning the use of such empirical proxies. We have determined the control on driving chemical variations in biogenic carbonates using STXM at the absorption edge of Mg, B, and Na in the shells of modern plankton. The power of these observations lies in their ability to link changes in chemistry, microstructure, and growth process in biogenic carbonate to environmental influences. We have seen that such changes occur at length scales of tens of nanometres and demonstrated that STXM provides an invaluable route to understanding chemical environment and key heterogeneity at the appropriate length scale. This new understanding provides new routes for future measurements of past climate variation in the sea floor fossil record

Ternary hypervalent silicon hydrides via lithium at high pressure

Hydrogen is rarely observed as ligand in hypervalent species, however, we find that high-pressure hydrogenation may stabilise hypervalent hydrogen-rich materials. Focussing on ternary silicon hydrides via lithium doping, we find anions composed of hypervalent silicon with H ligands formed under high pressure. Our results reveal two new hypervalent anions: layered-SiH−5 and tricapped triangular prismatic SiH2−. These differ from octahedral SiH2− described in earlier studies. In addition, there are further hydrogen-rich structures, Li3SiH10 and Li2SiH6+δ, which may be stabilised at high pressure. Our work provides pointers to future investigations on hydrogen rich materials

Mineralogical and geochemical characteristics of triassic lithium-rich K-Bentonite deposits in Xiejiacao section, South China

Widespread alteration in the Early–Middle Triassic volcanic ash of the Xiejiacao section, south China, has resulted in significant occurrences of lithium-rich K-bentonite deposits with economic potential. Detailed mineralogical and geochemical investigations of Li-rich K-bentonite deposits from the Xiejiacao section of Guangan city, South China, are presented here. The X-ray diffraction (XRD) data and major element chemistry indicates that the Li-rich K-bentonite deposits contain quartz, clay minerals, feldspar, calcite and dolomite, and the clay minerals are dominated by illite and ordered (R3) illite/smectite (I/S). The concentrations of major and trace elements in Li-rich K-bentonite deposits altered from volcanic ashes are most likely derived from felsic magmas, associated with intense volcanic arc activity. The composition of the clay components suggests that the Li-rich K-bentonite deposits are probably altered from the smectite during diagenesis, whereas smectite is mainly formed by submarine alterations of volcanic materials and subsequently the I/S derived from the volcanogenic smectite illitization. Moreover, accurate determination of the structure in I/S reveals that the temperatures reached by the sedimentary series are around 180 °C with a burial depth of ~6000 m. The widely distributed lithium-rich clay deposits strongly indicate widespread eruptions of volcanic ashes in the Early–Middle Triassic, which released huge amounts of volcanic ash. Lithium fixed in the illite and I/S is considered to have leached from the volcanogenic products by a mixed fluid source (i.e., meteoric, porewater and hydrothermal fluids). These Li-rich clay minerals in the marine basin contain economically extractable levels of metal and are a promising new target for lithium exploration.</jats:p

Pressure-Induced Amorphization of Small Pore Zeolites-the Role of Cation-H2O Topology and Anti-glass Formation

Systematic studies of pressure-induced amorphization of natrolites (PIA) containing monovalent extra-framework cations (EFC) Li+, Na+, K+, Rb+, Cs+ allow us to assess the role of two different EFC-H2O configurations within the pores of a zeolite: one arrangement has H2O molecules (NATI) and the other the EFC (NAT(II)) in closer proximity to the aluminosilicate framework. We show that NAT(I) materials have a lower onset pressure of PIA than the NAT(II) materials containing Rb and Cs as EFC. The onset pressure of amorphization (P-A) of NAT(II) materials increases linearly with the size of the EFC, whereas their initial bulk moduli (P-1 phase) decrease linearly. Only Cs- and Rb-NAT reveal a phase separation into a dense form (P-2 phase) under pressure. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) imaging shows that after recovery from pressures near 25 and 20 GPa long-range ordered Rb-Rb and Cs-Cs correlations continue to be present over length scales up to 100 nm while short-range ordering of the aluminosilicate framework is significantly reduced-this opens a new way to form anti-glass structuresopen

The distribution and coordination of trace elements in Krithe ostracods and their implications for paleothermometry

The Mg and Sr content of ostracod valves have been used to reconstruct past temperature and salinity, and their stable isotopes have been used to reveal aspects of marine, lake and estuary hydrology. However, significant uncertainties surround the ostracod calcification processes, the incorporation mechanisms of trace elements, and the sensitivity of proxy tracers to complex confounding factors. The valves of most ostracods are composed of micron-scale crystal grains embedded in an organic matrix. The fine-scale geochemistry of these structures, and the nature of the influence of biological mineralisation processes on valve chemistry remain poorly constrained. We have performed sub-micron resolution X-ray microscopy of a marine Krithe ostracod valve, and determined the chemical coordination of Mg, and the distribution of Mg, Na and S throughout the crystal-organic valve structure. These trace elements display systematic sub-micron-scale compositional variations within the mineral grains and iner-granular matrix of the valve ultrastructure. These patterns imply that Krithe biomineralisation processes significantly modulate trace element incorporation at the sub-micron scale. Thus Krithe chemical composition is likely to be decoupled to some extent from the water in which they calcified. Most importantly, Mg K-edge Near- Edge X-Ray Absorption Fine Structure (NEXAFS) spectra, and the coincidence of high-Mg regions with S-rich organic layers reveal that Mg is not primarily hosted in the calcite structure in the valve. Our results highlight the need to understand the processes that drive this fine-scale chemical heterogeneity and their influence on connections between the external environment and valve geochemistry, if ostracods are to be used as sources of paleoenvironmental proxies

Carbon network evolution from dimers to sheets in superconducting ytrrium dicarbide under pressure

Carbon-bearing compounds display intriguing structural diversity, due to variations in hybrid bonding of carbon. Here, first- principles calculations and unbiased structure searches on yttrium dicarbide at pressure reveal four new structures with varying carbon polymerisation ， in addition to the experimentally-obsersed high- temperature low-pressure I4/mmm dimer phase. At low pressures, a metallic C2/m phase (four-member single chain carbide) is stable, which transforms into a Pnma phase (single chain carbide) upon increasing pressure, with further transformation to an Immm structure (double chain carbide) at 54 GPa and then to a P6/mmm phase (sheet carbide) at 267 GPa. Yttrium dicarbide is structurally diverse, with carbon bonded as dimers (at lowest pressure), four- member single chains, infinite single chains, double chains and eventually sheet structures on compression. Electron-phonon coupling calculations indicate that the high-pressure phases are superconducting. Our results aid the understanding and design of new superconductors and illuminate pressure-induced carbon polymerisation in carbides.NERC (NE/P012167/1) EPSRC (EP/P022596/1

Low-density superhard materials: Computational study of Li-inserted B-substituted closo-carboranes LiBC<inf>11</inf> and Li<inf>2</inf>B<inf>2</inf>C<inf>10</inf>

Insertion of Li atoms into a B-substituted carbon cage produces two superhard compounds with relatively low density: LiBC11 and Li2B2C10.X. F. and Q. L. acknowledge the funding supports from the National Natural Science Foundation of China under Grant No. 11474125, 11274136 and 11534003. J. T. and H. L. acknowledge the National Science Foundation of China (11474126) and support from the University of Saskatchewan research computing group and the use of the HPC resources (Plato machine).This is the author accepted manuscript. The final version is available from the Royal Society Of Chemistry via http://dx.doi.org/10.1039/C6RA10177