No ion is an island: Multiple ions involved during boron incorporation into CaCO3

<p>Boron isotope ratios, as measured in marine calcium carbonate, are a proven tracer of past seawater and calcifying fluid pH and thus a powerful tool for the reconstruction of past atmospheric CO<sub>2</sub> and monitoring of coral physiology. For such applications, understanding the inorganic baseline upon which foraminiferal vital effects or coral pH upregulation are superimposed should be an important prerequisite. Yet, investigations into boron isotope fractionation in synthetic CaCO<sub>3 </sub>polymorphs have often reported variable and even conflicting results, implying that we may not fully understand pathways of boron incorporation into calcium carbonate.  Here we address this topic with experimental data from calcite and aragonite precipitated across a range of pH in the presence of both Mg and Ca. We confirm the results of previous studies that the boron isotope composition of inorganic aragonite precipitates closely reflects that of aqueous borate ion, but that calcites display a higher degree of scatter, and diverge from the boron isotope composition of borate ion at low pH. We discuss these findings with reference to the simultaneous incorporation of other trace and minor elements, and highlight a number of mechanisms by which crystal growth mechanisms may influence the concentration and isotope composition of boron in CaCO<sub>3</sub>. In particular, we highlight the potential importance of surface electrostatics in driving variability in published synthetic carbonate datasets. Importantly for palaeo-reconstruction, however, these electrostatic effects are likely to play a much more minor role during natural precipitation of biogenic carbonates.</p>

Partitioning of chromium between garnet and clinopyroxene: first-principle modelling versus metamorphic assemblages

Understanding the geochemical behaviour of trace and minor elements in mineral assemblages is of primary importance to study small- and large-scale geological processes. Partition coefficients are frequently used to model the chemical evolution of minerals and fluids during melting and in metamorphic rocks of all grades. However, kinetic effects hampering equilibrium partitioning may invalidate the modelling. This study aims at calculating partition coefficients and testing their applicability in natural mineral assemblages, choosing Cr in garnet and clinopyroxene via exchange with Al as a case study. First-principle modelling has been combined with measurements and element mapping to estimate partition coefficients for Cr and the deviation from equilibrium. Results highlight the role of crystal chemistry over the strain field around point defects, controlling the dynamics of the Cr3+ = Al3+ exchange between clinopyroxene and garnet. Ab initio calculations allowed estimation of Cr partition coefficients between garnet and clinopyroxene, using a thermodynamic approach based on endmembers and mixing models simplified for trace element behaviour. The Cr3+ = Al3+ exchange reaction between garnet and the jadeite component of clinopyroxene depends on the grossular and pyrope content, with Cr preferentially incorporated into grossular over jadeite but preferentially incorporated into jadeite over pyrope. Comparison of predicted partition coefficients to measured concentrations in natural samples, together with element mapping, shows large disequilibrium. Cr-rich and Cr-poor sectors exhibit disequilibrium attributed to slow diffusivity of Cr during crystal growth and interface-coupled dissolution–precipitation, even for garnet–clinopyroxene assemblages crystallized around 850 ∘C.

An evolutionary system of mineralogy. Part I: Stellar mineralogy (>13 to 4.6 Ga)

Minerals preserve records of the physical, chemical, and biological histories of their origins and subsequent alteration, and thus provide a vivid narrative of the evolution of Earth and other worlds through billions of years of cosmic history. Mineral properties, including trace and minor elements, ratios of isotopes, solid and fluid inclusions, external morphologies, and other idiosyncratic attributes, represent information that points to specific modes of formation and subsequent environmental histories—information essential to understanding the co-evolving geosphere and biosphere. This perspective suggests an opportunity to amplify the existing system of mineral classification, by which minerals are defined solely on idealized end-member chemical compositions and crystal structures. Here we present the first in a series of contributions to explore a complementary evolutionary system of mineralogy—a classification scheme that links mineral species to their paragenetic modes. The earliest stage of mineral evolution commenced with the appearance of the first crystals in the universe at >13 Ga and continues today in the expanding, cooling atmospheres of countless evolved stars, which host the high-temperature (T > 1000 K), low-pressure (P < 10-2 atm) condensation of refractory minerals and amorphous phases. Most stardust is thought to originate in three distinct processes in carbon- and/or oxygen-rich mineral-forming stars: (1) condensation in the cooling, expanding atmospheres of asymptotic giant branch stars; (2) during the catastrophic explosions of supernovae, most commonly core collapse (Type II) supernovae; and (3) classical novae explosions, the consequence of runaway fusion reactions at the surface of a binary white dwarf star. Each stellar environment imparts distinctive isotopic and trace element signatures to the micro- and nanoscale stardust grains that are recovered from meteorites and micrometeorites collected on Earth’s surface, by atmospheric sampling, and from asteroids and comets. Although our understanding of the diverse mineral-forming environments of stars is as yet incomplete, we present a preliminary catalog of 41 distinct natural kinds of stellar minerals, representing 22 official International Mineralogical Association (IMA) mineral species, as well as 2 as yet unapproved crystalline phases and 3 kinds of non-crystalline condensed phases not codified by the IMA.

Laser Induced Breakdown Spectroscopy and PIXE for Differentiation Between Different Tungsten Alloys

Tungsten is one of the hardest metals that has high melting point and high thermal conductivity. These unique properties make it suitable for many industrial applications. The increasing demand for using tungsten made the need for a fast and reliable analytical technique for tungsten to increase. In this paper we are comparing the ability of LIBS as a multi-elemental analysis technique to PIXE which is a well known established multi-elemental technique in the analysis of tungsten alloys. It was found that LIBS has the advantage over PIXE in the detection of the trace and minor elements. While PIXE is better than LIBS in the detection of major elements in the samples.