First-principle partitioning and disequilibrium of chromium in garnet – clinopyroxene assemblage

<p><span>Using partition coefficients is extremely useful to model melting processes and fluid-rock interactions. However, partition coefficients values remain scarce in regard of their sensitivity to mineral composition and to the variability of mineral composition. In addition, the inferred equilibrium between phases is not necessarily reached, even in high-grade metamorphic conditions associated to melting. Disequilibrium may dramatically hamper the effective mobility of species and lead to element distribution far from the predicted values.</span></p><p><span>This contribution aims at estimating partition coefficients for chromium (Cr) between garnet and clinopyroxene, and testing them in natural rocks of various metamorphic grades. As a poorly mobile trivalent element, Cr is chosen as a proxy to rare earth elements.</span></p><p><span>Theoretical partition coefficients for Cr between garnet and clinopyroxene are calculated </span><span><em>ab initio</em></span><span> from structures where Cr</span><sup><span>3+</span></sup><span> is modelled as a defect in Al</span><sup><span>3+</span></sup><span> sites using CRYSTAL17 (Dovesi et al., 2014) and the thermodynamic description of Dubacq and Plunder (2018). Results are compared to electron microprobe measurements in mineral assemblages containing tens to thousands of ppm of Cr, where element mapping brings much information. </span></p><p><span>Results of </span><span><em>ab initio</em></span><span> computations highlight the role of crystal-chemistry over the strain field around point defects, controlling the dynamics of the Cr</span><sup><span>3+</span></sup><span> = Al</span><sup><span>3+ </span></sup><span>exchange between clinopyroxene and garnet. As expected, the partitioning of Cr between garnet and clinopyroxene depends strongly on the grossular and pyrope content: Cr incorporates grossular preferentially to jadeite, but jadeite incorporates Cr preferentially to pyrope.</span></p><p><span>Comparison between predicted and measured partition coefficients allowed to estimate the deviation from equilibrium. Disequilibrium is evidenced even for samples metamorphosed around 850°C, as shown by the distribution of Cr-rich and Cr-depleted domains. Disequilibrium is attributed to slow diffusivity of Cr in fluid and at grain boundaries during crystal growth, leading to interface-coupled dissolution-precipitation.</span></p><p><span></span></p><p align="justify"><span>Dovesi, R., Orlando, R., Erba, A., Zicovich‐Wilson, C. M., Civalleri, B., Casassa, S., ... & </span><span>Noël, Y. (2014). CRYSTAL14: A program for the ab initio investigation of crystalline solids. </span><span><em>International Journal of Quantum Chemistry</em></span><span>, </span><span><em>114</em></span><span>(19), 1287-1317.</span></p><p><span>Dubacq, B., & Plunder, A. (2018). Controls on trace element distribution in oxides and silicates. </span><span><em>Journal of Petrology</em></span><span>, </span><span><em>59</em></span><span>(2), 233-256.</span></p>

How Plants Handle Trivalent (+3) Elements

Plant development and fitness largely depend on the adequate availability of mineral elements in the soil. Most essential nutrients are available and can be membrane transported either as mono or divalent cations or as mono- or divalent anions. Trivalent cations are highly toxic to membranes, and plants have evolved different mechanisms to handle +3 elements in a safe way. The essential functional role of a few metal ions, with the possibility to gain a trivalent state, mainly resides in the ion’s redox activity; examples are iron (Fe) and manganese. Among the required nutrients, the only element with +3 as a unique oxidation state is the non-metal, boron. However, plants also can take up non-essential trivalent elements that occur in biologically relevant concentrations in soils. Examples are, among others, aluminum (Al), chromium (Cr), arsenic (As), and antimony (Sb). Plants have evolved different mechanisms to take up and tolerate these potentially toxic elements. This review considers recent studies describing the transporters, and specific and unspecific channels in different cell compartments and tissues, thereby providing a global vision of trivalent element homeostasis in plants.

The synthesis of a silicalcyanide and of a felspar

In the course of an investigation which has occupied much time during some years the writer has obtained a considerable number of definite compounds including silicon and the nitrogen of diverse organic groups in direct chemical union. Several of these new substances resemble in composition and in their general relations certain well-known compounds of carbon with nitrogen, such as amides, imides, and nitriles, among them being a silicocyanogen group, SiN, in combination. The formation of such substances afforded complete proof that silicon has, like carbon, though in less degree, a marked affinity for trivalent nitrogen, even when the latter is associated with complex organic groups. In the mineral kingdom no definite compounds of silicon with nitrogen have yet been met with; nor are they likely to be found at any part of the earth's surface to which water has easy access, as it is probable that any such substances would be very speedily decomposed in presence of moisture into silica and ammonia, or their derivatives. On the other hand, the existence of the great group of "alumino-silicates," which constitute so large a proportion of granitic and other similar rocks, affords clear evidence of the strong attraction of silicon for aluminium, which latter acts as an essentially trivalent element in its high temperature relations, and seems capable under such conditions of doing chemical work somewhat similar to the nitrogen. These considerations raised the question whether some at least of the alumino-silicated may not be regarded as fully oxidised products of silicides of trivalent aluminium, somewhat analogous to SiN, which had been formed at high temperatures in the first instance below the oxidised crust of the earth. It is evident that such "nuclear" silicides should be obtained for study by the complete deoxidation of the corresponding native silicates, or by direct synthesis from the sutiable elements. The first method was found to be impracticable in the most important cases, i. e. of alumino-silicates including alkali or alkaline earth metals. The application of the second or synthetic method has proved satisfactory, as it has led to the discovery of substance or remarkable stability, provisionally named Calcium Silicalcyanide, and with which a new synthesis of the felspar Anorthite has been effected. The results recorded in the following pages tend to support the view suggested above as to the nitrogen rôle of aluminium in certain silicides, and afford some further clues to the constitution and natural relations of the plagioclasic felspars.