Some Features of Quantitative Analysis of Rock-Forming Minerals Using a JXA-8230 Electron Probe Microanalyzer

—The basic software package of a JXA-8230 microanalyzer, like its predecessor JXA-8100, uses the long-established ZAF correction method (with some differences) for a quantitative analysis: Calculation of mass absorption coefficients is based on Chantler’s theoretical data. The core of this method is quantum-mechanical calculation of the cross section of the interaction between an X-ray photon and atomic electrons. This innovation has had a positive influence on the trueness of X-ray microanalysis. Control tests on specimens where the absorption effect is dominant have demonstrated that the results of this analysis are slightly lower (by less than 2%) independently of the matrix absorption interval in which the analytical line is located. As a consequence, the selection of comparison specimens becomes easier: It is sufficient that the specimen under study and the comparison specimen belong to the same isomorphic series and that the intensity of the analytical line of the comparison specimen allows for the measurement with the required accuracy.

Maghemite in Brazilian Iron Ores: Quantification of the Magnetite-Maghemite Isomorphic Series by Χ-ray Diffraction and the Rietveld Method, and Confirmation by Independent Methods

Maghemite (γ-Fe2O3) is a mineral formed from magnetite oxidation at low temperatures, an intermediate metastable term of the magnetite to hematite oxidation and could be mixed with both. It has magnetic susceptibility similar to magnetite, crystal structure close to magnetite with which it forms a solid solution, while compositionally it equals hematite. Maghemite is thus easily misidentified as magnetite by Χ-ray diffraction and/or as hematite by spot chemical analysis in iron ore characterization routines. Nonstoichiometric magnetite could be quantified in samples of Brazilian soils and iron ores by the Rietveld method using a constrained refinement of the Χ-ray patterns. The results were confirmed by reflected light microscopy and Raman spectroscopy, thus qualitatively validating the method. Χ-ray diffraction with the refinement of the isomorphic substitution of Fe2+ by Fe3+ along the magnetite-maghemite solid solution could help to suitably characterize maghemite in iron ores, allowing for the evaluation of its ultimate influence on mineral processing, as its effect on surface and breakage properties.

Maghemite in Brazilian Iron Ores: Quantification of the Magnetite-Maghemite Isomorphic Series by X-ray Diffraction and the Rietveld Method, and Confirmation by Independent Methods

Maghemite (γ-Fe2O3) is a mineral formed from magnetite oxidation at low temperatures, an intermediate metastable term of the magnetite to hematite oxidation and could be mixed with both. It has magnetic susceptibility similar to magnetite, crystal structure close to magnetite with which it forms a solid solution, while compositionally it equals hematite. Maghemite is thus easily misidentified as magnetite by X-ray diffraction and/or as hematite by spot chemical analysis in iron ore characterization routines. Nonstoichiometric magnetite could be quantified in samples of Brazilian soils and iron ores by the Rietveld method using a constrained refinement of the X-ray patterns. The results were confirmed by reflected light microscopy and Raman spectroscopy, thus qualitatively validating the method. X-ray diffraction with the refinement of the isomorphic substitution of Fe2+ by Fe3+ along the magnetite-maghemite solid solution could help to suitably characterize maghemite in iron ores, allowing for the evaluation of its ultimate influence on mineral processing, by affecting its surface and breakage properties.

Electron Belt-to-σ-Hole Switch of Noncovalently Bound Iodine(I) Atoms in Dithiocarbamate Metal Complexes

Co-crystallization of the dithiocarbamate complexes [MII(S2CNEt2)2] (M = Cu 1, Ni 2, Pd 3, Pt 4) and 1,3,5-triiodotrifluorobenzene (FIB) gives an isomorphic series of (1–4)∙2FIB co-crystals exhibiting the quadruple Cu/Ni/Pd/Pt...

Mg-rich thomsonite-Ca from chromitites of the Lekkhoyla occurrence, Voykar-Syn’ya massif, Polar Urals

The paper describes a previously unknown isomorphic series between thomsonite-Ca and a hypothetical Mg-rich zeolite. The Mg-rich thomsonite-Ca was found for the frst time in hydrothermal veins of the Lekkhoyla chromite occurrence (Polar Urals, Russia). The mineral contains up to 6.5 wt. % MgO (up to 1.35 f.u. Mg, O = 20) and exhibits a continuous isomorphism between Mg2+ and Na+ and Ca2+. The formula of the Mg-richest thomsonite-Ca is Na0.37Mg1.35Ca1.59Si5.18 Al4.35O20 ? 6(?)H2O (O = 20). The Raman spectra of thomsonite-Ca and Mg-rich thomsonite-Ca are similar both showing no bands typical of other minerals. Figures 7. Tables 2. References 12.

Large-Scale Free Energy Calculations on a Computational MOF Database: Toward Synthetic Likelihood Predictions

Metal-organic frameworks (MOFs) have captivated the research community due to a modular crystal structure that is tailorable for many applications. However, with millions of possible MOFs to be considered, it is challenging to identify the ideal MOF for the application of choice. Although computational screening of MOF databases has provided a fast way to evaluate MOF properties, validation experiments on predicted “exceptional” MOFs are not common due to uncertainties on the synthetic likelihood of computationally constructed MOFs, hence hindering material discovery. Aiming to leverage the perspective provided by large datasets, here we created and screened a topologically diverse database of 8,500 MOFs to interrogate whether thermodynamic stability metrics such as free energy could be used to generally predict the synthetic likelihood of computationally constructed MOFs. To this end, we first evaluated the suitability of two methods and three force fields to calculate free energies in MOFs at large scale, settling on the Frenkel-Ladd path thermodynamic integration method and the UFF4MOF force field. Upon defining a relative free energy, Δ_LMF_FL, that corrects for some force field artifacts specific to MOF nodes, we found that previously synthesized MOFs tended to cluster in a region below Δ_LMF_FL = 4.4 kJ/mol per atom, suggesting a general first filter to discriminate between synthetically likely and unlikely MOFs. However, a second filter is needed when several MOF isomorphs are below the Δ_LMF_FL threshold. In 84% of the cases, the synthetically accessible MOF within an isomorphic series presented the lowest predicted free energy. The present; work suggests that crystal free energies could be key to understanding synthetic likelihood for MOFs in computational databases (and MOFs in general), and that the thermodynamics stability of the fully assembled MOF often determines synthetic accessibility.

Large-Scale Free Energy Calculations on a Computational MOF Database: Toward Synthetic Likelihood Predictions

Metal-organic frameworks (MOFs) have captivated the research community due to a modular crystal structure that is tailorable for many applications. However, with millions of possible MOFs to be considered, it is challenging to identify the ideal MOF for the application of choice. Although computational screening of MOF databases has provided a fast way to evaluate MOF properties, validation experiments on predicted “exceptional” MOFs are not common due to uncertainties on the synthetic likelihood of computationally constructed MOFs, hence hindering material discovery. Aiming to leverage the perspective provided by large datasets, here we created and screened a topologically diverse database of 8,500 MOFs to interrogate whether thermodynamic stability metrics such as free energy could be used to generally predict the synthetic likelihood of computationally constructed MOFs. To this end, we first evaluated the suitability of two methods and three force fields to calculate free energies in MOFs at large scale, settling on the Frenkel-Ladd path thermodynamic integration method and the UFF4MOF force field. Upon defining a relative free energy, Δ_LMF_FL, that corrects for some force field artifacts specific to MOF nodes, we found that previously synthesized MOFs tended to cluster in a region below Δ_LMF_FL = 4.4 kJ/mol per atom, suggesting a general first filter to discriminate between synthetically likely and unlikely MOFs. However, a second filter is needed when several MOF isomorphs are below the Δ_LMF_FL threshold. In 84% of the cases, the synthetically accessible MOF within an isomorphic series presented the lowest predicted free energy. The present; work suggests that crystal free energies could be key to understanding synthetic likelihood for MOFs in computational databases (and MOFs in general), and that the thermodynamics stability of the fully assembled MOF often determines synthetic accessibility.

Natural kaersutite: ftir, raman, thermal and thermochemical investigations

A thermochemical study of the natural oxo-amphibole ─ kaersutite Na0.4K0.3(Ca1.6Na0.4)(Mg2.9Fe0.82+Al0.7Ti0.6Fe0.53+)[Si6.1Al1.9O22](OH)0.2O1.8.(alkaline basalts of Mongolia) was performed on a Tian-Calvet microcalorimeter. The enthalpy of formation from the elements ∆fH el0(298.15 K) = – 12102 ± 16 kJ/mol) was obtained by the method of high-temperature melt solution calorimetry. The entropy, enthalpy and Gibbs energy of the formation of the end-members of the isomorphic series kaersutite NaCa2Mg3TiAl[Si6Al2O22]O2 – ferri-kaersutite NaCa2Mg3TiFe3+[Si6Al2O22]O2 were estimated.

Thermochemical study of sodium and sodium-calcium amphibols

A thermochemical study of six natural sodium and sodium-calcium amphiboles was carried out using the high-temperature melt solution calorimetry on a Tian-Calvet microcalorimeter. The enthalpies of formation from the elements have been obtained for arfvedsonites: K0.5(Na1.5Ca0.5)(Mg4.0Fe0.93+Al0.1)[Si8.0O22](OH)2 (Inaglinsky Massif, Central Aldan, Russia) (-11626.6 ± 8.9 kJ/mol), (Na0.5K0.1)(Na1.6Ca0.4)(Mg3.6Fe0.42+Fe0.83+Al0.2)[Si8.0O22](OH)2 (Khibiny Massif, Kola Peninsula, Russia) (-11520.8 ± 14.6 kJ/mol) and (Na0.7K0.3)Na2.0(Fe4.02+Fe0.63+Mn0.1Ti0.1Al0.2)[Si8.0O22](OH)2 (Katuginskoe deposit, Transbaikalia, Russia) (-11384.7 ± 17.1 kJ/mol); for riebeckites: Na2.0(Mg2.5Fe0.52+Fe1.53+Al0.5)Si8O22(OH)2 (Kumula deposit, Central Kazakhstan) (-10791.0 ± 10.1 kJ/mol) and Na2.0(Mg0.9Fe2.12+Fe1.93+Al0.1)[Si8O22](OH)2 (Krivoy Rog, Ukraine) (-10260.8 ± 10.9 kJ/mol) and richterite (Na0.7K0.3)(Ca1.2Na0.8)(Mg4.6Fe0.42+)[Si7.8Al0.2O22](OH)2 (Kovdorsky Massif, Kola Peninsula, Russia) (-12154.2 ± 9.7 kJ/mol). The values of the standard entropies, enthalpies, and Gibbs energies of formation are estimated for the end members of isomorphic series: arfvedsonite – magnesioarfvedsonite, riebeckite – magnesioriebeckite and richterite – ferrochichthite.

Geochemistry of carbonate rocks in early Precambrian and phanerozoic metamorphic complexes of East Siberia, north-west of Russia, Pamir

Geochemical study of carbonate rocks of Early-Precambrian and Phanerozoic metamorphic Complexes was carried out and their differences were revealed. Precambrian marbles and calciphyres studied in the Оnot Greenstone Belt, Kitoy and Sharyzhalgay granulitic Complexes Presayan uplift, Yenisei Series of the Angara-Kan Protrusion of the Siberian craton, the Belomorian and Lapland Complexes, North of the Pechenga Structure, Sortavala Suite of the Fennoscandian shield, Wakhan Complex of the Badakhshan Array; Phanerozoic – in Olkhon, Slyudyanka, Svyato nossky Complexes of the Baikal region, Boxon Series and Irkutnu Suite of the Eastern Sayan, Derbinsky Complex and Alchadur Suite of the Prisayan, Judin Suite and Panimba-Rybinsk Zone of the Yenisei Ridge, Muzkol Complex of the Eastern Pamirs. Precambrian carbonate rocks are enriched with Fe, Mn and depleted Sr, Ba in comparison with Phanerozoic rocks at a close low level of REE content. The Archean enrichment of Fe, Mn protoliths of marbles and calcifers is due to the dominance of basic and ultrabasic rocks in the feeding provinces. In the Paleoproterozoic, compared with the Phanerozoic, the proportion of Fe, Mn in carbonate rocks decreased, and Al, K, Ba, Sr have increased due to participation in occurrence their protoliths of the granite-metamorphic layer of the Earth. The distribution of petrogenic and rare elements in marbles and calcifirs is determined by the forms of their location: 1) in the isomorphic series of Ca-Mg carbonates with admixture of Fe, Mn, Ba, Sr, REE; 2) the presence of minerals Na, K, Ba, Sr in marbles; 3) location of thin (in marbles) and large (calcifications) fractions of minerals with iron, Mn Al, Ti, Zr, Cr, V, Ni, S. Performed paleoreconstruction behavior of REE in carbonate rocks in the Archean and Early-Paleoproterozoic dominated intracratonic shallow sea. Open oceans appeared at the turn of 2–1.9 billion years, but widespread development of carbonate rocks were in the Meso-Neoproterozoic and Phanerozoic. The revealed features are the basis of age-related paleoreconstructions of protoliths according to petrogeochemical characteristics of carbonate rocks of Precambrian and Phanerozoic metamorphic complexes.