Interaction of cations on octahedral and tetrahedral sites in simple spinels

The cation distribution in the partly inversed binary spinel is usually treated in terms of octahedral site preference energies. Such a model is based on the assumption of ideal cation mixing in each position. The present version of theory includes explicitly the interaction between altervalent cations on octahedral and tetrahedral sites by using a simple regular model of mixing. The values of octahedral site preference energies as well as the site interaction energies are readily obtained from experimental data on the temperature dependence of the inversion parameter.     

Jahn-Teller configurations in natural spinels

The local configurations and related hyperfine interactions of the tetrahedrally coordinated Fe²⁺ in two natural spinels were investigated. A special fitting procedure of the Mössbauer spectra is proposed in order to cover the involved mechanisms over the whole temperature range. The behavior of the quadrupole splitting vs temperature was successfully explained in terms of the split of the electronic 3d ground state doublet under a non-cooperative Jahn-Teller effect also providing estimates of the splitting energy for each configuration.     

Chromian spinels in lherzolite inclusions from Itinome-gata,Japan

Spinel, which constitutes from 0.7% to 3% of lherzolite inclusions, occurs as primary anhedral grains (chrome-rich variety) and as a secondary phase as breakdown products of garnet (alumina-rich variety). Although individual primary spinel grains are chemically homogeneous, spinels are characterized by a wide range of Cr/Al ratios and a relatively narrow range of Mg/Fe″ ratios, even in a single lherzolite sample. The chemical variations of spinels are considered to have the following origin: When garnet lherzolite enters the stability field of the spinel peridotite facies as a consequence of slow upward transport, both orthopyroxenes and clinopyroxenes are recrystallized with loss of jadeite and some Tschermak's component to reach equilibrium. A part of the Tschermak's component reacts with olivine to form pyroxene and spinel. This secondary spinel component is alloted to the primary chromian spinel. However, these reactions did not always reach equilibrium with the major constituent minerals in the lherzolites.     

Chromium spinels in North Chinese kimberlites,Huabei platform

Samples from diamondiferous pipes in the Mengyin and Fuxian regions were investigated. The chemical compositions of Cr spinels in kimberlites of China were found to be similar to those in kimberlites of the Arkhangelsk province in Russia. A long and complex evolution that was individual for each pipe was demonstrated. The kimberlites of the Shandong Province proved to be rich in high-Cr chromites. This means that the kimberlites formed at large depths in the field of diamond thermodynamic stability. Variations in the redox conditions were noted. They manifested themselves as a wide range of fluctuations of the chemical composition of microcrystalline spinels, up to formation to Ti-magnetite and magnetite.     

Experiments in long-range climate control

Extrapolation of the palaeoclimate chronology of the past million years or so seems to leave little doubt that another glacial stage can be expected in the near geologic future. On a shorter time scale it appears that the increasing greenhouse effect from the use of fossil fuel will raise atmospheric temperature by 2 to 3 °K within a hundred years. The possibility of man's reversing or at least weakening these portentous climate changes is investigated here with the use of a thermodynamic climate model. Preliminary results indicate that the climate effects referred to may be reversible if caught early and certainly suggest that further and more sophisticated attention be given the subject.     

X-ray single-crystal study of spinels: in situ heating

 Two MgAl₂O₄ stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized, and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts, the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample, which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range, only partially reversible order–disorder processes occurred in the time span used for the experiments. Received: 16 July 2001 / Accepted: 8 January 2002     

Experimental determination of the activity of chromite in multicomponent spinels

We determined activity-composition relationships in Pt-Cr and Pt-Fe-Cr alloys at 1300°C experimentally and used the results to constrain the thermodynamic properties of chromite-picrochromite spinels. The Pt-Cr binary is characterized by strong negative deviations from ideality throughout the investigated composition range and the activity-composition relationship can be fit by a four-suffix asymmetric regular solution with three binary interaction parameters. The ternary alloy was modeled as a four-suffix asymmetric regular solution; the three ternary interaction parameters in this model were constrained by combining interaction parameters for the three bounding binaries taken from this and previous work with results for a set of experiments in which the activity of Cr in Pt-Fe-Cr-alloys was fixed by coexisting Cr₂O₃ at known f_O2.The free energy of formation of FeCr₂O₄ at 1300°C was determined using the activities of Fe and Cr in Pt-alloys in equilibrium with oxide mixes of FeCr₂O₄ and Cr₂O₃. The free energy of formation of chromite from Fe+Cr₂O₃+O₂ is −202.7 ± 0.4 kJ/mol (1σ), indistinguishable from literature values. The corresponding free energy of formation of FeCr₂O₄ from the elements is −923.5 ± 2.1 kJ/mol (1σ), and the enthalpy of formation at 298 K is −1438 kJ/mol. The activity-composition relationship for the chromite component in (Fe,Mg)Cr₂O₄ solid solutions was determined from a set of experiments in which Pt-alloys were equilibrated with spinel + Cr₂O₃. (Fe,Mg)Cr₂O₄ spinels are nearly ideal at 1300°C; modeling our data with a one-site symmetric regular solution yields an interaction parameter of +2.14 ± 0.62 kJ/mol (1σ), similar to values based on data from the literature.     

Synthetic and natural chromium-bearing spinels: an optical spectroscopy study

Four samples of synthetic chromium-bearing spinels of (Mg, Fe²⁺)(Cr, Fe³⁺)₂O₄ composition and four samples of natural spinels of predominantly (Mg, Fe²⁺)(Al, Cr)₂O₄ composition were studied at ambient conditions by means of optical absorption spectroscopy. Synthetic end-member MgCr₂O₄ spinel was also studied at pressures up to ca. 10 GPa. In both synthetic and natural samples, chromium is present predominantly as octahedral Cr³⁺ seen in the spectra as two broad intense absorption bands in the visible range caused by the electronic spin-allowed ⁴ A _2g  → ⁴ T _2g and ⁴ A _2g  → ⁴ T _1g transitions (U- and Y-band, respectively). A distinct doublet structure of the Y-band in both synthetic and natural spinels is related to trigonal distortion of the octahedral site in the spinel structure. A small, if any, splitting of the U-band can only be resolved at curve-fitting analysis. In all synthetic high-chromium spinels, a couple of relatively narrow and weak bands of the spin-allowed transitions ⁴ A _2g  → ² E _g and ⁴ A _2g  → ² T _1g of Cr³⁺, intensified by exchange-coupled interaction between Cr³⁺ and Fe³⁺ at neighboring octahedral sites of the structure, appear at ~14,400 and ~15,100 cm^?1. A vague broad band in the range from ca. 15,000 to 12,000 cm^?1 in synthetic spinels is tentatively attributed to ^IVCr²⁺ + ^VICr³⁺ → ^IVCr³⁺ + ^VICr²⁺ intervalence charge-transfer transition. Iron, mainly as octahedral Fe³⁺, causes intense high-energy absorption edge in near UV-range (ligand–metal charge-transfer O^2? → Fe³⁺, Fe²⁺ transitions). As tetrahedral Fe²⁺, it appears as a strong infrared absorption band at around 4,850 cm^?1 caused by electronic spin-allowed ⁵ E → ⁵ T ₂ transitions of ^IVFe²⁺. From the composition shift of the U-band in natural and synthetic MgCr₂O₄ spinels, the coefficient of local structural relaxation around Cr³⁺ in spinel MgAl₂O₄–MgCr₂O₄ system was evaluated as ~0.56(4), one of the lowest among (Al, Cr)O₆ polyhedra known so far. The octahedral modulus of Cr³⁺ in MgCr₂O₄, derived from pressure-induced shift of the U-band of Cr³⁺, is ~313 (50) GPa, which is nearly the same as in natural low-chromium Mg, Al-spinel reported by Langer et al. (1997). Calculated from the results of the curve-fitting analysis, the Racah parameter B of Cr³⁺ in natural and synthetic MgCr₂O₄ spinels indicates that Cr–O-bonding in octahedral sites of MgCr₂O₄ has more covalent character than in the diluted natural samples. Within the uncertainty of determination in synthetic MgAl₂O₄ spinel, B does not much depend on pressure.     

The factors influencing cation site-preferences in spinels a new mendelyevian approach

Pseudopotential orbital radii r _s , r _p are used to construct an index, r _σ=r _s +r _p , which characterizes the average potential experienced by atomic valence electrons. A plot of r _A ^σ verses r _B ^σ for 172 chalogenide spinels (AB₂X₄, X=O, S, Se, Te) leads to two well defined areas, which separate normal and inverse spinels, with only four errors (a predictive success rate of 98%). The gross sorting is achieved without recourse either to the number of d-electrons or an orbital radius r _d , from which it is inferred that it is the energies and extents of the cation s and p-orbitals which primarily determine coordination number in these systems. This approach to the problem of cation distribution in spinels is contrasted with the less generally applicable, traditional, crystal field ideas. The relevance of both r _σ and crystal field stabilization energies to the thermodynamics of spinel reactions is also discussed.     

The occurrence of magnesioferrite-rich spinels in a trachyandesite from NE China

Magnesioferrite-rich spinels were found in the Cretaceous Jianguo trachyandesite in northeastern China. The trachyandesite is comprised of augite, plagioclase, K-feldspar, spinel, phlogopite, apatite, zircon, and pseudomorphed phenocrysts, which are interpreted as former olivine and/or orthopyroxene crystals and now consist of a mixture of chrysotile, antigorite and chlorite. Texturally, four stages of spinel growth are observed: magnesioferrite-rich spinel-I occurs within the pseudomorphs and magnesioferrite-rich spinel-II occurs in the matrix of the trachyandesite. Magnetite-rich spinel-III occurs either as rim around spinel-II or as distinct magnetite grains, whereas worm—like magnetite—rich spinel-IV occurs within the pseudomorphs. Chemically, spinel-I contains 51 to 82 mol.% magnesioferrite component, spinel-II contains a magnesioferrite component ranging from 60 to 79 mol% and spinel-III contains less than 15 mol.% magnesioferrite component. In contrast to spinel II, spinel-I is poor in TiO₂ but rich in Cr₂O₃, MnO and NiO. Two-feldspar thermometry yields temperatures of 880–1000°C for the formation of the trachyandesite matrix assemblage. In the absence of olivine and/or orthopyroxene, Schreinemakers analysis of T-fO₂ model phase relations indicate that magnesioferrite-rich spinels-I, and II are stable at high T and high fO₂. Magnetite-rich spinel-III formed under more reducing conditions, while spinel-IV most likely formed during subsolidus late-stage alteration of olivine and/or magnesioferrite phenocrysts. Semi quantitative fO₂ calculations using the oxygen barometer of Ballhaus et al. (1991) yielded fO₂ >7 log units above QFM, which excludes a mantle origin of these magnesioferrites. Thus oxidation of these trachyandesites most likely occurred at some stage during melt ascent, or in a differentiating magma chamber or even after emplacement of these still hot magmas at near surface conditions.     

Modern progress in solar paleoastrophysics and long-range solar-activity forecasts

Decade-averaged Wolf numbers are reconstructed for the time interval 8005 B.C.–1945 A.D. using radiocarbon data derived from tree rings. Comparisons of other paleoastrophysical reconstructions of solar activity with this temporal series verify its validity and reliability. A prediction of the mean solar activity for the next forty years is made using these reconstructed Wolf numbers. It is likely that the mean solar activity during 2005–2045 will be lower than the activity of recent decades. This conclusion is compared with the long-term predictions proposed by other researchers. The prospects for paleoastrophysical predictions for the long-term variations of solar activity in the future are discussed.     

The crystallisation trends of spinels in tertiary basalts from Rhum and Muck and their petrogenetic significance

Spinels are commonly observed in alkali olivine basalts and olivine basalts that form the Plateau Magma Series of the British Tertiary Province. The spinels are either partly or wholly enclosed within olivine or may have adhered to olivine surfaces, and have undergone cation exchange and reaction with the cooling basaltic melt. Detailed microprobe traverses indicate complex exchanges involving Fe-Mg, Cr-Al, Fe³⁺-R³⁺ and Fe²⁺ Ti-R³⁺ substitutions. Some of these changes are due to a reaction with liquid that produced plagioclase and resulted in Al depletion in the spinel. A complex series of solid solutions between hercynite-magnesioferrite-chromite and Al-Cr-titomomagnetite, is indicated in a combination that precludes the disappearance of spinel by a simple peritetic reaction with the melt. The initial spinels are compositionally distinct from the chromites found in the Rhum layered series and underline the great compositional variability of liquidus spinels that can crystallise from basaltic liquid. Some of this variability may relate to the changing solubility of Cr, which behaves as a trace element, in basaltic liquids in response to slight changes in the structure of the melt.LDGO Contribution no. 2575     

Fe2+–Fe3+ ordered distribution in chromite spinels

Mossbauer measurements have been carried out on three natural chromite minerals from different locations in China over the temperature range 50 to 750 K. The experiments showed these samples to be magnesioferrochromites. The Mossbauer spectra measured could be decomposed into three doublets: two attributed to the tetrahedral T-site Fe²⁺ ions and the third to the octahedral M-site Fe³⁺ ions. Thus for the chromite spinels the results strongly supported the ordered distribution with Fe²⁺ in the T-site and Fe³⁺ in M-site.     

Complex unmixed spinels in layered intrusions within an obducted ophiolite in the Natal-Namaqua mobile belt

Spinellids showing unmixed intergrowths of chromite or chromian spinel (sensu stricto) and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks. They do occur within metamorphosed rocks that attained temperatures sufficiently high (upper amphibolite facies) for the formation of homogeneous Al-Cr-Fe³⁺-Ti spinel phases with compositions not matched in slowly cooled igneous rocks. In the Tugela Rand intrusion complex intergrowths of chromian spinel, chromian magnetite, ulvöspinel, ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks. Compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed Tugela Rand and Mambulu Complexes confirm the extension of the magnetite-hercynite solvus (Turnock and Eugster 1962) towards magnesium- and chromium-rich compositions. The Tugela Rand spinellids are compared with those from the Carr Boyd Complex (Purvis et al. 1972) and the ultramafic rocks of the Giant Nickel Mine (Muir and Naldrett 1973) and the Red Lodge district (Loferski and Lipin 1983). Significant differences between the spinels from the Red Lodge district compared to the other three occurrences may reflect the different metamorphic histories of these areas.     

Detrital spinels from alpinotype source rocks in middle Devonian sediments of the Rhenish Massif

Middle Devonian sandy siltstones of the Gummersbach area (Rhenish Massif) show increased Cr contents (up to 675 ppm). The chromium is mainly associated with detrital Cr-Al spinels. The spinels are characterized by a wide spectrum of chemical composition.In more than 50% of 134 analyzed spinel grains the Cr₂O₃ content exceeds 50 wt %. The A1₂O₃ content which is negatively correlated to Cr₂O₃ reaches 40 wt %. The MgO contents are, with few exceptions, lower than the FeO contents; both elements are negatively correlated. The Fe₂O₃ contents which were calculated from a quarter of the grains rarely exceed 5 wt %. Besides Cr, Al, Fe, Mg TiO₂ contents were determined.The chemical composition of the detrital spinels indicates provenance from peridotites of an ophiolite suite.Provenance analysis shows that the source area of the Middle Devonian clastic sediments was situated in the north.

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Zusammenfassung In sandigen Siltsteinen des Mitteldevons des Bergischen Landes (Rheinisches Schiefergebirge) wurden erhöhte Cr-Gehalte von maximal 675 ppm festgestellt. Das Chrom ist überwiegend an detritische Spinelle gebunden. Mikrosondenuntersuchungen erbrachten eine große Variationsbreite im Chemismus dieser Spinelle. Über die Hälfte der 134 analysierten Körner weist mehr als 50 Gew % Cr₂O₃ auf. Der Al₂O₃-Gehalt, umgekehrt proportional zum Cr₂O₃-Gehalt, erreicht 40 Gew %. Die MgO-Gehalte sind bis auf wenige Ausnahmen kleiner als die FeO-Gehalte; beide Elemente sind negativ korreliert. Die Fe₂O₃-Gehalte, die von einem Viertel der Körner berechnet wurden, liegen meist unter 5 Gew %. Neben Cr, Al, Fe, Mg treten in einigen Spinellen meßbare TiO₂-Gehalte auf. Von ihrem Chemismus her lassen sich die detritischen Cr-Al-Spinelle von Peridotiten alpinotyper Ophiolite ableiten. Sedimentologische Befunde zeigen an, daß das Liefergebiet der klastischen mitteldevonischen Sedimente im Norden des Rheinischen Schiefergebirges gelegen hat.

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Résumé Les siltites sableuses du Dévonien moyen de la région de Gummersbach (Massif Schisteux Rhénan) montrent des teneurs en Cr élevées, jusqu'à un maximum de 675 ppm. Le chrome est lié principalement à des spinelles détritiques. La microsonde révèle une grande variation du chimisme de ces spinelles.Plus de la moitié des 134 grains analysés contiennent plus de 50% en poids de Cr₂O₃. La teneur en Al₂O₃ est enversement proportionnelle à celle en Cr₂O₃ et atteint 40 % en poids. Les teneurs en MgO sont à quelques exceptions près plus petites que celles en FeO; les deux éléments ont une corrélation négative. Les teneurs en Fe₂O₃, déterminées dans un quart des grains, ne dépassent généralement pas 5 % en poids. En plus de Cr, Al, Fe, Mg, on rencontre, dans quelques spinelles, des teneurs mesurables en TiO₂.La composition chimique des spinelles détritiques permet de leur assigner comme origine des péridotites appartenant à une série ophiolitique de type alpin.Les études sédimentologiques montrent que la région d'origine des sédiments clastiques du Dévonien moyen se trouve au nord du Massif Schisteux Rhénan.

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