A revision of the garnet-clinopyroxene Fe2+-Mg exchange geothermometer

A comprehensive experimental dataset was used to analyse the compositional dependence of the garnet-clinopyroxene Fe²⁺/Mg partition coefficient (K _d). The Mg no. of garnet was found to have a significant effect on the K _d, in addition to calcium content of garnet. An empirical model was developed to relate these effects with equilibrium temperature and pressure in the form of a conventional geothermometer, T(K) = { – 1629[X^Gt _Ca]² + 3648.55[X^Gt _Ca] – 6.59[Mg no. (Gt)] + 1987.98 + 17.66P (kbar)}/(In k_d + 1.076). Application of this thermometer produced reasonable temperature estimates for rocks from the lower crust (garnet amphibolites, granulites and eclogites) and the upper mantle (eclogite and lherzolite xenoliths in kimberlites, mineral inclusions in diamonds).     

Kinetics of Fe2+-Mg distribution in aluminous orthopyroxenes

Kinetic studies of isothermal heating experiments (600–800° C) on aluminous pyroxenes (Mg_0.942Fe _0.880 ²⁺ Fe _0.068 ³⁺ Mn_0.016Ca_0.010Al_0.084) (Si_1.848Al_0.152) permit the determination of rate constant of isothermal disordering as 2.5457 E13(±1.4 E13) min^?1. The activation energy is determined as 278 (±23) kJ/mol. Data on two other aluminous pyroxenes at 700° C indicate that the rate constant decreases significantly with increasing amount of trivalent cations. There is a similar but reverse correlation between the concentration of trivalent cations and the Fe²⁺-Mg equilibrium distribution between sites. The site distribution coefficient increases with increasing concentration of trivalent cations at constant temperature.     

The dependence of the Fe2+-Mg cation-partitioning between olivine and basaltic liquid on pressure,temperature and composition

The equilibrium partitioning of Fe²⁺ and Mg between olivine and liquid along a liquid line of descent has been determined for a calc-alkaline system, ranging in composition from picritic to andesitic. Experiments were conducted between 1000–1450° C and 1 bar to 30 kbar. Within the compositional range investigated and , the compositional dependence of the Fe²⁺ and Mg partitioning is a function of the Mg-content of the liquid. The Mg-content of the liquid correlates strongly with temperature. The variation of the Fe²⁺ and Mg partitioning were therefore evaluated individualy as functions of composition and temperature alone. The composition dependence of the cation-partitioning coefficients (Kd) is given by the following two equations:

Fe2+-Mg2+ partition between coexisting cordierite and garnet — a discussion of the experimental data

The partition of iron and magnesium between cordierite and garnet depends on as well as temperature. The apparently conflicting experimental data on the values of K _D may be reconciled by considering the pertaining during the different experiments.     

Fe2+-Mg order-disorder in orthopyroxene: equilibrium fractionation between the octahedral sites and thermodynamic analysis

The equilibrium intracrystalline distribution coefficient, k _D ^*, of Fe^* (i.e. Fe²⁺ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe²⁺-Mg orthopyroxene crystals (Fs₁₄, Fs₁₅ and Fs₄₉) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs₂₃), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k _D ^* as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS ^XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a “simple mixture”. The calculated ΔS ^XS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution. Received: 25 August 1998 / Accepted: 10 March 1999     

The role of Fe2+ and Fe3+ in synthetic Fe-substituted tetrahedrite

Summary Tetrahedrites with the composition between Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ were synthesized at 457 °C and 500 °C from the elements and carefully studied by Mössbauer spectroscopy of⁵⁷Fe. Between Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ iron is predominantly ferric. Between Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ iron is predominantly ferrous and occupies the tetrahedral M1-sites.

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Zusammenfassung Die Rolle von Fe²⁺ und Fe³⁺ in synthetischen Tetraedriten mit Fe-Substitution Tetraedrite mit einer Zusammensetzung zwischen Cu₁₂Sb₄S₁₃ and Cu₁₀Fe₂Sb₄S₁₃ wurden bei 457 °C und 500 °C aus den Elementen synthetisiert und sorgfdltig mit Mössbauer-Spektroskopie von⁵⁷Fe untersucht. Zwischen Cu₁₂Sb₄S₁₃ and Cu₁₁Fe₁Sb₄S₁₃ ist Eisen überwiegend dreiwertig. Zwischen Cu₁₁Fe₁Sb₄S₁₃ and Cu₁₁Fe₂Sb₄S₁₃ ist Eisen überwiegend zweiwertig und besetzt die tetraedrisch koordinierten M1-Plätze.

     

X-ray diffraction study of Fe2+-Mg order-disorder in orthopyroxene. Some kinetic results

Kinetic rates of Fe²⁺-Mg disordering in three orthopyroxenes (mean value of X_Fe = Fe²⁺/(Fe²⁺+Mg) = 0.175,0.482,0.770 respectively) have been determined employing heating experiments and single crystal X-ray structural refinements. Disordering rate constants \((\vec K)\) (550800° C) for two pyroxenes are given by: ln \((\vec K)\) = 27.107(±5.177)?32062(±783)T^?1(X_Fe = 0.175) ln \((\vec K)\) = 16.142(±0.057)?18227(±423)T^?1(X_Fe = 0.770) The distribution coefficients K_D (representing a steady state of disordering Fe_M2 + Mg_M1 ? Fe_M1 + Mg_M2) are given by: ln K_D = 5.016(±0.223)-7033(±1473) T^?1(X_Fe = 0.175) ln K_D = 1.988(±0.122)-3809(±913)T^?1(X_Fe = 0.770) These distribution coefficients provide the constraint of the disordering reaction on the value of the equilibrium constant for Fe²⁺-Mg order-disorder. Until the low temperature dependence of K_D is well constrained, the calculation of cooling rates of pyroxenes and host rocks cannot be done reliably.     

Thermodynamics and behavior of γ-Mg2SiO4 at high pressure: Implications for Mg2SiO4 phase equilibrium

Raman spectra of γ-Mg₂SiO₄ taken to 200 kbar were used to calculate entropy and heat capacity at various P-T conditions. These new thermodynamic data on γ-MgSiO₄, similar data on MgSiO₃ perovskite (pv), previous data on β-MgSiO₄ and MgO (mw), and previous volumetric data of all phases were used to calculate the phase boundaries in the Mg₂SiO₄ phase diagram. Our resulting slope for the β→γ transition (50±4 bar K^-1) is in excellent agreement with recent multi-anvil studies. The slopes for the β→pv+MgO and γ→pv+MgO are-7±3 and -25±4 bar K^-1, respectively, and are consistent with our CO₂ laser heated diamond anvil studies. These slopes result in a β-γ-MgO+pv triple point at approximately 229 kbar and 2260 K for the iron free system.     

Sign of the magnetic coupling of Fe2+ and Fe3+ in biotite

Mössbauer spectra of biotite at 4 K are reported. The biotite crystals were oriented with the c-axis parallel to the γ-ray direction and some spectra were recorded with external magnetic fields of 40 kOe applied at right angles to the c-axis. Decrease of the magnetic-hyperfine field of both Fe²⁺ and Fe³⁺ ions on application of the external field shows that both Fe³⁺-Fe³⁺ pairs and Fe²⁺-Fe²⁺ pairs are coupled ferromagnetically.     

Fe2+-Ti4+ charge transfer in stoichiometric Fe2+,Ti4+-minerals

Optical absorption spectra are presented for taramellite, traskite and neptunite, all of which have both Fe²⁺ and Ti⁴⁺ as major elements. The spectra of each of these minerals are dominated by a single, intense absorption band in the 415 to 460 nm region with 7000 to 9000 cm^?1 halfwidth. These transitions, assigned to Fe²⁺-Ti⁴⁺ intervalence charge transfer, showed little difference in intensity at 80 and 300 K and have molar absorptivities which range from ～100 to ～1300 M^?1 cm^?1. The Fe²⁺-Ti⁴⁺ absorptions in these standards generally compare well to other mineral spectra in which Fe²⁺ — Ti⁴⁺ intervalence absorption has previously been proposed with the exception of the most cited example, blue corundum.     

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.     

Raman spectra of hydrous γ -Mg2SiO4 at various pressures and temperatures

 One well-defined OH Raman band at 3651 ± 1 cm⁻¹ and one weak feature near 3700 ± 5 cm⁻¹ are recognized for the hydrous γ-phase of Mg₂SiO₄. Like the hydrous β-phase, the H₂O content in the γ-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous γ-phase were investigated up to about 200 kbar at room temperature and in the range 81–873 K at atmospheric pressure. Unlike the anhydrous γ-phase, which remains intact up to at least 873 K, the hydrous γ-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous γ-phase remains intact up to at least 800 K, Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands. Received: 27 April 2001 / Accepted: 12 September 2001     

Fe2+-Fe3+ interactions in tourmaline

The color and spectroscopic properties of ironbearing tourmalines (elbaite, dravite, uvite, schorl) do not vary smoothly with iron concentration. Such behavior has often been ascribed to intervalence charge transfer between Fe²⁺ and Fe³⁺ which produces a new, intense absorption band in the visible portion of the spectrum. In the case of tourmaline, an entirely different manifestation of the interaction between Fe²⁺ and Fe³⁺ occurs in which the Fe²⁺ bands are intensified without an intense, new absorption band. At low iron concentrations, the intensity of light absorption from Fe²⁺ is about the same for E∥c and E⊥c polarizations, but at high iron concentrations, the intensity of the E⊥c polarization increases more than ten times as much as E∥c. This difference is related to intensification of Fe²⁺ absorption by adjacent Fe³⁺. Extrapolations indicate that pairs of Fe²⁺-Fe³⁺ have Fe²⁺ absorption intensity ～200 times as great as isolated Fe²⁺. Enhanced Fe²⁺ absorption bands are recognized in tourmaline by their intensity increase at 78 K of up to 50%. Enhancement of Fe²⁺ absorption intensity provides a severe limitration on the accuracy of determinations of Fe²⁺ concentration and site occupancy by optical spectroscopic methods. Details of the assignment of tourmaline spectra in the optical region are reconsidered.     

Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy

Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe²⁺ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe²⁺/Fe total ratio on An content demonstrates that Fe²⁺ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe³⁺ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe³⁺ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe³⁺, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.     

Mutual Pb2+/Sn2+ substitution in sulfosalts

Summary In naturally occurring sulfides and sulfosalts, the elements tin and lead can substitute for each other and form solid solution series. The best known example is the montesite series between teallite and herzenbergite. Within a coordinated research program including ore microscopy, X-ray and microprobe studies, mineral syntheses and Mössbauer spectroscopy, other lead sulfosalts were discovered which show considerable solid solution series.Natural tin-bearing jamesonites which have been recently discovered have also been confirmed experimentally.The present work reveales a broad solid solution field for franckeite ranging from Pb-rich potosiite to Sn-rich incaite. The well-established silver-free franckeite formula exhibits an ideal composition of(Pb, Sn) ₆ ²⁺ Fe²⁺ Sn ₂ ⁴⁺ Sb ₂ ³⁺ S ₁₄ ^2– . Valencies of the Fe, Sn, and Sb in franckeite shown in this formula have been confirmed by Mössbauer spectroscopy. Frequently observed trace amounts of Ag, Zn, Ge, and In are not essential in the structure.Numerous microprobe analyses of cylindrites indicate that they form a restricted solid solution series with slight Pb/Sn²⁺ variations. Cylindrite forms at increased sulfur fugacity. Experiments and microprobe analyses on ores demonstrate the direct replacement of tin-rich franckeite or incaite by pseudomorphic lamellae of cylindrite during controlled sulfdization but no characteristic cylinders were observed. In the replacement reaction most or all of the bivalent tin was oxidized to quadrivalent tin to form cylindrite Pb₄FeSn₄Sb₂S₁₆.Contribution to the Ore Mineralogy Symposium (IMA/COM) at the 14th General Meeting of the International Mineralogical Association, at Stanford, California, in July, 1986.     

Raman spectroscopic features of Al- Fe3+- poor magnesiochromite and Fe2+- Fe3+- rich ferrian chromite solid solutions

Mineralogy and Petrology - Naturally occurring Al- Fe3 +- poor magnesiochromite and Fe2+- Fe3 +- rich ferrian chromite solid solutions have been analyzed by micro-Raman spectroscopy....     

The crystal structure of CaFe3+SiAlO6 and the crystal chemistry of Fe3+—Al3+ substitution in calcium Tschermak's pyroxene

The crystal structure of a synthetic CaFe³⁺Al-SiO₆ pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe³⁺ and 0.18 Al³⁺. Within the tetrahedral T site, Si⁴⁺ (0.50), Al³⁺ (0.41) and Fe³⁺ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe³⁺ ions with respect to Al³⁺ ions in CaFe³⁺AlSiO₆ is about 10 kcal/mole, which is much higher than that found in Y₃Al _x Fe_5–2O₁₂ garnets. Topologically the structure of CaFe³⁺AlSiO₆ is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO₆. For CaM³⁺ AlSiO₆ clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain.     

Fe2+ -F avoidance in silicates

Fe²⁺-F avoidance, reported in the literature in micas and amphiboles, can be accounted for by crystal field theory. The crystal field splitting parameter, Δ_O, of Fe²⁺ octahedrally coordinated to F^? is significantly smaller than its value when (OH)^? is the coordinating anion. Thus, the presence of Fe²⁺ is not favored at sites where F^? substitutes for (OH)^? due to smaller crystal field stabilization energy.     

Synthesis and stability of deerite,Fe 12 2+ Fe 6 3+ [Si12O40](OH)10, and Fe3+⇋Al3+ substitutions at 15–28 kb

Deerite, Fe ₁₂ ²⁺ Fe ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀, thus far known from ten localities in glaucophane schist terranes, was synthesized at water pressures of 20–25 kb and temperatures of 550–600 °C under the of the Ni/NiO buffer. The X-ray powder diagram, lattice constants and infrared spectrum of the synthetic phase are closely similar to those of the natural mineral. A solid solution series extends from this ferri-deerite end member to some 20 mole % of a hypothetical alumino-deerite, Fe ₁₂ ²⁺ Al ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀. The upper temperature breakdown of ferri-deerite to the assemblage ferrosilite +magnetite+quartz+water occurs at about 490 °C at 15 kb, and 610 °C at 25 kb fluid pressure for the of the Ni/NiO buffer. Extrapolation of these data to lower water pressures indicates that deerite can be a stable mineral only in very low-temperature, high-pressure environments.