Mg2+_Fe2+ order-disorder in orthopyroxene and the Mg2+_Fe2+ distribution between coexisting minerals (original) (raw)
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Contributions to Mineralogy and Petrology, 2005
We determined the forward rate constant (K + ) for the Fe 2+ -Mg order-disorder between the M2 and M1 sites of orthopyroxene (OPx), which is described by the homogeneous reaction Fe 2+ (M2) + Mg(M1) M Mg(M2) + Fe 2+ (M1), by both ordering and disordering experiments at isothermal condition and also by continuous cooling experiments. The rate constant was determined as a function of temperature in the range of 550-750°C, oxygen fugacity between quartz-fayaliteiron and Ni-NiO buffers, and at compositions of 16 and 50 mol% ferrosilite component. The K + value derived from disordering experiment was found to be larger than that derived from ordering experiment at 550°C, while at T>580°C, these two values are essentially the same. The fO 2 dependence of the rate constant can be described by the relation K + a (fO 2 ) n with n=5.5-6.5, which is compatible with the theoretically expected relation. The Arrhenius relation at the WI buffer condition is given by
Contributions to Mineralogy and Petrology, 1989
A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-FezSiO 4 and MgzSizO6-FezSi20 6. The formulation makes explicit provision for nonconvergent ordering of Fe z+ and Mg 2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene _+ quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and M6ssbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)~*Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe (M2)Fe (M 1) ~*Fe (MZ)Mg (M 1) + Mg (MZ)Fe (M 1)]
Contributions to Mineralogy and Petrology, 1999
Detailed phase relations have been determined within the systems Fe 2 O 3 -MgO-TiO 2 and FeO-MgO-TiO 2 . Experiments were performed over the temperature interval 1173±1473 K by equilibrating pelletized, ®negrained oxide mixtures in either inert calcia-stabilized zirconia pots (Fe 2 O 3 -MgO-TiO 2 system) or evacuated silica tubes (FeO-MgO-TiO 2 system). Equilibrium phase assemblages were determined by combined optical microscope, X-ray diraction and EMP examination. Phase relations in the Fe 2 O 3 -MgO-TiO 2 ternary are dominated by the instability of the M 2 O 3 solid solution relative to the phase assemblage M 3 O 4 + M 3 O 5 . A miscibility gap along the M 2 O 3 binary also gives rise to two, 3-phase ®elds (a-) separated by the M 3 O 4 + M 3 O 5 phase ®eld. Phase relations in the FeO-MgO-TiO 2 ternary were divided into two sub-systems. For the FeTiO 3 -MgTiO 3 -TiO 2 sub-ternary, there is complete solid solution along the M 2 O 3 and M 3 O 5 binary joins at high temperature. At low temperatures (T < 1373 K) the M 3 O 5 pseudobrookite solid solution decomposes to M 2 O 3 + TiO 2 . Increasing the concentration of MgO in M 3 O 5 phase results in a decrease in the temperature at which M 3 O 5 becomes unstable and compositional tie lines linking M 2 O 3 and TiO 2 fan out, before the appearance of a three-phase region where M 2 O 3 , M 3 O 5 , and TiO 2 coexist. Within the expanded FeO-MgO-TiO 2 system, at temperatures above 1273KthereisacontinuoussolidsolutionalongtheM3O4binary.Atlowtemperatures(T<1273K)theMg2TiO4end−memberbreaksdowntoMgOandMgTiO3.TheM3O4phaseshowssigni®cantnon−stoichiometry,downtoatleast1173K.Fe2+−MgpartitioningdatawereobtainedforcoexistingM2O3−M3O5andM2O3−M3O4pairsintheFeO−MgO−TiO2ternary.Assumingaregularsolutionmixingmodelforallphases,theM2O3andM3O4solidsolutionswerebothfoundtoexhibitmoderatepositivedeviationsfromideality(1273 K there is a continuous solid solution along the M 3 O 4 binary. At low temperatures (T < 1273 K) the Mg 2 TiO 4 end-member breaks down to MgO and MgTiO 3 . The M 3 O 4 phase shows signi®cant non-stoichiometry, down to at least 1173 K. Fe 2+ -Mg partitioning data were obtained for coexisting M 2 O 3 -M 3 O 5 and M 2 O 3 -M 3 O 4 pairs in the FeO-MgO-TiO 2 ternary. Assuming a regular solution mixing model for all phases, the M 2 O 3 and M 3 O 4 solid solutions were both found to exhibit moderate positive deviations from ideality (1273KthereisacontinuoussolidsolutionalongtheM3O4binary.Atlowtemperatures(T<1273K)theMg2TiO4end−memberbreaksdowntoMgOandMgTiO3.TheM3O4phaseshowssigniR◯cantnon−stoichiometry,downtoatleast1173K.Fe2+−MgpartitioningdatawereobtainedforcoexistingM2O3−M3O5andM2O3−M3O4pairsintheFeO−MgO−TiO2ternary.Assumingaregularsolutionmixingmodelforallphases,theM2O3andM3O4solidsolutionswerebothfoundtoexhibitmoderatepositivedeviationsfromideality(2600 J/mol), whereas the data for the M 3 O 5 binary suggest close to ideal behaviour. Nomenclature End-member phases in the FeO-Fe 2 O 3 -MgO-TiO 2 subsystem of the Fe-Mg-Ti-O quaternary are represented in Fig. 1. In the FeO-Contrib Mineral Petrol (1999) 135: 198 ± 211 Ó Springer-Verlag 1999 M.I. Pownceby á M.J. Fisher-White CSIRO Minerals,
Journal of Metamorphic Geology, 1990
Manganese-rich and manganese-poor iron formations which occur as thin layers in the Halagum-Satnum area, south of Kabbaldurga. Kamataka, India are chemically intermediate between the 'Algoma' and 'Lake Superior' types, but higher in their MnO and TiO, contents. The rocks are of four petrographic varieties: (a) quartz-magnetite-orthopyroxene-clinopymxene, (b) quartz-magnetite-ort~pyroxeneclinopyroxene-garnet, (c) quartz-magnetite-clinopyroxenc-garnet, and (d) quartz-magnetiteclinopyroxene-garnet-plagiodase. In the orthopyroxene-clinopyroxene pairs, Mn-Mg and Mn-Fe exchange is ideal irrespective of the MnSi03 contents of orthopyroxenes (0.61.8 mol. % in Mn-poor and 15-25 mol. % in Mn-rich compositions). Mg-Fe exchange in the same pair is however non-idcal. Mn-Fe exchange in orthopyroxenegarnet pain is ideal. The distribution patterns in the other binaries are inconclusive regarding idcality of exchange. Orthopyroxene-garnet and clinopyroxene-garnet geothermometers, modified for high spessartine contents, give temperatures of 800 f 30" C. A modified version of the Harley (1984) geothennometer registen 740 f 60" C, in agreement with the consensus temperature value. The equilibrium log f s values in the iron formations, as calculated from the reaction 6FeSi0, + 0, = 2Fe3O,+6SiO2 are in the range of-14.2 to-15.5. Algebraic analysis of variations of f q with composition of phases indicates buffering of 0, in the rocks. The absence of grunerite in these assemblages is compatible with X,, being less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C-0-H system, however, predict high X, values (>0.7) at uc = 1 .O, implying that the activity of graphite must have been greatly reduced-this is in accordance with the absence of graphite in these rocks. Key wor& fluid composition; geothermobarometry; manganiferous iron formations; pyroxenes.
Geochimica et Cosmochimica Acta, 2000
We have determined the cooling rates of orthopyroxene crystals from two group IVA stony iron meteorites-Steinbach (ST) and São João Nepomuceno (SJN)-on the basis of their Fe-Mg ordering states. The rate constant was calibrated as a function of temperature by controlled cooling experiments using orthopyroxene crystals separated from ST. These data were used along with earlier calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature for crystals separated from both meteorites to calculate their cooling rates. The site occupancies of the orthopyroxene crystals were determined by single-crystal X-ray diffraction subject to the bulk compositional constraints. The closure temperatures (T c ) of cation ordering for the untreated crystals from SJN are ϳ400°C, whereas those from ST vary between ϳ430 and 470°C. Reconciliation of the metallographic and orthopyroxene cooling rate data, within the framework of the metal-silicate mixing model of , suggests that these two stony irons had cooled at a similar rate of ϳ400°C/Ma through the closure temperatures for cation ordering in the orthopyroxenes. This was followed by slow cooling for ST at ϳ50°C/Ma at T Ͻ 425°C. Similar slow cooling was not recorded by the metals in SJN, which implies that if this stony iron were subjected to slow cooling, it must have been below 350°C. The similar cooling rates above 425°C for both ST and SJN, as required to reconcile the metal and orthopyroxene cooling rate data, is at variance with the earlier notion of distinctly different cooling rates for the high and low Ni IVA irons and stony irons. The cation ordering and metallographic cooling rate data are also amenable to an alternative interpretation, which requires two different parent bodies for the two stony irons, and mixing of the metal and silicate components of ST after the metals had cooled below the closure temperature of Fe-Ni interdiffusion. However, the available textural data for ST seems to argue against such metal-silicate mixing model.
Phase relations inferred from field data for mn pyroxenes and pyroxenoids
Contributions to Mineralogy and Petrology, 1980
Electron microprobe analysis of manganese silicates from Balmat, N.Y., has helped elucidate phase relations for Mn-bearing pyroxenes and pyroxenoids. A compilation of these data along with published and unpublished analyses for phases plotting on the CaSiO3-MgSiO3-MnSiO3 and CaSiO3-FeSiO3-MnSiO3 faces of the RSiO3 tetrahedron has constrained the subsolidus phase relations. For the system CaSiO3-FeSiO3-MnSiO3, the compositional gaps between bustamite/hedenbergite, bustamite/ rhodonite and rhodonite/pyroxmangite are constrained for middle-upper amphibolite facies conditions and extensive solid solutions limit possible three phase fields. For the CaSiO3-MgSiO3-MnSiO3 system much less data are available but it is clear that the solid solutions are much more limited for the pyroxenoid structures and a continuum of compositions is inferred for clinopyroxenes fi'om diopside to kanoite (MnMgSi206) for amphibolite facies conditions (T=650 ~ C). At lower temperatures, Balmat kanoites are unstable and exsolve into C2/c calciumrich (Cao.68Mno.44Mgo.88Si206) and C2/c calciumpoor (Cao.tzMnl.ozMgo.86Si206) phases. At temperatures of 300 400~ the calcium-poor phase subsequently has undergone a transformation to a P21/c structure; this exsolution-inversion relationship is analogous to that relating augites and pigeonites in the traditional pyroxene quadrilateral. Rhodonite coexisting with Mn-clinopyroxenes is compositionally restricted to . For the original pyroxene + rhodonite assemblage, the Mg and Ca contents of the rhodonite are fixed for t Present address." U.S. Geological Survey, a specific P (6kbars)-T (650 ~ C)--X(H20 ) --X(CO2) by the coexistence of talc + quartz and calcite + quartz respectively.
X-ray diffraction study of Fe2+-Mg order-disorder in orthopyroxene. Some kinetic results
Physics and Chemistry of Minerals, 1989
Kinetic rates of Fe 2 +-Mg disordering in three orthopyroxenes (mean value of XF~ = Fe 2 +/(Fe 2 + + Mg) = 0.175, 0.482, 0.770 respectively) have been determined employing heating experiments and single crystal X-ray structural refinements. Disordering rate constants (/{) (550-800 ~ C) for two pyroxenes are given by: In/(= 27.107(• 5.177)-32062(+ 783) T-I(XFe = 0.175) In/~ = 16.142(___ 0.057)-18 227(___ 423) r-1 (XFo = 0.770) The distribution coefficients KD (representing a steady state of disordering Fe~2 + MgM1-~FeM1 + MgM2) are given by: in KD = 5.016(• 0.223)-7033 (_+ 1473) T-1 (XFe =0.175) In KD = 1.988 (• 0.122)-3809 (_+ 913) T-1 (XFe = 0.770) These distribution coefficients provide the constraint of the disordering reaction on the value of the equilibrium constant for Fe 2 +-Mg order-disorder. Until the low temperature dependence of KD is well constrained, the calculation of cooling rates of pyroxenes and host rocks cannot be done reliably.
Contributions to Mineralogy and Petrology, 1999
The equilibrium intracrystalline distribution coecient, k à D , of Fe à (i.e. Fe 2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe 2+ -Mg orthopyroxene crystals (Fs 14 , Fs 15 and Fs 49 ) were determined by annealing experiments at several temperatures between 550 and 1000°C and single crystal X-ray structure re®nements. In addition, the X-ray data of an orthopyroxene crystal (Fs 23 ), which were collected earlier by Molin et al. (1991) between 700 and 1000°C, were re-re®ned. The data were processed through two dierent re®nement 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 signi®cantly better goodness of ®t and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted re®nements yielded signi®cantly smaller standard deviations of the site occupancies than the unweighted re®nements even when the same set of re¯ections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k à D as a function of temperature and composition. Calculation of the excess con®gurational entropy, DS XS , suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a``simple mixture''. The calculated DS 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.
Thermochemistry of (Fe2+, Mg)SiO3 orthopyroxene
Geochimica et Cosmochimica Acta, 1983
Enthalpies of solution in eutectic (Li, Na)*B204 melts at 1023 K were measured for five synthetic orthopyroxenes on the join MgSiO,-FeSiOj. The pyroxenes were synthesized at 1 120°C and 20 kbar and thus were presumed to be highly disordered. The measurements indicate a small positive enthalpy of mixing, with W, = 950 cal/MSi03. Enthalpy of solution measurements were made on a natural, well-ordered orthopyroxene near the composition En52.TFsd7.5 and on this material after heat-treatment at 1 150°C and 20 kbar. Irreversible expansion of the unit-cell constants of the natural pyroxene after heat-treatment at various temperatures was used to characterize the degree of M-site disorder. The observed enthalpy of solution decrement of 0.85 kcal/MSiOj between the natural En s2.5 and the same material heated at 1150" corresponds to about half of the maximum possible disordering, or AX F",' z 0.25, which leads to a AIY of 7.5 kcal/MzSizOh for the exchange reaction: Fe(M2) + Mg(M1) = Fe(M1) + Mg(M2) if M-site interaction energy terms are ignored. This m is larger than inferred from any of the analyses of site-occupancy data except that of BESANCON (198 l), who gave a very similar value. The measured AH of disorder and the U;, of mixing together indicate a large AH as great as 3.2 kcal for the reciprocal reaction: Fe&O6 + Mg&06 = Fe(M2)Mg(Ml)Si20, + Fe(M1)Mg(M2)Siz0, as anticipated by SACK (1980). As a consequence of the inferred magnitudes of m of the exchange and reciprocal reactions, departures from ideality of Gibbs energy of mixing of orthopyroxene are very small at 700"-1000°C. Activities of MgSiOS and FeSiO, may be replaced by their mol fractions at all temperatures in most petrologic calculations.