Importance of considerations of mixing properties in establishing an internally consistent thermodynamic database: thermochemistry of minerals in the system Mg2SiO4-Fe2SiO4-SiO2 (original) (raw)
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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.
Mg2+_Fe2+ order-disorder in orthopyroxene and the Mg2+_Fe2+ distribution between coexisting minerals
Lithos, 1971
An approximate temperature of crystallization of rocks and important information on the thermodynamic nature of a crystalline solution, coexisting with orthopyroxene, can be obtained by using the experimental Mg 2+Fe 2+ intracrystalline distribution data in orthopyroxenes at various temperatures and the inter-crystalline Mg 2+Fe 2+ distribution data in the minerals of the natural assemblage. Ca-pyroxene Ca (Mg, Fe) Si 2O 6 with the M2 site nearly filled by Ca, is a 'simple mixture' of Mg 2+ and Fe 2+ components with an energy constant W approximately 1023 cal/mole at 680°C. The estimated temperature of ion-exchange equilibrium between coexisting cummingtonite and orthopyroxene in Biwabik Iron Formation, Minnesota is 620°C. Cummingtonite also approximates to a 'simple mixture' with W as 834 cal/mole at 620°C.
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.
Computers & Geosciences, 1993
Al~traet-Program QUILF assesses equilibria among Ti-magnetite, ilmenite, augite, pigeonite, orthopyroxene, olivine, and quartz (or subassemblages thereof). Oxide and silicate equilibria are related through the QUIIF equilibrium: SiO 2 + 2Fe2TiO 4 = 2FeTiO 3 + Fe2SiO4 Quartz Ulvrspinel llmenite Fayalite. Depending on the assemblage, QUILF can provide information on temperature, pressure, oxygen fugacity, and the activities of SiO 2, TiO2, and Fe ° at which the phases were last in equilibrium. For many low-variance assemblages, the system is overdetermined; thus quantitative information can be extracted even if one phase is altered or has reequilibrated. QUIIF equilibria can reduce the uncertainties in temperature and oxygen fugacity as determined from coexisting ilmenite and Ti-magnetite alone. QUILF is written in Turbo Pascal for IBM PC and compatibles. The compiled program is approximately 210 kbyte; it also requires two data files that total approximately 10 kbyte. The compositions of the phases first must be projected into seven-component space (CaO MgO-MnO-FeO-Fe203 TiO2-SiO2) before they are used in program QUILF; routines to accomplish this are included in the program, and also are available as separate BASIC programs.
Contributions to Mineralogy and Petrology, 2007
We have determined the equilibrium Fe 2+ -Mg fractionation between orthopyroxene and spinel in the ferromagnesium system at 0.9-1.4 GPa, 850-1,250°C, and also as a function of the Cr/Al ratio of spinel at 1.24 GPa, 1,000°C. At each P-T condition, the equilibrium value of the distribution coefficient, K D (Fe-Mg), was constrained by experiments with crystalline starting mixtures, and approaching from both higher and lower initial values. The experimental data have been cast, within a thermodynamic framework, in the form of a geothermometer in the system FeO-MgO-Al 2 O 3 -Cr 2 O 3 -SiO 2 (FMACrS). Using the data of O'Neill and Wall (1987) on the thermodynamic properties of Fe 3+ and Ti 4+ bearing spinels, we extended the thermometric formulation to account for the effect of these components. However, practical application of the extended formulation is beset with the problem of accurate determination of Fe 3+ content of natural minerals. Using published data, the thermometric formulation in the FMACrS system has been applied to a number of natural assemblages that have small Fe 3+ content. The retrieved temperatures are generally higher, on the average by $60°C, than those obtained from the olivine-spinel Fe 2+ -Mg exchange thermometer of O'Neill and Wall, as modified by , but are more compatible with the original temperature estimates by the authors of the publications. The smaller Fe 2+ -Mg interdiffusion coefficient, D(Fe-Mg), in orthopyroxene compared with those in both olivine and spinel is expected to yield higher temperatures from orthopyroxene-spinel than from olivine-spinel thermometry.
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 Geophysical Research, 1980
The A1203 solubility in orthopyroxene (OPx) coexisting with pyrope has been determined at several P-T conditions in the system MgO-AI203-SiO2 (MAS), using synthetic crystalline starting mixtures and approaching from both high-and low-alumina sides. These results and the reversed experimental data of on AI-OPx + spinel + forsteritc equilibrium have been treated thermodynamically, using the solution model of AI-OPx proposed by , to express A1203 solubility in OPx as a function of P and T in both assemblages. The calculated alumina isopleths are in very good agreement with all available reversed experimental data. In addition, the univariant boundary between spinel peridotitc and garnet peridotitc in the MAS system, which is defined by the intersection of complementary alumina isopleths in these two fields, is in excellent agreement with experimental reversals of the boundary by Danckwerth and Newton. The spinel field isopleths have somewhat steeper dT/dP slopes than those calculated by previous workers but are still not sufficiently sensitive to pressure to be useful as a practical geobarometer. The garnet field isopleths are nearly equally sensitive to both P and T but are somewhat different from those of MacGregor (1974) and show mild curvature, concave toward the pressure axis. The univariant boundary shows moderate curvature, convex toward the temperature axis, with a pressure minimum of 19 kbar at 840øC. The 'pyroxene geotherm' of Boyd (1973) has been reexamined in the light of the results of this study and the garnet-clinopyroxene geothermometer of Ganguly (1979). Boyd's conclusion that the sheared garnet lherzolite nodules in kimberlite pipes have formed at higher P-T conditions than the granular ones is reinforced; however, the inferred P-T conditions do not seem to suggest an inflection of the geotherm between the two groups of nodules. The pyroxene or nodule geotherm overlaps the shield geotherm of Clark and Ringwood but progressively diverges from the latter toward higher temperature with increasing depth. INTRODUCTION Natural orthopyroxenes may contain up to about 10 wt % AleO3, depending on pressure, temperature, and the compositions of the coexisting phases. Because of its potential geothermobarometric applications, especially to samples transported from the earth's mantle [e.g., Boy& 1973; MacGregor and Basu, 1974], there has been a considerable amount of work attempting to relate the AleO3 content of orthopyroxene to the above intensive variables. MacGregor [1974] published the first comprehensive phase diagram showing the variation of AleO3 solubility in orthopyroxene in the presence of Fo 4-Sp and/or garnet as a function of P and T in the simple ternary system MgO-AI•O3-SiO2(MAS). These results were, however, based only on synthesis experiments. Wood [1975] and Obata [1976] have independently calculated the alumina isopleths (
Mineralogy and Petrology, 2011
Replacement of olivine by orthopyroxene is a frequently observed phenomenon in mantle metasomatism. In order to study element redistribution in SiO 2 metasomatism we synthesised orthopyroxene reaction rims at the contacts between forsterite-rich olivine and quartz. The orthopyroxene rims grew from the original quartz-olivine interface into both directions implying counterdiffusion of iron/magnesium and silicon. Following local equilibrium partitioning the X Fe is lower in the orthopyroxene than in the reactant olivine at the olivine-orthopyroxene replacement front. The resulting local iron excess is compensated by formation of orthopyroxene with a higher X Fe at the quartz-orthopyroxene interface, which is out of equilibrium with the reactant olivine. This is facilitated through short circuit diffusion along grain boundaries within the orthopyroxene rim. Due to the low capacity of orthopyroxene to accommodate Ni, this component is forced to diffuse back into the olivine producing a Ni enriched zone ahead of the replacement front. This leads to Ni contents in the orthopyroxene rim, which are higher than what is expected in equilibrium with the unaltered olivine. Taking quartz as a proxy for a silica rich fluid or liquid metasomatising agent, we conclude that the overall element fractionation between olivine and the silica rich phase may deviate from equilibrium partitioning so that the Fe and Ni concentrations in the orthopyroxene which is in contact with quartz are higher than in equilibrium with the reactant olivine. This indicates that kinetic fractionation is important for the chemical evolution of both the mantle rocks and the metasomatising agents.