Plagioclase Solid Solution Research Articles

An anisotropic equation of state for solid solutions, with application to plagioclase

Summary This paper presents a framework for building anisotropic equations of state for solid solutions. The framework satisfies the connections between elastic and thermodynamic properties required by Maxwell’s relations. It builds on a recent anisotropic equation of state for pure phases under small deviatoric stresses, adding a dependence on a vector n, whose components ni contain the molar amounts of independent endmembers in the solid solution. These endmembers may have distinct chemical compositions, site species occupancies or electronic spin states. The high albite-anorthite (C$\bar{\text{1}}$) plagioclase solid solution is used to illustrate the formulation.

Local strain heterogeneity associated with Al/Si ordering in anorthite, CaAl2Si2O8, with implications for thermodynamic mixing behavior and trace element partitioning in plagioclase feldspars

Abstract Hard Mode IR powder absorption spectroscopy has been used to characterize local strain relaxation associated with Al/Si ordering in a suite of synthetic anorthite samples with structural states that vary from a high degree of Al/Si order through a metastable incommensurate structure at intermediate states of order to long-range order with I1 symmetry. The dominant feature accompanying the changing structural states is line broadening, which has been quantified by autocorrelation analysis and is attributed to local heterogeneous strain variations on a length scale of at least 1–5 unit cells. The autocorrelation results are consistent with contributions to the line broadening as being due to order parameters for both the C1 → I1 and I1 → P1 transitions, which couple biquadratically, λQod2Qdispl2. Close correlation with enthalpy variations from previously published calorimetric data indicates that the driving force for ordering can be understood in terms of elimination of strain fields arising from accommodating more or less rigid AlO4 and SiO4 tetrahedra in the feldspar framework. The metastable incommensurate structure of anorthite is closely analogous to the stable incommensurate structure that develops at intermediate compositions in the plagioclase solid solution, confirming that the same strain relaxation mechanism dominates the properties and behavior of all structural states across the solid solution. Elimination of strain heterogeneity by ordering on the basis of I1 symmetry determines the form of non-ideal mixing shown by the solid solution at high temperatures, and changes in elastic properties may contribute to a break in the slope of partitioning of trace elements between crystals and melt.

Quantifying the Effects of Non‐Hydrostatic Stress on Multi‐Component Minerals

AbstractMineral compositions are used to infer pressures, temperatures, and timescales of geological processes. The thermodynamic techniques underlying these inferences assume a uniform, constant pressure. Nonetheless, convergent margins generate significant non‐hydrostatic (unequal) stresses, violating the uniform pressure assumption and creating uncertainty. Materials scientists F. Larché and J. Cahn derived an equation suitable for non‐hydrostatically stressed geologic environments that links stress and equilibrium composition in elastic, multi‐component crystals. However, previous works have shown that for binary solid solutions with ideal mixing behavior, hundreds of MPa to GPa‐level stresses are required to shift mineral compositions by a few hundredths of a mole fraction, limiting the equation's applicability. Here, we apply Larché and Cahn's equation to garnet, clinopyroxene, and plagioclase solid solutions, incorporating for the first time non‐ideal mixing behavior and more than two endmembers. We show that non‐ideal mixing increases predicted stress‐induced composition changes by up to an order of magnitude. Further, incorporating additional solid solution endmembers changes the predicted stress‐induced composition shifts of the other endmembers being considered. Finally, we demonstrate that Larché and Cahn's approach yields positive entropy production, a requirement for any real process to occur. Our findings reveal that stresses between tens and a few hundred MPa can shift mineral compositions by several hundredths of a mole fraction. Consequently, mineral compositions could plausibly be used to infer stress states. We suggest that stress‐composition effects could develop via intracrystalline diffusion in any high‐grade metamorphic setting, but are most likely in hot, dry, and strong rocks such as lower crustal granulites.

Constraints on fluids in the continental crust from laboratory-based electrical conductivity measurements of plagioclase

Aqueous fluids are regarded as the crucial origin of high conductivity and low velocity anomalies observed by magnetotelluric and seismic surveys in the middle to lower continental crust. Integration of field geophysical methods with experimental conductivity data is essential to accurately constrain the fluid fraction and salinity. Studies of fluid content and salinity in the crust from the experimental or theoretically calculated conductivity of fluid–minerals systems and saline-bearing fluid have demonstrated that crustal constituent minerals play a prominent role in the conductivity of fluid–mineral systems. We performed systematic measurements on electrical conductivity of dry and hydrous plagioclase solid solutions at high temperature and pressure. A positive dependence of electrical conductivity on Na contents is observed in dry samples, and discontinuous variation in conductivity is associated with the degree of Al/Si order–disorder. For hydrous samples, trace amounts of water considerably enhance the electrical conductivity. The dominant charge carriers in dry and hydrous plagioclase are Na ions and hydrogen, respectively, both of which migrate via the Frenkel mechanism. If there are no free fluids present in minerals, the crustal major constituent minerals containing structure water cannot account for the high-conductivity anomalies in the middle to lower crust based on our model of the rock conductivity–depth profiles in the continental crust. The calculated bulk conductivities of fluid–rock systems further suggest that at least 1 vol% of fluid with seawater salinity (3.5 wt%) is necessary in the middle to lower crust to interpret the high conductivity anomalies.

The incommensurately modulated structures of low-temperature labradorite feldspars: a single-crystal X-ray and neutron diffraction study.

Labradorite feldspars of the plagioclase solid solution series have been known for their complicated subsolidus phase relations and enigmatic incommensurately modulated structures. Characterized by the irrationally indexed e-reflections in the diffraction pattern, e-labradorite shows the largest variation in the incommensurate ordering states among the e-plagioclase structures. The strongly ordered low-temperature e-labradorite is one of the last missing pieces of the e-plagioclase puzzle. Nine plutonic and metamorphic labradorite feldspar samples from Canada, Ukraine, Minnesota (USA), Tanzania and Greenland with compositions ranging from An52.5 to An68 were studied with single-crystal X-ray diffraction. Two crystals from Labrador, Canada, and Duluth, MN, USA, with wide enough twin lamellae were analyzed with single-crystal neutron diffraction. The incommensurately modulated structures of e-plagioclase are refined for the first time with neutron diffraction data, which confirmed that the T-O distance modulation in the low-temperature e-plagioclase results from the Al-Si ordering in the framework. Detailed configurations of the M site are also observed in the structures refined from neutron diffraction data, which were not possible to see with X-ray diffraction data. The relation between the q-vectors and the mole% An composition is revealed for the entire compositional range of e-plagioclase, from An25 to An75. The previously proposed two-trend relation depending on the cooling rate and phase transition path is confirmed. A new classification of e-plagioclase (eα, eβ and eγ) is proposed based on the q-vector of the structure, which makes it an independent character from the presence/absence of density modulation. New parameters are proposed to quantify the ordering states of these complicated aperiodic structures of e-plagioclases, such as the difference between 〈T1o-O〉 and 〈T1m-O〉 at phase t = 0.2 or the normalized intensity of the (071\bar 1) reflection.

Thermal infrared emissivity of felsic-rich to mafic-rich analogues of hot planetary regoliths

Inner Solar System bodies with absent or extremely tenuous atmospheres such as Mercury and several high-eccentricity Near Earth Objects (NEOs) are subject to a wide range of surface temperatures. Here we show how the thermal expansion induced by strong diurnal insolation variations affects the thermal infrared spectra of felsic-rich to mafic-rich regoliths. 1400-700 cm−1 spectra of granular plagioclase-pyroxene mixtures were measured in the laboratory at four temperatures between 425 K and 725 K, simulating different daily surface temperatures within the Solar System. At the same time, we describe for the first time how two minerals with different thermal expansion coefficients combined in the same granular sample appear in a non-ambient (i.e., non-298 K) thermal infrared spectrum. The spectra of plagioclase-rich analogues appear to be dominated by felsic features regardless of the daily surface temperature, and do not exhibit any of the short-wavelength absorptions of pyroxenes. When detectable, on the other hand, the 870-cm−1 pyroxene absorption, which is not affected by thermal variations, functions instead as a proxy for the solid-solution composition (i.e., the Mg-Fe relative contents). In contrast, indications of the plagioclase solid solution occurring at ambient temperatures (i.e., Ca-Na relative contents) disappear at temperatures over 525 K. Christiansen feature position and spectral contrast, which show a clear dependency on composition even at high temperatures, require an accurate reproduction of the shallow thermal gradient in order to be used as indices. In general, in order to decipher planetary hyperspectral data, surface temperature acquisition seems to be a prerequisite, as well as a comparison with analogue spectra obtained under realistic environmental conditions.

Plagioclase disequilibrium induced during fluid‐absent biotite‐breakdown melting in metapelites

AbstractIn the Bandelierkop Quarry (South Marginal Zone of the Limpopo belt), two generations of leucosome contain peritectic garnet produced by incongruent melting of biotite‐bearing assemblages, either in the presence or absence of sillimanite, respectively. This study focuses on mineral inclusions in large garnet crystals from both leucosome generations, which, in rare cases, host an abundance of euhedral plagioclase inclusions with variable compositions. Plagioclase in the residuum adjacent to the leucosomes ranges from An32 to An39, while the euhedral plagioclase inclusions are considerably more calcic with compositions that range from An50 to An83. These grains are also significantly zoned, with inclusions displaying both Na‐ and Ca‐enriched rims. Garnet zoning directly adjacent to the inclusions demonstrates that the Ca‐rich nature of the inclusions is not a consequence of Ca‐uptake from garnet. The euhedral plagioclase inclusions are interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase. Thus, the kinetic pathway followed by the incongruent melting reaction appears to be one where only the outer portions of the reactant plagioclase grains were involved, via dissolution, not diffusion, and with the precipitation of new Ca‐rich peritectic plagioclase as a consequence. This is predictable, as the coupled substitutions involved in plagioclase solid solution result in very slow intracrystalline diffusion of the major elements, while plagioclase dissolution in granitic melt is faster by several orders of magnitude. Phase equilibrium modelling demonstrates that the Ca‐content of the peritectic plagioclase is inversely proportional to the amount of plagioclase that participates in the fluid‐absent incongruent melting reaction in biotite‐bearing metapelites, with the specific compositions of the peritectic grains, reflecting local variations in effective bulk composition as the melting reaction progressed. The variation in composition and zoning patterns within plagioclase inclusions from individual garnet grains is interpreted to reflect melt segregation and flow simultaneously with garnet growth, allowing individual garnet grains to trap euhedral peritectic plagioclase grains that had formed within a range of effective bulk compositions. The preservation of this unusual garnet, which was most likely due to rapid and efficient melt loss from the residual source, as well as rapid cooling, provides a rare insight into the details of the anatectic process.

Ferroan Dolomitization by Seawater Interaction with Mafic Igneous Dikes and Carbonate Host Rock at the Latemar Platform, Dolomites, Italy: Numerical Modeling of Spatial, Temporal, and Temperature Data

Numerous publications address the petrogenesis of the partially dolomitized Latemar carbonate platform, Italy. A common factor is interpretation of geochemical data in terms of heating via regional igneous activity that provided kinetically favorable conditions for replacement dolomitization. New field, petrographic, XRD, and geochemical data demonstrate a spatial, temporal, and geochemical link between replacement dolomite and local mafic igneous dikes that pervasively intrude the platform. Dikes are dominated by strongly altered plagioclase and clinopyroxene. Significantly, where ferroan dolomite is present, it borders dikes. We hypothesize that seawater interacted with mafic minerals, causing Fe enrichment in the fluid that subsequently participated in dolomitization. This hypothesis was tested numerically through thermodynamic (MELTS, Arxim-GEM) and reactive flow (Arxim-LMA) simulations. Results confirm that seawater becomes Fe-enriched during interaction with clinopyroxene (diopside-hedenbergite) and plagioclase (anorthite-albite-orthoclase) solid solutions. Reaction of modified seawater with limestone causes ferroan and nonferroan replacement dolomitization. Dolomite quantities are strongly influenced by temperature. At 40 to 80°C, ferroan dolomite proportions decrease with increasing temperature, indicating that Latemar dolomitization likely occurred at lower temperatures. This relationship between igneous dikes and dolomitization may have general significance due to the widespread association of carbonates with rifting-related igneous environments.

Solved: The enigma of labradorite feldspar with incommensurately modulated structure

![][1] Intermediate plagioclase feldspars are the most abundant minerals in the Earth’s crust. Their incommensurately modulated structure has puzzled geologists and crystallographers for decades since the phenomenon in a labradorite was reported in 1940. Solving the structure is a necessary step toward mapping the complex subsolidus phase relations of plagioclase solid solution. The structure of a homogeneous labradorite (An51) single crystal from a metamorphic rock is solved and refined from single-crystal X-ray diffraction. The result structure can be simplified as alternating I 1-like lamellae domains related by inversion twins. The inversion boundary shows an anorthite-like structure with ![Formula][2] symmetry and is richer in Ca than the neighboring domains with opposite polarity. No albite-like subunits appear in the e -plagioclase structure. The modulated structure displays a unique Al-Si ordering pattern. A density modulation with a variation of 17 mol% in composition is also observed and can be properly described only by applying second-order harmonic waves for the atomic modulation functions. The modulated structure reveals details that cannot be observed from refinement with only main reflections and may be used to assess the ordering state and cooling rate of its host rock. The homogeneity of the crystal indicates the closure of the solvus for Boggild intergrowth at low temperature. The highly ordered modulation supports the thermodynamic stability of e -plagioclase. Both Al-Si ordering and Ca-Na ordering are the driving force for formation of the incommensurately modulated structure. [1]: /embed/graphic-1.gif [2]: /embed/mml-math-1.gif

Determination of Hashin–Shtrikman bounds on the isotropic effective elastic moduli of polycrystals of any symmetry

Although methods to determine optimal Hashin–Shtrikman bounds for polycrystals of cubic to monoclinic symmetry have been described, the calculation of bounds for triclinic crystals has not previously been possible. The recent determination of elastic moduli of common minerals with low symmetry provides motivation to extend the Hashin–Shtrikman formulation to lower symmetry. Here, Hashin–Shtrikman moduli, valid for crystals of any symmetry, are calculated as a function of the properties of a reference isotropic material. Defining the difference between moduli of the crystal and the moduli of the reference isotropic material as the residual tensor, the optimal lower (and upper) bounding moduli are found by a search along the boundary of positive (or negative) definite regimes of the residual elasticity tensor. The new numerical approach reproduces earlier results for higher symmetry crystals and successfully provides optimal bounds for triclinic crystals that have previously not been subject to analysis. The algorithm is sufficiently compact that implementation is relatively easy within any modern computational environment. Hashin–Shtrikman bounds for triclinic minerals in the plagioclase solid solution series are reported. These bounds are significantly narrower than extremal Voigt–Reuss bounds. The Hill averages moduli lie within the Hashin–Shtrikman bounds.

A predictive thermodynamic model for element partitioning between plagioclase and melt as a function of pressure, temperature and composition

One-atmosphere experiments in the system diopside-albite-anorthite (Di-Ab-An) have been used to explore the controls on plagioclase-melt partitioning of trace elements (Li, K, Rb, Cs, Mg, Zn, Sr, Ba, La, Sm, Y). By performing experiments along isotherms it was possible to isolate compositional controls from those of temperature. The lattice strain model accounts for variations in partition coefficients (<i>K</i>_<i>p</i>(i)) for isovalent series of cations (<i>i</i>^<i>n</i>+) and the experimental data are used to optimize the lattice strain parameters so that partition coefficients for <i>n</i> = 1 to 3 can be derived from <i>K</i>_<i>p</i>(Na), <i>K</i>_<i>p</i>(Ca), and <i>K</i>_<i>p</i>(La), respectively, at the conditions of interest. The optimized lattice strain parameters have magnitudes and compositional and thermal dependences that are consistent with elastic data for plagioclases. <i>K</i>_<i>p</i>(La) is parameterized via an exchange reaction involving CaAl₂Si₂O₈ and the La-feldspar component La_0.5Na_0.5Al₂Si₂O₈. The free energy change of this reaction is small, but well constrained by the data, such that for a given <i>P-T-X K</i>_<i>p</i>(La) can be derived from a combination of <i>K</i>_<i>p</i>(Na) and <i>K</i>_<i>p</i>(Ca). In the case where <i>K</i>_<i>p</i>(Na) and <i>K</i>_<i>p</i>(Ca) are independently constrained, the models reproduce the experimental values of 749 <i>K_p</i>s for Li, Mg, K, Rb, Sr, Y, Cs, Ba, and REE (obtained from 91 individual experiments varying over five orders of magnitude) within a factor of 1.5 for 53 percent of the data and a factor of 2 for 74 percent of the data. The isothermal experimental data reveal that additional compositional effects, arising from non-ideal mixing in plagioclase and in melt, also influence <i>K</i>_{<i>p</i>(<i>i</i>)}. A thermodynamic model based on the free energy of fusion of albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈), and taking into account plagioclase and melt non-ideality in Di-Ab-An, is developed to express <i>K</i>_<i>p</i>(Na), <i>K</i>_<i>p</i>(Ca) as functions of pressure (<i>P</i>), temperature (<i>T</i>) and composition (<i>X</i>). The thermodynamic models are extended to a wider compositional range by means of an empirical parameterization of 919 <i>K</i>_<i>p</i>(Na) and 920 <i>K</i>_<i>p</i>(Ca) from a database of 991 published experiments. These models are able to reproduce 919 measured partitioning values for Li, Na, Mg, Ca, K, Rb, Sr, Y, Cs, Ba, and REE (from 91 experiments) with 47 percent of the model values within a factor of 1.5 of the measured values and 69 percent within a factor of 2 for any <i>P-T-X</i> condition. The models can also be used to constrain the dependence of the chemical potential of trace cations in plagioclase solid solutions (μ_i^pl) for use in diffusion chronometry where the fully equilibrated trace element concentration profile must be known in order to calculate timescales. It is shown that μ_i^pl is not adequately described by existing empirical models used to predict <i>K_p</i> as a function of anorthite because of the conflating effects of <i>T</i> and <i>X</i> in the polythermal experimental datasets from which those models were derived.

Experimental determination of plagioclase dissolution rates as a function of its composition and pH at 22 °C

The steady-state, far-from-equilibrium dissolution rates of nine distinct plagioclases ranging in composition from An2 to An89 were measured in mixed flow reactors at 22±2°C and pH from 2 to 11. The dissolution rates of all plagioclases based on silica release show a common U-shaped behaviour as a function of pH, where rates decrease with increasing pH at acid condition but rise with increasing pH at alkaline conditions. Consistent with literature findings, constant pH plagioclase dissolution rates increase with increasing anorthite content at acidic conditions; measured anorthite dissolution rates are ∼2.5 orders of magnitude faster than those of albite at pH ∼2. Perhaps more significantly, rates are independent of plagioclase composition at alkaline conditions. Interpretation and data fitting suggests that plagioclase dissolution rates are consistent with their control by the detachment of Si-rich activated complexes formed by the removal of Al from the mineral framework. Taking account of this mechanism and transition state theory yields equations describing plagioclase dissolution rates (r+) as a function of both the mineral and aqueous fluid compositions found in natural Earth surface systems. For pH⩾6 rates are consistent with Log(r+/(mol/cm2/s))=0.35Log(aH+3/aAl3+)-11.53 and for pH<6 rates are consistent with Log(r+/(mol/cm2/s))=nacidLog(aH+3/aAl3+)+0.033An%-14.77 where An% represents the percent anorthite in the plagioclase solid solution, ai corresponds to the activity of the ith aqueous species, and nacid is given by nacid=0.004An%+0.05.

Laboratory emissivity measurements of the plagioclase solid solution series under varying environmental conditions

New laboratory thermal infrared emissivity measurements of the plagioclase solid solution series over the 1700 ∼ 400 cm−1 (6–25 μm) spectral range are presented. Thermal infrared (TIR) spectral changes for fine‐particulate samples (0–25 μm) are characterized for the first time under different laboratory environmental conditions: ambient (terrestrial‐like), half‐vacuum (Mars‐like), vacuum, and vacuum with cooled chamber (lunar‐like). Under all environmental conditions the Christiansen Feature (CF) is observed to vary in a systematic way with Na‐rich end‐member (albite) having a CF position at the highest wave number (shortest wavelength) and the Ca‐rich end‐member (anorthite) having a CF position with the lowest wave number (longest wavelength). As pressure decreases to &lt;10−3 mbar four observations are made: (1) the CF position shifts to higher wave numbers, (2) the spectral contrast of the CF increases relative to the RB, (3) the spectral contrast of the RB in the ∼1200–900 spectral range decreases while the spectral contrast of the RB in the ∼800–400 spectral range either increases or remains the same and (4) the TF disappears. A relationship between the wavelength position of the CF measured under simulated lunar conditions and plagioclase composition (An#) is developed. Although its exact form may evolve with additional data, this linear relationship should be applied to current and future TIR data sets of the Moon. Our new spectral measurements demonstrate how sensitive thermal infrared emissivity spectra of plagioclase feldspars are to the environmental conditions under which they are measured and provide important constraints for interpreting current and future thermal infrared data sets.

A thermodynamic model for the plagioclase-liquid hygrometer/thermometer

A new thermodynamic model for the plagioclase-liquid exchange reaction between the albite (NaAlSi3O8) and anorthite (CaAl2Si2O8) components is presented, which can be used as a plagioclaseliquid hygrometer or thermometer. The model incorporates calorimetric and volumetric data for the pure liquid and crystalline components, which permits the effect of temperature and pressure on the exchange reaction to be calculated independently from the effect of composition. This allows a more accurate assessment of the effect of melt composition (including dissolved water concentration) on the exchange reaction from plagioclase-liquid equilibrium experiments. Activity-composition relations for the plagioclase solid solution are taken from Holland and Powell (1992). The new hygrometer is calibrated on 71 plagioclase-liquid experiments, of which 45 are hydrous and 26 are anhydrous. Three filters were applied to the phase-equilibrium data: (1) crystallanities <30%; (2) pure H2O fluidsaturated; and (3) compositional totals (including H2O component) of 97-101% for hydrous quenched glasses. The final data set spans a wide range of liquid compositions (46-74 wt% SiO2), plagioclase compositions (An93-An37), temperatures (825-1230 °C), pressures (0-300 MPa), and dissolved melt water concentrations (0-7 wt% H2O). The standard error of estimate (SEE) for the model is ±0.32 wt% H2O, and all liquid compositions are fitted equally well. When the model is used as a thermometer, all measured temperatures are recovered equally well within ±14 °C on average. The model is only recommended for applications that fall within the compositional bounds of the calibration data set (i.e., metaluminous basalts through rhyolites in equilibrium with An95-An35). It is not yet calibrated for rhyolites crystallizing plagioclase more sodic than An30, owing to an absence of phase-equilibrium experiments on rhyolites that pass the required filters. The new plagioclase-liquid hygrometer/thermometer is available as a Visual Basic program that runs on Excel 2004.

Theoretical approach to evaluating plagioclase dissolution mechanisms

The more rapid dissolution of Ca-rich feldspars relative to Na, K-rich feldspars has been attributed to the preferential leaching of Al deep within the feldspar structure. Evidence from surface microanalysis (e.g., Hellmann et al., 2003), however, shows that preferential dissolution of Al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution. It is thus critical to examine the extent of preferential Al removal. Here we present a theoretical study of plagioclase dissolution behavior using parameterized Monte Carlo simulations. Two different dissolution mechanisms, a mechanism involving preferential leaching of Al and an interfacial dissolution–reprecipitation mechanism, are tested using compositions representing the entire plagioclase solid solution series. Our modeling results indicate that under the control of the preferential Al leaching mechanism, the influence of (Al, Si) disorder on the dissolution rate is significant. At a fixed composition, an increase in the degree of (Al, Si) disorder yields an increased dissolution rate, with an 8-fold increase in dissolution rate observed for highly disordered albite ( An 0) compared to low albite. Increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder. The difference in the dissolution rate of 293 tested oligoclase configurations with a composition of An 20 is 3-fold, and the difference is reduced to 2-fold among 107 andesine configurations of An 30. Furthermore, feldspar configurations with completely disordered (Al, Si) distributions yield a consistent log-linear dependence of dissolution rate on the anorthite content ( An), while other feldspar configurations with modest degrees of (Al, Si) disorder exhibit rates less than this trend. In contrast, when Al removal is confined to the top surface layers, a variety of feldspar configurations with different (Al, Si) disorder but a single fixed composition have similar dissolution rates; and the dissolution rate of Ca-rich feldspars departs positively from its log-linear relationship with anorthite content. This departure occurs around An 80 and is in good agreement with previous experimental studies. Subsequent modeling results of aluminum inhibition, Δ G dependence, and formation of altered surface layers in the framework of the interfacial dissolution–reprecipitation mechanism are all comparable with experimental investigations, and these results suggest that an interfacial dissolution–reprecipitation mechanism governs the dissolution of plagioclase feldspars.

Sub‐Solidus Synthesis and Microwave Dielectric Characterization of Plagioclase Feldspars

NaxCa1−xAl2−xSi2+xO8 plagioclase solid solutions (0≤x≤1) were synthesized under sub‐solidus conditions using a solid‐state reaction technique. The plagioclase formation and the sintering temperature decreased with an increase in x from the anorthite (CaAl2Si2O8; x=0) to the albite (NaAlSi3O8; x=1). Microwave (MW) dielectric measurements revealed that slow‐cooled (P1) anorthite exhibited higher Q×f values than fast‐cooled (I1) anorthite. Slow cooling also considerably improved the Q×f values of the sodium‐rich NaxCa1−xAl2−xSi2+xO8 solid solutions (0.8≤x≤1), where the highest Q×f value of 17 600 GHz was obtained for slow‐cooled Na0.8Ca0.2Al1.2Si2.8O8. The temperature coefficient of resonant frequency (τf) approached zero for 0.8≤x≤1.

Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field

Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid, lower Na out of both the mineral and fluid, easier dissolution of the mineral. Similarly, lower-K fluid is more powerful to dissolve K-feldspar than the higher. The anorthite component of plagioclase is most easily dissolved in ground water-rock system, the albite is the secondary, and K-feldspar is the most stable. Selective dissolution of plagioclase occurs in diagenesis because of the plagioclase solid solution that is mainly composed of albite and anorthite end-members, Ca-rich part of which is preferentially dissolved by the pore water, in contrast to the Na-rich parts. Based on investigation of reservoir quality, secondary pore, dissolution structures of feldspar, clay minerals and ground water chemistry of the Kela 2 gas field of Kuqa Depression in the Tarim basin of Western China, we discovered that the secondary pore is very well developed in the highest quality section of the reservoir, and the plagioclase of which was obviously selectively dissolved, in contrast to the overgrowth of K-feldspar. Chemistry of the ground water out of the highest quality section of the reservoir is consistent with the complete dissolution field of anorthite, partial dissolution field of albite component of plagioclase, and the precipitation of K-feldspar on the temperature-logK phase equilibrium diagram drawn from this study. The thermodynamic modeling calculation gave us an insight into formation of the secondary pore within the reservoir, and the dissolution of plagioclase an important cause of the secondary pore. It is assumed that the solvent to dissolve the plagioclase frame grains originated from the origin-enriched compacted fluid acids out of the mudstone buried beneath the top of geopressured zones in the Kuqa Depression.

Determination of planetary basalt parentage: A simple technique using the electron microprobe

Many basaltic meteorites are being discovered in old and new meteorite suites including those from cold- deserts (e.g., Antarctica) and hot-desert environments. It is important to establish the specific planetary body source. Proven techniques for establishing planetary parentage include stable-isotopic signatures (especially oxygen), certain elemental ratios in bulk samples, and certain elemental ratios in specific minerals. Some of these techniques are expensive, require considerable sample preparation, and are adversely affected by weathering processes on the parent body or on Earth. We have been seeking key major and minor elemental ratios (in pyroxene, olivine, and feldspar) that can be measured by the electron microprobe on standard thin sections. These ratios may be preserved in unweathered portions of mineral grains and thus “see through” weathering processes. In addition, if the sample is too small to provide a representative bulk composition, it may still have key information recorded in individual minerals. We have found that some of the most useful chemical parameters are Fe/Mn (atomic) in olivine or pyroxene and the percent anorthite (%An) in plagioclase solid solutions. A plot of Fe/Mn in pyroxene and/or olivine verses %An defines compositional fields that are significantly different for Earth, Mars, Moon, 4 Vesta, and the angrite parent body. This method may be especially powerful when combined with oxygen isotope data.

Production of silicate thin films using pulsed laser deposition (PLD) and applications to studies in mineral kinetics

In this paper we describe a setup to efficiently deposit thin films (up to several micrometers) of complex silicate and oxide compositions. We describe the production and characterization of thin films of a variety of natural mineral compositions ( e.g. olivine, garnet, pyroxene and plagioclase solid solutions). The films have been characterized using different techniques ( e.g. RBS, TEM, SEM and white light interference microscopy) and are found to be amorphous, highly uniform (± 1nm) in thickness and of the desired solid solution stoichiometries, as deposited. Small amounts of excess oxygen after deposition can be readjusted by subsequent annealing and such anneals may crystallize the films in some cases ( e.g. several hours at 900°C for olivine). The films have an excellent contact with the substrate even at the HRTEM scale and open the possibility for a variety of studies in mineral kinetic and surface sciences, which are outlined. Some examples of the production of diffusion couples in minerals for measurements that were difficult or impossible before are illustrated. These include Fe-Mg diffusion in olivine at low temperatures, Si and O diffusion in olivine and diffusion of Lu in garnets.

Ferric iron partitioning between plagioclase and silicate liquid: thermodynamics and petrological applications

A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO2–Al2O3–Fe2O3–FeO–MgO–CaO–Na2O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe3+,Al3+)Si2O8–Ca(Fe2+,Mg)Si3O8 system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeOt, MgO content and Mg/(Fet+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ±0.34 wt% (relative error =9%) and ±0.08 wt% (14%) and ±0.7 (8%) respectively. Calculated Fe3+–Al exchange chemical potentials of plagioclase, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} $$ agree with those calculated using reported thermodynamic models for multicomponent spinel, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} $$ and clinopyroxene, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} $$ . The FeOt content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe3+/(Fe3++Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase–liquid oxygen barometer are scattered, this study found that plagioclase–spinel–clinopyroxene–oxygen and plagioclase–olivine–oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and fO2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies.