Feldspar Solid Solution Research Articles

Thermodynamic mixing properties of disordered alkali feldspar solid-solution from Na–K partitioning and low-temperature calorimetry

The equilibrium partitioning of Na and K between alkali feldspar and NaCl–KCl salt melt was determined at 800 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}C, 850 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}C, 900 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}C, 950 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}C and 1000 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}C and close to ambient pressure. Four different natural gem-quality alkali feldspars with low degree of Al–Si ordering covering the range from orthoclase to high sanidine and with slightly different minor element concentrations were used as starting materials. The partitioning curves obtained for the four feldspars are indistinguishable indicating that Na–K partitioning independent of the differences of Al–Si ordering state and minor element concentrations existing amongst these feldspars. A sub-regular two parameter Margules type solution model was fitted to the partitioning data, and the excess Gibbs energy describing the thermodynamic non-ideality of the alkali feldspar solid-solution and the respective Margules parameters WgK\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W_{\ ext {g}\ ext {K}}$$\\end{document} and Wg\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W_{\ ext {g}\ ext {Na}}$$\\end{document} including their temperature dependence expressed as Wg=Wh-TWs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$W_g=W_h-TW_s$$\\end{document} were determined: WgK=19754±3140J·mol-1-T·2.33±2.67J·mol-1·K-1Wg=14916±4272J·mol-1-T·3.55±3.64J·mol-1·K-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} W_{\ ext {g}\ ext {K}}&= 19754 \\pm 3140 J\\cdot \\,\\,{\\hbox {mol}}\\,\\,^{-1} - T \\cdot 2.33 \\pm 2.67 J\\cdot \\,\\,{\\hbox {mol}}\\,\\,^{-1}\\cdot K^{-1} \\\\ W_{\ ext {g}\ ext {Na}}&= 14916 \\pm 4272 J\\cdot \\,\\,{\\hbox {mol}}\\,\\,^{-1} - T \\cdot 3.55 \\pm 3.64 J\\cdot {\\hbox {mol}}\\,\\,^{-1}\\cdot K^{-1} \\\\ \\end{aligned}$$\\end{document}The corresponding solvus has a critical temperature slightly above 650 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}C and is well comparable with earlier direct experimental determinations of the low-sanidine-albite solvus curve. Comparison of the vibrational excess entropy determined from low-temperature heat capacity measurements with the total excess entropy derived from the temperature dependence of the excess Gibbs energy yields a negative configurational contribution to the excess entropy pointing towards short-range Na–K ordering on the alkali site.

Revisiting the Bøggild Intergrowth in Iridescent Labradorite Feldspars: Ordering, Kinetics, and Phase Equilibria

The enigmatic Bøggild intergrowth in iridescent labradorite crystals was revisited in light of recent work on the incommensurately modulated structures in the intermediated plagioclase. Five igneous samples and one metamorphic labradorite sample with various compositions and lamellar thicknesses were studied in this paper. The lamellar textures were characterized with conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The compositions of individual lamellae were analyzed with high-resolution energy-dispersive X-ray spectroscopy (EDS) mapping and atom probe tomography (APT). The average structure states of the studied samples were also compared with single-crystal X-ray diffraction data (SC-XRD). The Na-rich lamellae have a composition of An44–48, and the Ca-rich lamellae range from An56 to An63. Significant differences between the lamellar compositions of different samples were observed. The compositions of the Bøggild intergrowth do not only depend on the bulk compositions, but also on the thermal history of the host rock. The implications on the subsolidus phase relationships of the plagioclase feldspar solid solution are discussed. The results cannot be explained by a regular symmetrical solvus such as the Bøggild gap, but they support an inclined two-phase region that closes at low temperature.

Exploring sources of variation in thermoluminescence emissions and anomalous fading in alkali feldspars

Alkali feldspars are routinely used in retrospective dosimetry using luminescence methods. However there is a signal loss over time, termed ‘anomalous fading’, which results in age underestimation if uncorrected. Although significant improvements have been made in recent years, luminescence dating of feldspars remains challenging. This paper investigates the relationships between chemistry, structural state and the scale of exsolution with thermoluminescence (TL) emission spectra and infrared stimulated luminescence (IRSL) fading rates.We measure TL emission spectra, where possible linking the recombination site to physical features of the feldspar crystals. We show that fading rates are lowest in ordered end-member Na- and K-feldspars but significantly greater in disordered end-members, showing that Al–Si order influences fading. As well as having very low fading rates, ordered end-member samples have distinctive TL emission spectra, with the yellow-green emission dominant, while all other samples have a dominant blue emission. Perthites, i.e. exsolved members of the (Na,K)-feldspar solid solution, show greater fading than disordered end-members and fading is greatest in semi-coherent macroperthite. We propose that the state of Al–Si-order, and the occurrence of defects and dislocations at the perthite lamellar interfaces influence anomalous fading rates in feldspars.

Excess enthalpy of mixing of mineral solid solutions derived from density-functional calculations

Calculations using the density-functional theory (DFT) in combination with the single defect method were carried out to determine the heat of mixing behaviour of mineral solid solution phases. The accuracy of this method was tested on the halite–sylvite (NaCl–KCl) binary, pyrope–grossular garnets (Mg3Al2Si3O12–Ca3Al2Si3O12), MgO–CaO (halite structure) binary, and on Al/Si ordered alkali feldspars (NaAlSi3O8–KAlSi3O8); as members for coupled substitutions, the diopside–jadeite pyroxenes (CaMgSi2O6–NaAlSi2O6) and diopside–CaTs pyroxenes (CaMgSi2O6–CaAlAlSiO6) were chosen for testing and, as an application, the heat of mixing of the tremolite–glaucophane amphiboles (Ca2Mg5Si8O22(OH)2–Na2Mg3Al2Si8O22(OH)2) was computed. Six of these binaries were selected because of their experimentally well-known thermodynamic mixing behaviours. The comparison of the calculated heat of mixing data with calorimetric data showed good agreement for halite–sylvite, pyrope–grossular, and diopside–jadeite binaries and small differences for the Al/Si ordered alkali feldspar solid solution. In the case of the diopside–CaTs binary, the situation is more complex because CaTs is an endmember with disordered cation distributions. Good agreement with the experimental data could be, however, achieved assuming a reasonable disordered state. The calculated data for the Al/Si ordered alkali feldspars were applied to phase equilibrium calculations, i.e. calculating the Al/Si ordered alkali feldspar solvus. This solvus was then compared to the experimentally determined solvus finding good agreement. The solvus of the MgO–CaO binary was also constructed from DFT-based data and compared to the experimentally determined solvus, and the two were also in good agreement. Another application was the determination of the solvus in tremolite–glaucophane amphiboles (Ca2Mg5Si8O22(OH)2–Na2Mg3Al2Si8O22(OH)2). It was compared to solvi based on coexisting amphiboles found in eclogites and phase equilibrium experiments.

Addressing the Difficulties of Differentiating Members of the Feldspar Solid Solution Series via Electron Backscatter Diffraction

Addressing the Difficulties of Differentiating Members of the Feldspar Solid Solution Series via Electron Backscatter Diffraction

Investigations of the phase relations among e1, e2 and C{\bar {\bf 1}} structures of Na-rich plagioclase feldspars: a single-crystal X-ray diffraction study.

The subsolidus phase relations of plagioclase feldspar solid solution have been puzzling mineralogists and petrologists for decades, mainly due to the complicated structures of intermediate plagioclase at low temperature. The crystal structures of 12 Na-rich plagioclase samples are investigated by single-crystal X-ray diffraction analyses. The samples studied cover a compositional range from An21 to An49 (An is anorthite, CaAl2Si2O8), as well as a wide variety of origins, from extremely slow-cooled gabbroic rocks to pegmatite and metamorphic rocks. The structures fall into three different types: C{\bar 1}, e2 and e1, with an obviously increasing trend in the ordering states of the structures. The phase transitions from C{\bar 1} to e2 and e2 to e1 are both continuous in nature, as no abrupt structure change is required for the transformation. However, the structural difference between C\bar 1 and e1 is large enough to create a miscibility gap causing the Bøggild intergrowth. As the plagioclase structure becomes more and more ordered, Al-Si reorganization in the framework would occur before the ordering of Ca and Na in M sites. Dramatic variations of Na occupancy would only appear in e1 structure with density modulation. This result confirms that Al-Si ordering is the major driving force of the formation of e-plagioclase structure. The composition of the lower end of the Bøggild intergrowth is precisely constrained to An44-An45, based on the structural differences between two samples from the same pegmatite crystal. The modulation periods and directions of e-plagioclase are dependent on the conditions at which e-ordering starts to happen, other than the composition of the plagioclase. However, the three components (δh, δk and δl) of the q vector show strong linear correlations among one another, indicating some crystallographic constraint on the modulation direction which might be independent from the composition. The detailed subsolidus phase relations among e1, e2 and C{\bar 1} are illustrated with a local phase diagram, and schematic free energy curves at different temperatures are provided.

First-principles investigation of the lattice vibrations in the alkali feldspar solid solution.

The heat capacities of Al, Si ordered alkali feldspars of different Na, K compositions were calculated using the density functional theory. The effect of the Na, K distribution, if random, ordered or clustered, on the resulting heat capacity was investigated on different cells with Ab50Or50 composition. For all compositions and distributions studied, the excess heat capacity of mixing is positive at low temperatures with a maximum at ~60 K. This produces an increasing excess vibrational entropy that reaches a constant value above ~200 K. The amount of the excess heat capacity of Ab50Or50, however, depends on the Na, K distribution. Best agreement with measured excess heat capacities is achieved, if the distribution of Na and K is either ordered or clustered. The positive excess heat capacities can be attributed to a strong softening of the acoustic and the lowest optical modes related to a strong increase of Na–O bond lengths in samples with intermediate compositions. The softening of the lowest optical mode is, however, not reflected by thoroughly measured literature IR data. Comparing calculated and measured IR spectra suggests that the resolution of the measured spectra was insufficient for detecting the lowest IR-active modes.Electronic supplementary materialThe online version of this article (doi:10.1007/s00269-014-0715-8) contains supplementary material, which is available to authorized users.

Electrical conductivity of alkali feldspar solid solutions at high temperatures and high pressures

The electrical conductivities of alkali feldspar solid solutions ranging in chemical composition from albite (NaAlSi3O8) to K-feldspar (KAlSi3O8) were measured at 1.0 GPa and temperatures of 873–1,173 K in a multi-anvil apparatus. The complex impedance was determined by the AC impedance spectroscopy technique in the frequency range of 0.1–106 Hz. Our experimental results revealed that the electrical conductivities of alkali feldspar solid solutions increase with increasing temperature, and the linear relationship between electrical conductivity and temperature fits the Arrhenius formula. The electrical conductivities of solid solutions increase with the increasing Na content at constant temperature. At 1.0 GPa, the activation enthalpy of solid solution series shows strong dependency on the composition, and there is an abrupt increase from the composition of Or40Ab60 to Or60Ab40, where it reaches a value of 0.96 eV. According to these results in this study, it is proposed that the dominant conduction mechanism in alkali feldspar solid solutions under high temperature and high pressure is ionic conduction. Furthermore, since the activation enthalpy is less than 1.0 eV for the alkali feldspar solid solutions, it is suggested to be a model where Na+ and K+ transport involves an interstitial mechanism for electrical conduction. The change of main charge carriers can be responsible for the abrupt increase in the activation energy for Or60Ab40. All electrical conductivity data were fitted by a general formula in order to show the dependence of activation enthalpy and pre-exponential factor on chemical composition. Combining our experimental results with the effective medium theory, we theoretically calculated the electrical conductivity of alkali feldspar granite, alkali feldspar quartz syenite, and alkali feldspar syenite with different mineral content and variable chemical composition of alkali feldspar at high temperatures at 1.0 GPa, and the calculated results are almost in agreement with previous experimental studies on silicate rocks.

Origin of felsic granulite microstructure by heterogeneous decomposition of alkali feldspar and extreme weakening of orogenic lower crust during the Variscan orogeny

AbstractThis study answers the question of origin and evolution of a granulitic microstructure typically developed in felsic granulites of the European Variscan belt. It shows that the precursor of the Variscan felsic granulites was a high‐pressure alkali feldspar‐rich coarse‐grained layered orthogneiss. Its S1 subhorizontal layering is defined by the alignment of alkali feldspar porphyroclasts alternating with monomineralic bands of quartz and bands rich in plagioclase and garnet. The alkali feldspar porphyroclasts contain inclusions of quartz, garnet, kyanite, biotite and rutile, reflecting peak P–T conditions of 1.6–1.8 GPa and 850 °C during S1 formation. Superimposed steep folds and steep cleavage, S2, are associated with recrystallization of alkali feldspar, plagioclase and quartz, and garnet chemistry modifications that correspond to 0.9–1.0 GPa and 800 °C. During exhumation, involving 0.8 GPa decompression and cooling, the probably perthitic alkali feldspar underwent an unusual process of heterogeneous decomposition along irregular reaction fronts forming a fine‐grained matrix composed of plagioclase and K‐feldspar grains. Regular grain distributions in the matrix, nucleation‐dominated crystal size distribution and preservation of lattice orientation of the parental perthite crystals are all explained by a discontinuous precipitation process. This heterogeneous decomposition of alkali feldspar solid solution is controlled by chemically and strain induced grain‐boundary migration. During exhumation and decompression, the fine‐grained matrix underwent viscous deformation, forming the typical microstructure of the Variscan granulites. Random phase distributions, minor coarsening and feldspar textures are interpreted as a result of strain softening due to diffusion creep‐accommodated grain‐boundary sliding. Subordinate large quartz ribbons were rheologically stronger than the feldspar‐dominated matrix due to the activity of different deformational mechanisms. Finally, in mid‐crustal levels, the subvertical structure was overprinted by a perpendicular steep fabric associated with the growth of sillimanite, heterogeneous hydration and local partial melting, development of aggregate phase distributions and significant coarsening. This evolution is accompanied with the development of a strong lattice preferred orientation of quartz, K‐feldspar and plagioclase, reflecting a switch to dislocation creep mechanism and a general hardening of the granulites under amphibolite facies conditions.

Tunable afterglow color in Eu 2+ and Dy 3+ co-activated alkaline earth feldspar solid solutions phosphors

The Eu 2+ and Dy 3+ co-activated Ca–Sr feldspar solid solution compounds, (Ca 0.98− x Sr x )Al 2Si 2O 8:Eu 0.01,Dy 0.01 ( x = 0 – 0.98 ), have been prepared by solid-state reaction in weak reductive atmosphere. The fluorescence spectra showed that the emission peaks of these phosphors shifts to shorter wavelength with an increase of Sr 2+ solid solution quantity and correspondingly the afterglow color changes from blue to purple. All the changes can be attributed to the various crystal fields around Eu 2+ ions. The afterglow with special short wavelength in near ultraviolet region can be obtained by adjusting the host chemical composition.

Molecular dynamics simulation of Al/Si-ordered plagioclase feldspar

Molecular dynamics (MD) simulations of the thermodynamic properties of Al/Si-ordered plagioclase were performed for two different crystallographic configurations: a solid solution structure and a lamellar structure. MD-simulated plagioclase feldspar solid solution has three space groups— \(\mathit{C}\overline{1}\) , \(\mathit{I}\overline{1}\) , and \(\mathit{P}\overline{1}\) —at 300 K, and two space groups— \(\mathit{C}\overline{1}\) and \(\mathit{I}\overline{1}\) —at higher temperatures. In addition, the MD-simulated, composition-dependent \(\mathit{I}\overline{1}-\mathit{C}\overline{1}\) and \(\mathit{P}\overline{1}-\mathit{I}\overline{1}\) phase transitions are non-first order. However, the temperature-dependent \(\mathit{I}\overline{1}-\mathit{C}\overline{1}\) phase transition is not observed for any plagioclase composition. The calculated excess enthalpy and excess Gibbs free energy of the MD-simulated solid solution structure are convex upward, and for a given plagioclase composition the excess Gibbs free energy of the MD-simulated lamellar structure is smaller than that of the MD-simulated solid solution structure. These results indicate that a lamellar structure (created by phase separation) consisting of fully ordered albite and anorthite is more stable than the solid solution structure, which consists of randomly distributed clusters of both phases. Therefore, when plagioclase feldspar is heated for a long period of time at a low temperature, phase separation between pure albite and pure anorthite can be expected if equilibrium is achieved.

Static lattice simulation of feldspar solid solutions: Ferroelastic instabilities and order/disorder

Lattice energy minimisation calculations with empirical shell model potentials have been performed on feldspar solid solutions as a function of composition and tetrahedral Al/Si order. The monoclinic-triclinic phase transitions have been simulated across the KxNa1-xAlSi3O8 and SrxCa1-xA12Si2O8 solid solutions. In both cases it is found that the transitions are driven by an elastic softening without critical softening of an optic phonon. The results for the ferroelastic spontaneous strains are consistent with experimental observation, and explain the nature of the coupling between the ferroelastic instability and Al/Si ordering.

Excess free energies of mixing of Sr,Ba-bearing binary feldspar solid solutions (experimental data)

AbstractThe distribution of potassium and strontium, potassium and barium between feldspars and aqueous chloride solutions was studied at 700°C, 2 kbar and 800°C, 2 kbar respectively. The distribution of Sr and Ba between coexisting feldspar and chloride solutions is not ideal; Sr and Ba are enriched in feldspar relative to solution. In order to estimate the phase state of the chloride solutions, special experiments were conducted using the synthetic fluid-inclusion method in the systems KCl-BaCl2-H2O and KCl-SrCl2-H2O at 700–800°C, 2 kbar. It was shown that (0.5M KCl+0.5M SrCl2) aqueous solution is homogeneous at 800°C, 2 kbar. The 0.25–0.5M SrCl2 solutions are homogeneous at 700°C 2, kbar. The (0.5M KCl+0.5M BaCl2) solution is homogeneous at 700°C, 2 kbar and heterogeneous at 800°C, 2 kbar. Margules parameters for describing the excess energy of mixing of the (K,Sr)- and (K,Ba)-feldspar solid solutions were calculated. The integrated values of excess energy of mixing () were estimated for the different binary feldspar solid solutions, and a correlation of with the crystallochemical parameter ρ = (RA-RB)/D(Me-O) was observed. Feldspar solid solutions can be subdivided into three groups based on the relationship between , p and the stability of the solid solution.

Textural evolution of the rapakivi granites, south Greenland ?Sr, O and H isotopic investigations

The development of rapakivi texture in feldspars from the Ketilidian granitoids of south Greenland has been investigated using Sr, O and H isotopes. A low temperature signature is found in the Sr and O data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the K-feldspar ovoids. An origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed, which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar. This mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of K-feldspar to microcline. Rapakivi granites with both white and dark green feldspar occur in south Greenland but show no major isotopic differences, although the dark alkali feldspars contain significantly more fluid. Equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution.

Incommensurate structures

Incommensurate structures occur in a very wide range of crystalline materials and illustrate a number of interesting aspects of the physics of atomic interaction in the context of crystal structure and bonding. Formal theories that deal with the origin of these phases are reviewed and shown to have a great deal in common. Several important examples of incommensurate structures in simple compounds are described and these include the minerals quartz and nepheline. Nepheline, which has not been analysed previously shows an interaction between oxygen displacements associated with the loss of a triad axis, a three-state Potts model, and potassium- vacancy ordering. The important role of incommensurate structures in mineral solid solutions is discussed and illustrated with reference to the plagioclase feldspar solid solution.

Phase relations in the system anorthite–potassium-feldspar at 10 kbar with emphasis on their solid solutions

AbstractPhase relations in the binary system anorthite-K-feldspar at 10 kbar were studied with a solid-media piston cylinder apparatus. The eutectic character of this system is confirmed, with the eutectic point located at An30Kf70 (± 3 mol%), and 1215 ± 15°C, 10 kbar. The mutual solubilities between anorthite and K-feldspar at the solidus are greatly increased at 10kbar compared with those at 1 atm. The maximum solubility of K-feldspar in anorthite is c. 5% at 1 atm, c. 18% at 10 kbar; and that of anorthite in K-feldspar is c. 3% at 1 atm, c. 7% at 10 kbar. The increased solubilities are attributed to the increase in the eutectic temperature at high isostatic pressures. The results imply increased ternary feldspar solid solutions at high pressures and support the existence of homogeneous feldspars with extensive ternary composition under high-P, T and anhydrous conditions.

Distribution of strontium between plagioclases and 1 molar aqueous chloride solutions at 600 °C, 1.5 kbar and 750 °C, 2 kbar

The distribution of trace amounts of Sr between plagioclase solid solutions and a chloride-bearing hydrothermal solution decreases almost exponentially by a factor of 20-25 between the albite (Ab) and anorthite (An) end-members. A simple mixing model in the ternary system Ab-An-SrF (where SrF is strontian feldspar) leads to a thermodynamic interpretation of the experimental results. This interpretation requires a knowledge x>f the three equilibrium constants concerned in the binary exchange reactions Ab-An, Ab-SrF and An-SrF. In addition, the activity coefficients of trace strontian feldspar in albite and anorthite must be determined as well as the activity coefficients of albite and anorthite in plagioclase solid solution. The results of experiments in this study on the three binary systems have enabled the calculation of these constants and coefficients. In this way, a mixing model can be established which yields a calculated partition coefficient for trace amounts of Sr in plagioclase. This calculated coefficient is near the experimentally determined coefficient. The variation of this partition coefficient between albite and anorthite can be explained by near ideal mixing behaviour in the solid solution. This is in contrast with alkali feldspar solid solutions which are not ideal and generally show an irregular variation of trace element partition coefficients from one end-member to the other.

Experimental delineation of the ?e? ? I $$\bar 1$$ and ?e? ? C $$\bar 1$$ transformations in intermediate plagioclase feldspars

Approximately 125 hydrothermal annealing experiments have been carried out in an attempt to bracket the stability fields of different ordered structures within the plagioclase feldspar solid solution. Natural crystals were used for the experiments and were subjected to temperatures of ∼650°C to ∼1,000°C for times of up to 370 days at $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ =600 bars, or $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ =1,200 bars. The structural states of both parent and product materials were characterised by electron diffraction, with special attention being paid to the nature of type e and type b reflections (at h+k=(2n+1), l=(2n+1) positions). Structural changes of the type C $$\bar 1$$ → I $$\bar 1$$ , C $$\bar 1$$ → “e” structure, I $$\bar 1$$ → “e” and “e” structure → I $$\bar 1$$ have been followed. There are marked differences between the ordering behaviour of crystals with compositions on either side of the C $$\bar 1$$ ⇌ I $$\bar 1$$ transition line. In the composition range ∼ An50 to ∼ An70 the e structure appears to have a true field of stability relative to I $$\bar 1$$ ordering, and a transformation of the type I $$\bar 1$$ ⇌ e has been reversed. It is suggested that the e structure is the more stable ordered state at temperatures of ∼ 800°C and below. For compositions more albite-rich than ∼ An50 the upper temperature limit for long range e ordering is lower than ∼ 750°C, and there is no evidence for any I $$\bar 1$$ ordering. The evidence for a true stability field for “e” plagioclase, which is also consistent with calorimetric data, necessitates reanalysis both of the ordering behaviour of plagioclase crystals in nature and of the equilibrium phase diagram for the albite-anorthite system. Igneous crystals with compositions of ∼ An65, for example, probably follow a sequence of structural states C $$\bar 1$$ → I $$\bar 1$$ → e during cooling. The peristerite, Boggild and Huttenlocher miscibility gaps are clearly associated with breaks in the albite, e and I $$\bar 1$$ ordering behaviour but their exact topologies will depend on the thermodynamic character of the order/disorder transformations.

A TEM and microprobe study of a two-perthite alkali gabbro: Implications for the ternary feldspar system

An unusual magmatic three-feldspar syenogabbro occurs 3 m inside the contact of the Klokken gabbro-syenite stock. It contains plagioclase, mesoperthite and cryptoperthite, together with a low temperature symplectite intergrowth. Textural relationships have been investigated by cathodoluminescence, bulk chemistry by microprobe, and exsolution microtextures and intracrystalline boundaries by TEM. The mesoperthite has a bulk composition around Or26Ab52An22, well outside accepted limits of ternary feldspar solid solution. It is a mainly coherent, lamellar intergrowth of sodic andesine and orthoclase with incipient Mtwinning. The cryptoperthite, bulk composition around Or61Ab33An6, is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase. The compositions of the exsolved phases have been estimated. The meso- and cryptoperthite crystals have sharp boundaries with each other.

Chemical interaction of aqueous solutions with epidote-feldspar mineral assemblages in geologic systems; II, Equilibrium constraints in metamorphic/geothermal processes

Thermodynamic analysis of the system Na/sub 2/O-K/sub 2/O-CaO-FeO-Fe/sub 2/O/sub 3/-Al/sub 2/O/sub 3/-SiO/sub 2/-H/sub 2/O-HCl-CO/sub 2/ at pressures and temperatures to 5 kb and 600/sup 0/C affords quantitative description and interpretation of phase relations among epidote, garnet, plagioclase, and alkali feldspar solid solutions in hydrothermal systems. Comparison of computed and observed compositions of these minerals and coexisting fluids suggests that the calculations afford close approximation of equilibrium and mass transfer in metamorphic/geothermal processes. The standard molal enthalpies of decomposition reactions for plagioclase are of the order of -4 to -50 kcal mole/sup -1/, which requires the activity of the anorthite component of plagioclase coexisting with epidote solid solutions to increase with increasing temperature at constant pressure. In contrast, because the standard molal volumes of these reactions are also negative, a/sub CaAl/sub 2/Si/sub 2/O/sub 8// decreases with increasing pressure at constant temperature. Either stoichiometric epidote or clinozoisite is compatible with plagioclase, quartz, calcite, and an aqueous phase, but only at X/sub CO/sub 2// less than or equal to 0.2. The complex zoning commonly exhibited by both plagioclase and epidote solid solutions in geologic systems can be attributed to minor isothermal/isobaric changes in the composition of coexisting aqueous solutions. Logarithmic activity and fugacitymore » diagrams constructed with provision for solid solution permit documentation of such changes and facilitate thermodynamic interpretation of mineral and fluid compositions in metamorphic/geothermal systems.« less