High Structural State Research Articles

Smaller Hippocampal Volume Among Black and Latinx Youth Living in High-Stigma Contexts.

To determine whether structural and individual forms of stigma are associated with neurodevelopment in children. Stigma related to gender, race, and Latinx ethnicity was measured at the structural level using objective state-level indicators of social policies and prejudicial attitudes and at the individual level using self-reports of perceived discrimination. Respective associations of stigma with hippocampal volume and amygdala reactivity to threat were examined using data from the Adolescent Brain Cognitive Development (ABCD) Study (N= 11,534, mean age 9.9 years), the first multisite neuroimaging study that provided substantial variability in sociopolitical contexts and that included individual-level measures of stigma among youth. In a preregistered analysis, Black (B=-58.26, p= .023) and Latinx (B=-40.10, p= .044) youths in higher (vs lower) structural stigma contexts were found to have smaller hippocampal volume, controlling for total intracranial volume, demographics, and family socioeconomic status. This association was also observed at a trend-level among girls (p= .082). The magnitude of the difference in hippocampal volume between high and low structural stigma states was equivalent to the predicted impact of a $20,000 difference in annual family income in this sample. As hypothesized, structural stigma was not associated with hippocampal volume in nonstigmatized youths, providing evidence of specificity. Perceived discrimination was unrelated to hippocampal volume in stigmatized groups. No associations between perceived discrimination or structural stigma and amygdala reactivity to threat were observed. This study provides novel evidence that an objective measure of structural stigma may be more strongly related to hippocampal volume than subjective perceptions of stigma, suggesting that contextual approaches to stigma could yield new insights into neurodevelopment among marginalized youth.

Microstructure and mechanical properties of V–Cr–Zr alloy with carbide and oxide strengthening

A comparative study of the effectiveness of carbide and oxide types of strengthening of V–Cr–Zr alloy was carried out by means of a comprehensive certification of structural-phase state parameters and measuring the mechanical properties characteristics. It has been shown that the use of chemical-heat treatment contributes to a significant increase in the thermal stability of the microstructure and mechanical properties of V–Cr–Zr alloy in comparison with carbide strengthening under the conditions of thermomechanical treatment. A controlled increase in the volume fraction of fine particles based on ZrO2, along with an increase in the concentration of oxygen in the solid solution, leads to a decrease in the rate of oxides coagulation and an increase in the thermal stability of high disperse heterophase structure. These effects contribute to the retention of high defect structural states with nonzero values of crystal lattice curvature even after high-temperature (0.67 Tmelt) anneals. The high efficiency of dispersion and substructural strengthening is a consequence of blocking dislocation slip by fine particles stabilized by oxygen in a solid solution.

Cumulative exposure to state-level structural sexism and risk of disordered eating: Results from a 20-year prospective cohort study

BackgroundGendered inequities in disordered eating are well-documented, yet few studies have examined their structural drivers. To help fill this gap, we investigated whether cumulative exposure to state-level structural sexism from childhood through young adulthood potentiates differences in disordered eating risk between cisgender girls/women and boys/men. MethodsParticipants came from the Growing Up Today Study (N = 16,875), a cohort of children aged 9–14 years in 1996 who we followed through 2016. Using a composite index of relevant state policies and social inequalities from the Institute for Women's Policy Research, we categorized states as having high or low levels of structural sexism and summed the number of years participants had lived in a high structural sexism state during the study period to quantify their cumulative exposure. We fit sequential conditional mean models to estimate the effect of cumulative exposure on risk of four outcomes (chronic dieting, purging, binge eating, and overeating), controlling for individual- and state-level confounders via propensity scores. We then tested whether effects differed between girls/women and boys/men by including cumulative-exposure-by-gender-identity interaction terms and calculating the relative excess risk due to interaction (RERI). ResultsIn the full sample, each additional year of living in a high structural sexism state was associated with a 5% increased risk of purging (95% confidence interval (CI): 3%, 7%), an 8% increased risk of binge eating (95% CI: 6%, 10%), and a 9% increased risk of overeating (95% CI: 8%, 11%). Risk increases were larger on average for girls/women than for boys/men, and girls/women who had lived in a high structural sexism state for four or more years had excess risk of chronic dieting (RERI: 0.64, 95% CI: 0.18, 1.10), purging (RERI: 2.64, 95% CI: 1.24, 4.30), and binge eating (RERI: 2.21, 95% CI: 0.93, 3.50). ConclusionsStructural sexism may contribute to inequities in disordered eating between cisgender girls/women and boys/men. Future research should include transgender and gender diverse participants, explore intersectional effects, and identify underlying mechanisms to inform policy-oriented interventions.

Exploring How U.S. High School Staff Support, Protect, and Affirm Sexual and Gender Minority Youth: Methods and Lessons Learned from a Qualitative Interview Study

Background. Sexual and gender minority youth (SGMY) experience health inequities compared with cisgender heterosexuals, and these inequities are heightened in areas with high structural stigma. Quantitative research shows school assets (e.g., adult support) are associated with better health for SGMY. Though some qualitative studies elucidated how school staff support SGMY, none have triangulated such strategies in a geographically and sociodemographically diverse sample of school staff and SGMY. This paper describes a multi-perspective qualitative study design and offers lessons learned from conducting such a study. Methods. Using a novel stratified sampling frame, we interviewed 60 SGMY and 29 school staff who attended/worked at high schools in U.S. states with low, medium, and high structural stigma. To ensure sociodemographic diversity, we constructed sampling quotas, and recruited SGMY using social media and staff using a multi-pronged approach. Results. The stratified sampling strategy met our goal of enrolling diverse SGMY and staff participants. SGMY participants attended schools in low ( n = 20), medium ( n = 22), and high ( n = 18) structural stigma states. We enrolled 18 cisgender girls, 18 cisgender boys, and 24 gender minority youth. Fifty-three percent of SGMY were youth of color, and 45% attended schools in rural areas. School staff participants worked at schools in low ( n = 11), medium ( n = 11), and high ( n = 7) structural stigma states. School staff participants were 55% heterosexual, 91% cisgender, and had diverse roles (e.g., teacher, principal, librarian, and nurse). Conclusions. This paper describes new methods for collecting qualitative data from diverse SGMY and school staff. Some lessons learned from this study include the importance of using trauma-informed interviewing methods, having a suicidality safety protocol, establishing a priori sampling quotas, and creating tailored social media advertisements. With these data we will explore the heterogeneity of SGMY and school staff experiences across varying structural stigma levels, yielding foundational information for future school-based interventions.

Effect of High Strength Structural Steel Structural State on Fracture Resistance

Effect of High Strength Structural Steel Structural State on Fracture Resistance

Influence of V–Cr–W–Zr alloy microstructure on features of oxidation during chemical-heat treatment

Investigation results of microstructure influence on oxidation coefficient during chemical-heat treatment of V–Cr–W–Zr alloy are presented. It is shown that the formation of a high defect structural state in the alloy leads to a twofold increase in the oxidation coefficient and a significant increase in the oxygen concentration. The maximum effects of the microstructure on the oxidation processes are detected at short treatment times.

High‐induction nanocrystalline soft magnetic FeXTiYBZ films prepared by magnetron sputtering

AbstractTo design films with the Fe/TiB2 nanocomposite structure, which are characterized by high saturation induction Bs, the phase and structural states and static magnetic properties of Fe‐TiB2 films prepared by magnetron sputtering and subjected to subsequent annealing have been studied. According to X‐ray diffraction data, either amorphous or nanocrystalline single‐phase structure (an α‐Fe(Ti,B) supersaturated solid solution with a bcc crystal lattice) is formed in the as‐sputtered films. Depending on the film composition, the grain size of the α‐Fe(Ti,B) phase varies from 45.6 to 6.5 nm; grains are characterized by high microstrain (0.21‐4.96%). The annealing at 200‐500 °C leads to a decrease in the lattice parameter of the α‐Fe(Ti,B) phase, i.e. to its depletion of titanium and boron and to the formation of two‐phase α‐Fe + Fe3B structure after annealing at 500 °C. The annealing at 200‐500 °C almost does not affect the grain size and microstrain of the bcc α‐Fe‐based phase. The amorphous state of the films is stable up to 500 °C. All studied films are ferromagnets; the saturation induction Bs(0.95‐2.13 T) and coercive field Hc (0.4‐5 kA/m) of the films were determined. Correlations between the Bs and Hc magnitudes and the chemical composition of the films, their phase and structural states and magnetic structure are discussed. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural racism and myocardial infarction in the United States

There is a growing research literature suggesting that racism is an important risk factor undermining the health of Blacks in the United States. Racism can take many forms, ranging from interpersonal interactions to institutional/structural conditions and practices. Existing research, however, tends to focus on individual forms of racial discrimination using self-report measures. Far less attention has been paid to whether structural racism may disadvantage the health of Blacks in the United States. The current study addresses gaps in the existing research by using novel measures of structural racism and by explicitly testing the hypothesis that structural racism is a risk factor for myocardial infarction among Blacks in the United States. State-level indicators of structural racism included four domains: (1) political participation; (2) employment and job status; (3) educational attainment; and (4) judicial treatment. State-level racial disparities across these domains were proposed to represent the systematic exclusion of Blacks from resources and mobility in society. Data on past-year myocardial infarction were obtained from the National Epidemiologic Survey on Alcohol and Related Conditions (non-Hispanic Black: N = 8245; non-Hispanic White: N = 24,507), a nationally representative survey of the U.S. civilian, non-institutionalized population aged 18 and older. Models were adjusted for individual-level confounders (age, sex, education, household income, medical insurance) as well as for state-level disparities in poverty. Results indicated that Blacks living in states with high levels of structural racism were generally more likely to report past-year myocardial infarction than Blacks living in low-structural racism states. Conversely, Whites living in high structural racism states experienced null or lower odds of myocardial infarction compared to Whites living in low-structural racism states. These results raise the provocative possibility that structural racism may not only harm the targets of stigma but also benefit those who wield the power to enact stigma and discrimination.

Heat capacity and entropy of low structural state plagioclases

The heat capacity of 11 different compositions of the low structural state plagioclase binary was measured between 5 and 773 K. The results are compared to those obtained from synthetic high structural state samples investigated in a previous study and obtained from a heat-treated natural sample of this study. The heat capacity of anorthite samples with slightly different order parameters shows large differences at ~500 K, where the Ibar1-Pbar1 phase transition occurs, which affects petrological calculations. At T = 298.15 K, the vibrational entropy versus composition behaviour of the low structural state plagioclases is almost ideal, in contrast to the high structural state plagioclases with positive excess vibrational entropies. At higher temperatures, the low structural state plagioclases show a negative deviation from ideal vibrational entropy composition behaviour in the Na-rich region.

Subtle Monte Carlo Updates in Dense Molecular Systems.

Although Markov chain Monte Carlo (MC) simulation is a potentially powerful approach for exploring conformational space, it has been unable to compete with molecular dynamics (MD) in the analysis of high density structural states, such as the native state of globular proteins. Here, we introduce a kinetic algorithm, CRISP, that greatly enhances the sampling efficiency in all-atom MC simulations of dense systems. The algorithm is based on an exact analytical solution to the classic chain-closure problem, making it possible to express the interdependencies among degrees of freedom in the molecule as correlations in a multivariate Gaussian distribution. We demonstrate that our method reproduces structural variation in proteins with greater efficiency than current state-of-the-art Monte Carlo methods and has real-time simulation performance on par with molecular dynamics simulations. The presented results suggest our method as a valuable tool in the study of molecules in atomic detail, offering a potential alternative to molecular dynamics for probing long time-scale conformational transitions.

Excess heat capacity and entropy of mixing in ternary series of high-structural-state feldspars

Low- and high-temperature heat capacities were measured for ternary series of synthetic, high structural state feldspars in the NaAlSi 3 O 8 –KAlSi 3 O 8 –CaAl 2 Si 2 O 8 system that had been characterised by X-ray diffraction and transmission light microscopy. The compositions of the phases lie in the NaAlSi 3 O 8 -rich part of the system where the stability is less limited by the ternary miscibility gap than in other compositional fields. The heat capacities of the end-members had been measured in previous studies using the same calorimeters as in this study (relaxation calorimeter and differential scanning calorimeter). Below 300 K, all samples showed strong positive excess heat capacities of mixing giving rise to excess entropies of mixing up to Δ S ex = 5.8 ± 1.6 J mol −1 K −1 at T = 298.15 K. Above 300 K, further contributions to the excess entropies of mixing do not appear to be significant. The non-ideal entropic mixing behaviour was described by a ternary asymmetric Margules model, resulting in the ternary interaction parameter W AbOrAn S = 93.9 J mol −1 K −1 .

A ternary feldspar-mixing model based on calorimetric data: development and application

A mixing model for high structural state ternary feldspars in the NaAlSi3O8–KAlSi3O8–CaAl2Si2O8 system is presented based exclusively on calorimetric and volumetric measurements. Comparisons with existing mixing models, which are based on phase-equilibrium experiments, reveal distinct differences. The incorporation of K into Ca-rich plagioclase and of Ca into K-rich alkali feldspar is more strongly limited by our calorimetry-based model, whereas the stability field of Na-rich feldspars is broadened. Natural feldspar assemblages from well-studied magmatic and high-grade metamorphic rocks (i.e. a teschenite sill in Scotland, the Klokken syenogabbro in Greenland, and a granulite-facies metapelite in Sri Lanka) were used to test the mixing models. It was found that the new model largely eliminates discrepancies between observed and predicted feldspar compositions that were present in earlier attempts. The reasons for the problems associated with phase-equilibrium based mixing models are discussed.

Excess heat capacity and entropy of mixing in the high-structural state (K,Ca)-feldspar binary

Low- and high-temperature heat capacities were measured for a series of synthetic high-structural state (K,Ca)-feldspars (Or–An) using both a relaxation and a differential scanning calorimeter. The data were collected at temperatures between 5 and 800 K on polycrystalline samples that had been synthesised and characterised in a previous study. Below T = 300 K, Or90An10, and Or80An20 showed excess heat capacities of mixing with maximum values of ~3 J mol−1 K−1. The other members of this binary (An > 20 mol%) had lower excess heat capacity values of up to ~1 J mol−1 K−1. Above T = 300 K, some compositions exhibited negative excess heat capacities of mixing (with maximum values of −2 J mol−1 K−1). The vibrational entropy at 298.15 K for Or90An10 and Or80An20 deviated strongly from the behaviour of a mechanical mixture, with excess entropy values of ~3.5 J mol−1 K−1. More An-rich members had only small excess vibrational entropies at T = 298.15 K. The difference in behaviour between members with An > 20 mol% and those with An ≤ 20 mol% is probably a consequence of the structural state of the (K,Ca)-feldspars, i.e., (K,Ca)-feldspars with An ≤ 20 mol% have monoclinic symmetry, whereas those with An > 20 mol% are triclinic. At T = 800 K, the vibrational entropy values were found to scatter around the values expected for a mechanical mixture and, thus, correspond to a quasi-ideal behaviour. The solvus for the (K,Ca)-feldspar binary was calculated based on the entropy data from this study in combination with enthalpy and volume of mixing data from a previous study.

Excess heat capacity and entropy of mixing in high structural state plagioclase

Low- and high-temperature heat capacities for a series of synthetic high structural state plagioclase crystals (Ab-An) were measured using both a relaxation calorimeter and a differential scanning calorimeter. The measurements were performed at temperatures between 5 and 800 K on milligram-sized polycrystalline samples that had been characterized in a previous study. The data show positive excess heat capacities of mixing at temperatures below 300 K with a maximum value of ~2 J/(mol·K). Below ~70 K, the excess heat capacities exceed two standard deviations and are thus significant. Above 300 K, the measurements indicate negative excess heat capacities with a maximum of ca. −1.5 J/(mol·K) at about 400 K, and do not exceed two standard deviations. The excess vibrational entropies of mixing are positive with an asymmetric variation. At T = 298.15 K, the largest deviation from ideal behavior occurs at Ab 20 An 80 amounting to Δ S vib ex = 2.8 ± 2.4 J/(mol·K). An asymmetric Margules mixing model was found to adequately describe the vibrational entropy-composition behavior, yielding W AbAn Svib = 16.4 J/(mol·K) and = W AbAn Svib 4.7 J/(mol·K).

Flexibility of Actin Filaments During Myosin Induced Sliding

The bending flexibility of actin filaments, freely suspended in solution or propelled by heavy meromyosin (HMM) in the in vitro motility assay, was studied under different experimental conditions using both phalloidin-stabilized and phalloidin-free filaments (visualized by N-Hydroxysuccinimide-Rhodamine). For actin filaments propelled by HMM, the persistence length (inversely related to the bending flexibility) was obtained from exponential fits to the cosine correlation function for sliding paths. In solution (in the absence of HMM) both phalloidin free and phalloidin stabilized filaments exhibited a nearly rectilinear increase (slope ≈ 0.046 μm/mM) in persistence length upon increased ionic strength of the observation solution (from 40 to 130 mM). For phalloidin-free and phalloidin-stabilized actin filaments (80 mM ionic strength), LP in solution was 9.2 ± 0.8 μm (mean ± 95 % confidence interval) and 15.7 ± 1.1 μm, respectively (> 80 filaments). For phalloidin free filaments, the persistence length was similar whether the filaments were propelled by HMM or freely suspended in solution in the absence of HMM. In contrast, the persistence length for phalloidin stabilized actin filaments was reduced to the same level as for phalloidin free filaments during HMM induced sliding. The results accord with the idea that a high flexibility structural state of the actin filament is a perquisite for actomyosin motility. However, the results do not support ideas that a low-to-high flexibility transition of the filament should be an important component of the force-generating step per se. Finally, our results challenge the general view that phalloidin stabilized filaments behave as native actin filaments in their interaction with myosin.

Enthalpies in (Na,Ca)- and (K,Ca)-feldspar binaries: a high-temperature solution calorimetric study

A series of high structural state plagioclases (Ab–An) was crystallized from glasses. By exchanging Na for K in KCl melts, metastable K-plagioclases (Or–An) were prepared which possess the same structural state as the starting plagioclases. Both series were investigated at 980 K by lead borate solution calorimetry. Continuing the ideas of Carpenter and McConnell (1984) and Carpenter (1992a), the results can be interpreted as follows. In the high plagioclase series, the enthalpies of solution, ΔHsol, reflect the schemes of Al,Si ordering: (1) analbite-like (C2/m) ordering in the An-poor region 0≤XAn≤0.2, (2) high albite-like (C1) ordering in intermediate plagioclases, and (3) anorthite-like (I1) ordering in the An-rich region 0.7 0.7) can be "corrected" for the C1→I1 ordering effect (Carpenter 1992a). When combining the corrected values with the ΔHsol data which were actually measured on the C1 plagioclases (XAn<0.7), negative excess enthalpies are generated in the plagioclase binary. This may be expected when C1 ordering occurs relative to topochemically monoclinic reference states of analbite and hypothetical anorthite devoid of I1 order. The solution experiments on the K-plagioclases resulted in similar characteristics as those found for the plagioclases. However, in addition to the ordering effects observed in the plagioclase binary, volume mismatch effects contribute to ΔHex in the K-plagioclase series. ΔHex can be represented by a Margules model with WHAnOr=60±10 kJ/mol and WHOrAn=91±14 kJ/mol when the phase transitions at XAn=0.2 and XAn=0.7 are again neglected. The contribution of the volume mismatch effect to ΔHex is considerable, as appears from the large difference between the K-plagioclase and the plagioclase Margules parameters. Their difference corresponds to a practically symmetrical dependence of ΔHexvolmism on composition, with WHvolmism=48±13 kJ/mol.

Microstructures and high-temperature phase transitions in kalsilite

Other| June 01, 1996 Microstructures and high-temperature phase transitions in kalsilite Michael A. Carpenter; Michael A. Carpenter University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom Search for other works by this author on: GSW Google Scholar Daniela Cellai Daniela Cellai Search for other works by this author on: GSW Google Scholar American Mineralogist (1996) 81 (5-6): 561–584. https://doi.org/10.2138/am-1996-5-604 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Michael A. Carpenter, Daniela Cellai; Microstructures and high-temperature phase transitions in kalsilite. American Mineralogist 1996;; 81 (5-6): 561–584. doi: https://doi.org/10.2138/am-1996-5-604 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract A diversity of twinning and exsolution textures has been observed by transmission electron microscopy in natural kalsilite from volcanic and metamorphic rocks (Mount Nyiragongo, Zaire; Alban Hills, Italy; Kerala, southern India). The phase transitions responsible for these microstructures were examined by high-temperature powder and singlecrystal X-ray diffraction. Pure kalsilite has P63mc symmetry at high temperatures but transforms to its P63 state through a two-phase field, between ∼870 and ∼920 °C. In this two-phase field, the low form coexists with a structure that has a sixfold repeat of its a unit-cell dimension. For ∼Ks88, the high-temperature state is high tetrakalsilite with possible space group P63mc; a two-phase field between ∼890 and ∼930 °C involves the coexistence of this phase with P63 kalsilite. High tetrakalsilite with composition ∼Ks74 reverts to low tetrakalsilite on quenching to room temperature from 950 °C. Lattice-parameter variations, and estimates of the Al-O-Si angles that can be derived from them, imply that the stability limit for the high structural states occurs when the angles for apical and basal O atoms converge. At lower temperatures, pure P63 kalsilite appears to have transformed during a metamorphic evolution in nature to an intergrowth of the P63 structure and a structure for which P31 c symmetry is proposed. The latter can be thought of as a polytype of kalsilite, with (001) layers stacked in an eclipsed array rather than in the staggered array of normal low kalsilite. In this regard, KAlSiO4 seems to be remarkably similar to KLiSO4. If ∼ 3-12 mol% NaAlSiO4 is present in solid solution, a P63 structure with an a parameter √3 greater than normal low kalsilite develops. An anomaly in the lattice parameters of Ks88 suggests that the transition temperature below which this superstructure develops may be ∼500 °C. Natural nepheline exsolved from kalsilite displays merohedral twinning, which can be accounted for by a P63mc → P63 transition during cooling. Transformation behavior in the Ne-Ks system may be explained, at least qualitatively, in terms of Na-K ordering between cavity sites, ordering of basal O atoms between two sites on either side of the mirror plane parallel to the c axis of P63mc structures, ordering of apical O atoms between three sites, and coupling between all these processes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Tetrahedral site ordering in synthetic gallium albite: A 29Si MAS NMR study

The ordering of Si in the tetrahedral sites of gallium albite (NaGaSi 3O 8) has been studied by MAS NMR and Rietveld structure refinement of X-ray powder diffraction data. Low structural state (ordered) material was annealed at about 800°C under a load pressure of 1 kbar, and by Rietveld refinement has tetrahedral-site occupancies for Si of T 1 O = 0.24(3), T 1 m = 0.89(2), T 2 O = 0.98(2), and T 2 m = 0.89(2), respectively. Corresponding Si occupancies for high structural state (disordered) material are 0.71(2), 0.78(1), 0.76(2), and 0.74(2), respectively. The 29Si MAS NMR spectra of low gallium albite is equivalent to the three-peak spectrum of natural (Amelia) albite, with resonances at −89.6, −96.4, and −104.2 ppm but with relative peak areas of 0.79:1.0:0.77. The tetrahedral-site occupancies derived from the MAS NMR spectrum are in good agreement with those obtained by Rietveld refinement and, in particular, indicate that the −96.4 ppm peak must correspond to Si in T 2 O. This is the first independent assignment of the 29Si peak at −96 ppm in the spectrum of ordered albite to the T 2 O site. A peak at −96 ppm is also resolved in the spectrum of high gallium albite. The systematic differences in peak position between the 29Si MAS NMR spectra of low gallium albite and those of Amelia albite cannot be explained simply by the direct replacement of Al by Ga, without a change in angle at the bridging oxygen atoms.

K-Na exchange equilibria between muscovite-paragonite solid solution and hydrothermal chloride solutions

AbstractThree ion exchange equilibrium isotherms between muscovite-paragonite solid solution and 2-molal KCl-NaCl aqueous solutions have been studied at (1) 420°C 1 kbar, (2) 420°C, 2 kbar, and (3) 550°C, 2 kbar. The ΔG°(Joules) ± 2σ of the ion exchange reaction are: ΔG° (1) = −17 259±686, ΔG° (2) = −18 268 ± 560, ΔG° (3) = −16018 ± 336. The excess mixing parameters (‘subregular solution’) of the solid solution (at 1 bar) have been calculated:The corresponding binodal compositions are (muscovite mol fraction): 12–56% at 420 °C, 1 bar and 15–51% at 550 °C 1 bar. The compositions of micas in equilibrium with perthites (high structural state) at 400, 500, 600 °C and 2 kbar are respectively:Xmus= 91, 86, and 82%.The mixing properties of the solution were estimated using the speciation of two molal chloride solutions calculated from the dissociation constants of NaCl and KCl in aqueous solution. Although NaCl appears to be substantially more dissociated than KCl, the resulting excess free energy of mixing of the hydrothermal (Na,K)Cl solution was found less than 500 J at temperatures above 400 °C and pressures up to 2 kbar.The difference in Gibbs free energy of formation (from the elements at 25 °C, 1 bar) between NaCl and KCl in two molal aqueous solutions is proposed:

Enthalpies of ordering in the plagioclase feldspar solid solution

Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A “low” series contained the natural ordered material and a “high” series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole “e” structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il̄ structure →- Cl̄ high albite structure: ~0.7-1.9 kcal/mole Il̄ structure equilibrated at low temperature → Il̄ structure equilibrated at high temperature: ~ 1.8−0.8 kcal/mole. ΔH soln data for the high series overlap with the data of Newton et al. (1980) for synthetic high structural state plagioclases except in the composition range ~An 90–An 100. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl̄ symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il̄ and “e” structures. Values of the enthalpy change due to disordering ( ΔH ord ) also show a number of systematic trends. Firstly, the values for e → Cl̄ are larger than for Il̄ → Cl̄ in the composition range where both e and Il̄ structures are observed (~An 65-An 72). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔH ord , which occurs at ~An 50, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il̄ anorthites and bytownites to a Cl̄ structure have been estimated by assuming that the Cl̄ solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.