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

Our plain-English summary of Excess enthalpy of mixing of mineral solid solutions derived from density-functional calculations, by A. Benisek, E. Dachs (Physics and Chemistry of Minerals, 2020).

The study

• Authors: A. Benisek, E. Dachs
• Journal: Physics and Chemistry of Minerals
• Published: 2020;47(3):15
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

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.

© The Author(s) 2020.

In plain English

This was a computer-modelling study in mineral chemistry, not a study of asbestos exposure or human health. It did not examine fibers, workers, disease or any health outcome. The authors used quantum-mechanical calculations (density-functional theory with a 'single defect' method) to estimate the heat of mixing, the heat absorbed when two related minerals mix into one crystal. They tested the method on six mineral pairs already measured in laboratories, then applied it to a seventh pair, the amphiboles tremolite and glaucophane. Tremolite appears here only as a crystal composition, not as asbestos fibers. The calculated values agreed well with measurements for several pairs. For alkali feldspars they were slightly lower than the measured values. For tremolite–glaucophane, the calculated solvus (the temperature–composition boundary where the mixture separates into two minerals) matched natural and experimental evidence in height but not in its lopsided shape. The authors said this was most likely because of their simple entropy model. The authors concluded that the method can give accurate heat of mixing values, and needs additional information where ordering effects dominate.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Computational (theoretical) study using density-functional theory calculations; no human, animal or fiber exposure data
• Who was studied: Seven mineral solid solution pairs. Six were test cases: halite–sylvite (NaCl–KCl), pyrope–grossular garnets, MgO–CaO, Al/Si ordered alkali feldspars, diopside–jadeite pyroxenes, and diopside–CaTs pyroxenes. The seventh, tremolite–glaucophane amphiboles, was an application. No people were studied.
• Exposure: Not applicable. The study involved no exposure of people or animals. Tremolite was studied only as a chemical endmember of an amphibole solid solution, not as a fiber or as asbestos.
• Main results:
  • Halite–sylvite: the calculated heat of mixing agreed well with the experimental values
  • Pyrope–grossular garnets: calculations using the LDA functional agreed well with the calorimetric data of Newton et al. (1977)
  • MgO–CaO: the calculated solvus (the boundary at which the mixture separates into two phases) lay at slightly too low temperatures compared with the experimental solvus
  • Al/Si ordered alkali feldspars: calculated values were slightly lower than the calorimetric data. A difference of about 1 kJ/mol in heat of mixing produced solvi differing by almost 400 °C.
  • Diopside–CaTs: a partly disordered CaTs structure (4 Al–O–Al out of 32 T–O–T linkages) gave the best agreement with experiments
  • Tremolite–glaucophane: the solvus calculated from the DFT results had a similar height to natural and experimental evidence, but its asymmetry did not agree with that evidence
• The authors' conclusion: The authors concluded that DFT calculations combined with the single defect method can generate accurate heat of mixing values, especially for simple solid solutions, and that additional information is needed where ordering effects play a dominant role.
• Limitations the authors state:
  • For diopside–CaTs, the heat of mixing could not be determined unequivocally from the calculations alone without additional investigations
  • The mismatch in the shape of the tremolite–glaucophane solvus was most likely due to the simple model assumed for configurational entropy
  • Whether the method works for systems containing iron (Fe) was not established and remains under study

Topics

Medical terms in this study, explained in our Lexicon: Solid Solution, Diopside, Pyroxene, Tremolite, Amphibole Group, Cation.

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