On the mechanisms of cation diffusion processes in ternary feldspars

verfasst von: H. Behrens, W. Johannes, H. Schmalzried
Abstract: Cation diffusion processes have been studied in single crystals of intermediate plagioclase and albite composition by tracer-diffusion experiments and optical absorption spectroscopy. Tracer-diffusivities were determined by the residual activity method, using the radioactive isotopes ²²Na, ⁴⁵Ca and ⁵⁹Fe. In most cases, diffusion experiments were performed at 1 bar, at controlled oxygen activity and at temperatures between 750 and 1300°C. The obtained Na-diffusivities for plagioclases were much smaller then previously determined for albite. This indicates a strong composition dependence of Na-diffusion. In contrast, Ca-diffusivity in albite does not differ very much from that in intermediate plagioclases. The relative diffusivities determined for plagioclase of composition An62 at 1200° C (CO/CO₂ =50:50) were D_infNa^sup*:D_infFe^sup*:D_infCa^sup*=5000:10:1. Despite the an isotropic structure of feldspars, no difference was found for Na-and Ca-diffusion normal to (001) and normal to (010). Water pressure of 2 kbar has no influence on the Na-diffusivity. In contrast to the Ca-diffusion, a dependence on oxyggen activity was found for Na-and Fe-diffusion. Fe-diffusivity increases with decreasing oxygen activity. This can be correlated to changes in oxidation state of iron dissolved in the plagioclases. Optical absorption spectroscopy shows that iron is oxidized in the plagioclases by annealing in air. This effect can be reversed by annealing at reducing conditions. A model is proposed to explain the oxidation of iron by a chemical diffusion process in which A-vacancies are formed by out-diffusion of Na⁺. Preannealing of samples in air gives a temperature independent decrease of Na-diffusivity by a factor of about 2.5. This effect is explained with help of a simple disorder model for A-cations in ternary feldspars. It is concluded that Na⁺ diffuses via interstitials and that the A-vacancy concentration in the plagioclases is controlled extrinsically, probably by dissolved SiO₂.
Organisationseinheit(en): Institut für Mineralogie
Typ: Artikel
Journal: Physics and chemistry of minerals
Band: 17
Seiten: 62-78
Anzahl der Seiten: 17
ISSN: 0342-1791
Publikationsdatum: 01.1990
Publikationsstatus: Veröffentlicht
Peer-reviewed: Ja
ASJC Scopus Sachgebiete: Allgemeine Materialwissenschaften, Geochemie und Petrologie
Elektronische Version(en): https://doi.org/10.1007/BF00209227 (Zugang: Geschlossen)

BibTeX

@article{6489091b61a546debcc318faf008d2a6,
title = "On the mechanisms of cation diffusion processes in ternary feldspars",
abstract = "Cation diffusion processes have been studied in single crystals of intermediate plagioclase and albite composition by tracer-diffusion experiments and optical absorption spectroscopy. Tracer-diffusivities were determined by the residual activity method, using the radioactive isotopes 22Na, 45Ca and 59Fe. In most cases, diffusion experiments were performed at 1 bar, at controlled oxygen activity and at temperatures between 750 and 1300°C. The obtained Na-diffusivities for plagioclases were much smaller then previously determined for albite. This indicates a strong composition dependence of Na-diffusion. In contrast, Ca-diffusivity in albite does not differ very much from that in intermediate plagioclases. The relative diffusivities determined for plagioclase of composition An62 at 1200° C (CO/CO2 =50:50) were DinfNasup*:DinfFesup*:DinfCasup*=5000:10:1. Despite the an isotropic structure of feldspars, no difference was found for Na-and Ca-diffusion normal to (001) and normal to (010). Water pressure of 2 kbar has no influence on the Na-diffusivity. In contrast to the Ca-diffusion, a dependence on oxyggen activity was found for Na-and Fe-diffusion. Fe-diffusivity increases with decreasing oxygen activity. This can be correlated to changes in oxidation state of iron dissolved in the plagioclases. Optical absorption spectroscopy shows that iron is oxidized in the plagioclases by annealing in air. This effect can be reversed by annealing at reducing conditions. A model is proposed to explain the oxidation of iron by a chemical diffusion process in which A-vacancies are formed by out-diffusion of Na+. Preannealing of samples in air gives a temperature independent decrease of Na-diffusivity by a factor of about 2.5. This effect is explained with help of a simple disorder model for A-cations in ternary feldspars. It is concluded that Na+ diffuses via interstitials and that the A-vacancy concentration in the plagioclases is controlled extrinsically, probably by dissolved SiO2.",
author = "H. Behrens and W. Johannes and H. Schmalzried",
year = "1990",
month = jan,
doi = "10.1007/BF00209227",
language = "English",
volume = "17",
pages = "62--78",
journal = "Physics and chemistry of minerals",
issn = "0342-1791",
publisher = "Springer Verlag",
number = "1",
}