Details
| Original language | English |
|---|---|
| Pages (from-to) | 62-78 |
| Number of pages | 17 |
| Journal | Physics and chemistry of minerals |
| Volume | 17 |
| Issue number | 1 |
| Publication status | Published - Jan 1990 |
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.
ASJC Scopus subject areas
- Materials Science(all)
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Physics and chemistry of minerals, Vol. 17, No. 1, 01.1990, p. 62-78.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - On the mechanisms of cation diffusion processes in ternary feldspars
AU - Behrens, H.
AU - Johannes, W.
AU - Schmalzried, H.
PY - 1990/1
Y1 - 1990/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0002451809&partnerID=8YFLogxK
U2 - 10.1007/BF00209227
DO - 10.1007/BF00209227
M3 - Article
AN - SCOPUS:0002451809
VL - 17
SP - 62
EP - 78
JO - Physics and chemistry of minerals
JF - Physics and chemistry of minerals
SN - 0342-1791
IS - 1
ER -