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Rainer Abart - One of the best experts on this subject based on the ideXlab platform.
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effect of chemically induced fracturing on the ice nucleation activity of Alkali Feldspar
Atmospheric Chemistry and Physics, 2021Co-Authors: Alexei Kiselev, E. Petrishcheva, Alice Keinert, Tilia Gaedeke, T Leisner, Christoph Sutter, Rainer AbartAbstract:Abstract. Feldspar is an important constituent of airborne mineral dust. Some Alkali Feldspars exhibit particularly high ice nucleation (IN) activity. This has been related to structural similarities of the ice (10 1 ‾ 0) prism planes and the (100) planes of Alkali Feldspar. Here the effect of generating Feldspar surfaces with close to (100) orientation by means of chemically induced fracturing on the IN activity of Alkali Feldspar was investigated experimentally. To this end, gem-quality K-rich Alkali Feldspar was shifted towards more Na-rich compositions by cation exchange with an NaCl–KCl salt melt at 850 ∘ C. By this procedure, a system of parallel cracks with an orientation close to the (100) plane of the Feldspar was induced. Droplet-freezing assay experiments performed on grain mounts of the cation-exchanged Alkali Feldspars revealed an increase in the overall density of ice-nucleating active site (INAS) density with respect to the untreated Feldspar. In addition, annealing at 550 ∘ C subsequent to primary cation exchange further enhanced the INAS density and led to IN activity at exceptionally high temperatures. Although very efficient in experiment, fracturing by cation exchange with an Alkali halide salt is unlikely to be of relevance in the conditioning of Alkali Feldspars in nature. However, parting planes with similar orientation as the chemically induced cracks may be generated in lamellar microstructures resulting from the exsolution of initially homogeneous Alkali Feldspar, a widespread phenomenon in natural Alkali Feldspar known as perthite formation. Perthitic Alkali Feldspars indeed show the highest IN activity. We tentatively ascribe this phenomenon to the preferential exposure of Feldspar crystal surfaces oriented sub-parallel to (100).
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Spinodal decomposition in Alkali Feldspar studied by atom probe tomography
Physics and Chemistry of Minerals, 2020Co-Authors: E. Petrishcheva, Gerlinde Habler, Lisa Tiede, Kevin Schweinar, Baptiste Gault, Rainer AbartAbstract:We used atom probe tomography to complement electron microscopy for the investigation of spinodal decomposition in Alkali Feldspar. To this end, gem-quality Alkali Feldspar of intermediate composition with a mole fraction of $$a_{\text {K}}=0.43$$ a K = 0.43 of the K end-member was prepared from Madagascar orthoclase by ion-exchange with (NaK)Cl molten salt. During subsequent annealing at $$550\,^\circ \hbox {C}$$ 550 ∘ C and close to ambient pressure the ion-exchanged orthoclase unmixed producing a coherent lamellar intergrowth of Na-rich and K-rich lamellae. The chemical separation was completed, and equilibrium Na–K partitioning between the different lamellae was attained within four days, which was followed by microstructural coarsening. After annealing for 4 days, the wavelength of the lamellar microstructure was $$\approx 17\,\hbox {nm}$$ ≈ 17 nm and it increased to $$\approx 30\,\hbox {nm}$$ ≈ 30 nm after annealing for 16 days. The observed equilibrium compositions of the Na-rich and K-rich lamellae are in reasonable agreement with an earlier experimental determination of the coherent solvus. The excess energy associated with compositional gradients at the lamellar interfaces was quantified from the initial wavelength of the lamellar microstructure and the lamellar compositions as obtained from atom probe tomography using the Cahn–Hilliard theory. The capability of atom probe tomography to deliver quantitative chemical compositions at nm resolution opens new perspectives for studying the early stages of exsolution. In particular, it helps to shed light on the phase relations in nm scaled coherent intergrowth.
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Diffusion-controlled crack propagation in Alkali Feldspar
Physics and Chemistry of Minerals, 2019Co-Authors: E. Petrishcheva, Markus Rieder, Jožef Predan, F D Fischer, Gerald Giester, Rainer AbartAbstract:The chemically driven propagation of interacting parallel cracks in monoclinic Alkali Feldspar was studied experimentally. Single crystals of potassium-rich gem-quality sanidine were shifted towards more sodium-rich compositions by cation exchange with a NaCl–KCl salt melt at a temperature of $$850\,^{\circ }\hbox {C}$$ 850 ∘ C and close to ambient pressure. Initially, a zone with elevated sodium content formed at the crystal surfaces due to the simultaneous in-diffusion of sodium and out-diffusion of potassium, where the rate of cation exchange was controlled by sodium–potassium interdiffusion within the Feldspar. A chemical shift of potassium-rich Alkali Feldspar towards more sodium-rich compositions produces highly anisotropic contraction of the crystal lattice. This induced a tensile stress state in the sodium-rich surface layer of the crystals, which triggered the formation of a system of nearly equi-spaced parallel cracks oriented approximately perpendicular to the direction of maximum shortening. Crack propagation following their nucleation was driven by cation exchange occurring along the crack flanks and was controlled by the intimate coupling of the diffusion-mediated build-up of a tensile stress state around the crack tips and stress release by successive crack propagation. The critical energy release rate of fracturing was determined as 1.8–2.2 $$~ \hbox {J}\,\hbox {m}^{-2}$$ J m - 2 from evaluation of the near-tip J-integral. The mechanism of diffusion-controlled crack propagation is discussed in the context of high-temperature Feldspar alteration.
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Potassium self-diffusion in a K-rich single-crystal Alkali Feldspar
Physics and Chemistry of Minerals, 2017Co-Authors: Fabian Hergemöller, Rainer Abart, Matthias Wegner, Manfred Deicher, Herbert Wolf, Florian Brenner, Herbert Hutter, Nicolaas A. StolwijkAbstract:The paper reports potassium diffusion measurements performed on gem-quality single-crystal Alkali Feldspar in the temperature range from 1169 to 1021 K. Natural sanidine from Volkesfeld, Germany was implanted with $${}^{43}{\hbox {K}}$$ 43 K at the ISOLDE/CERN radioactive ion-beam facility normal to the $$\left( 001\right)$$ 001 crystallographic plane. Diffusion coefficients are well described by the Arrhenius equation with an activation energy of 2.4 eV and a pre-exponential factor of $$5 \times 10^{-6}\,{\hbox {m}}^2/{\hbox {s}}$$ 5 × 10 - 6 m 2 / s , which is more than three orders of magnitude lower than the $${}^{22}{\hbox {Na}}$$ 22 Na diffusivity in the same Feldspar and the same crystallographic direction. State-of-the-art considerations including ionic conductivity data on the same crystal and Monte Carlo simulations of diffusion in random binary alloy structures point to a correlated motion of K and Na through the interstitialcy mechanism.
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Ionic conductivity in gem-quality single-crystal Alkali Feldspar from the Eifel: temperature, orientation and composition dependence
Physics and Chemistry of Minerals, 2016Co-Authors: Hamid El Maanaoui, Rainer Abart, Fabian Wilangowski, Aditya Maheshwari, Hans-dieter Wiemhöfer, Nicolaas A. StolwijkAbstract:We measured the ion conductivity of single-crystal Alkali Feldspar originating from two different locations in the Eifel/Germany, named Volkesfeld and Rockeskyller sanidine and having potassium site fractions $$C_\mathrm{K}$$ C K of 0.83 and 0.71, respectively. The dc conductivities resulting from electrochemical impedance spectroscopy over the temperature range of 300–900 $$^{\circ }\hbox {C}$$ ∘ C show a weak composition dependence but pronounced differences between the b -direction [ $$\perp (010)$$ ⊥ ( 010 ) ] and $$c^{*}$$ c ∗ -direction [ $$\perp (001)$$ ⊥ ( 001 ) ] of the monoclinic Feldspar structure. Conductivity activation energies obtained from the observed linear Arrhenius plots are close to 1.2 eV in all cases, which is closely similar to the activation energies of the $$^{22}\mathrm{Na}$$ 22 Na tracer diffusivity in the same crystals. Taking into account literature data on K tracer diffusion and diffusion correlation effects, the present results point to a predominance of the interstitialcy mechanism over the vacancy mechanism in mass and charge transport on the Alkali sublattice in potassium-rich Alkali Feldspar.
Joshua H F L Davies - One of the best experts on this subject based on the ideXlab platform.
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diffusion and fluid interaction in itrongay pegmatite madagascar evidence from in situ 40ar 39ar dating of gem quality Alkali Feldspar and u pb dating of protogenetic apatite inclusions
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan Spikings, Stephane Scaillet, Gary Osullivan, David Chew, Eszter Badenszki, Stephen J Daly, Theodore Razakamanana, Joshua H F L DaviesAbstract:Alkali Feldspar 40Ar/39Ar and apatite UPb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40Ar and 206-208Pb. Some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures; others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration. These two end-member interpretations have fundamentally different implications for tectonic models derived from geochronological data, and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes. Here, we revisit the mechanisms of 40Ar loss in the famous gem-quality Alkali Feldspar from the Itrongay pegmatite in Madagascar. Previous studies have suggested that volume diffusion is the dominant mechanism of 40Ar loss, providing key evidence to support the use of 40Ar/39Ar dating of Alkali Feldspar for thermochronology. We attempted to verify these results by obtaining time-temperature paths from petrologically characterised cogenetic Feldspar and apatite from the Itrongay pegmatite and comparing them with each other. However, our results suggest that only a minor component in the variability of 40Ar/39Ar dates of Itrongay Feldspar is related to the diffusive loss of 40Ar, and that this loss was not compatible with the majority of previously proposed models, which hinders quantitative interpretations. The crystal studied here grew in five episodes related to the influx initially of co-existing dense SiO2-rich solution and CO2-dominated fluid (the first and supposedly the following two episodes) and subsequently of H2O-rich fluid (supposedly the final two episodes). Much greater component in the variability of the acquired 40Ar/39Ar dates is interpreted to reflect the differences in the ages of these growth episodes, which we estimate to span from 477 Ma to 176 Ma (the first four episodes). Apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the Itrongay pegmatite. These yield older UPb dates than the estimated age of their host Feldspar and have apparently experienced diffusive loss of 206,207Pb prior to entrapment. Our 40Ar/39Ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40Ar from Alkali Feldspar that can be readily interpreted for thermochronological purposes. However, in situ 40Ar/39Ar dating of Alkali Feldspar appears to be a promising tool for tracking fluid-flow events in the Earth's crust whose applicability is not restricted to sedimentary rocks. Our UPb results corroborate previous suggestions that UPb dating of apatite can be used for thermochronology.
Daniil Popov - One of the best experts on this subject based on the ideXlab platform.
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diffusion and fluid interaction in itrongay pegmatite madagascar evidence from in situ 40ar 39ar dating of gem quality Alkali Feldspar and u pb dating of protogenetic apatite inclusions
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan Spikings, Stephane Scaillet, Gary Osullivan, David Chew, Eszter Badenszki, Stephen J Daly, Theodore Razakamanana, Joshua H F L DaviesAbstract:Alkali Feldspar 40Ar/39Ar and apatite UPb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40Ar and 206-208Pb. Some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures; others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration. These two end-member interpretations have fundamentally different implications for tectonic models derived from geochronological data, and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes. Here, we revisit the mechanisms of 40Ar loss in the famous gem-quality Alkali Feldspar from the Itrongay pegmatite in Madagascar. Previous studies have suggested that volume diffusion is the dominant mechanism of 40Ar loss, providing key evidence to support the use of 40Ar/39Ar dating of Alkali Feldspar for thermochronology. We attempted to verify these results by obtaining time-temperature paths from petrologically characterised cogenetic Feldspar and apatite from the Itrongay pegmatite and comparing them with each other. However, our results suggest that only a minor component in the variability of 40Ar/39Ar dates of Itrongay Feldspar is related to the diffusive loss of 40Ar, and that this loss was not compatible with the majority of previously proposed models, which hinders quantitative interpretations. The crystal studied here grew in five episodes related to the influx initially of co-existing dense SiO2-rich solution and CO2-dominated fluid (the first and supposedly the following two episodes) and subsequently of H2O-rich fluid (supposedly the final two episodes). Much greater component in the variability of the acquired 40Ar/39Ar dates is interpreted to reflect the differences in the ages of these growth episodes, which we estimate to span from 477 Ma to 176 Ma (the first four episodes). Apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the Itrongay pegmatite. These yield older UPb dates than the estimated age of their host Feldspar and have apparently experienced diffusive loss of 206,207Pb prior to entrapment. Our 40Ar/39Ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40Ar from Alkali Feldspar that can be readily interpreted for thermochronological purposes. However, in situ 40Ar/39Ar dating of Alkali Feldspar appears to be a promising tool for tracking fluid-flow events in the Earth's crust whose applicability is not restricted to sedimentary rocks. Our UPb results corroborate previous suggestions that UPb dating of apatite can be used for thermochronology.
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pathways for 39ar loss during step heating of Alkali Feldspar megacrysts from the shap granite uk combined evidence from diffusion experiments and characterisation of heating induced texture modifications
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan Spikings, Kalin KouzmanovAbstract:Abstract This study aims to identify the dominant fast pathways for 39Ar loss during step-heating 40Ar/39Ar analysis of Alkali Feldspar from the Shap granite (UK). In our analysis we combined step-heating 40Ar/39Ar data acquired from variably sized Feldspar fragments (0.4 to 2.2 mm in the shortest dimension) and optical and electron microscopy observations of microtextural modifications caused by heating in a muffle furnace in air. Our optical and electron microscopy results suggest that laboratory heating of the Shap Feldspar causes it to fracture. The resulting cracks form an interconnected network, which may link with some of previously documented nanoscale defects such as nanotunnels, slots, pull aparts and bubbles. Our step-heating 40Ar/39Ar analyses yielded non-linear Arrhenius trajectories of 39Ar release. The non-linearity is best explained by heating-induced reduction of the effective diffusion length due to fracturing, likely with some contribution from nanotunnels, pull-aparts, slots and bubbles. Incoherent grain boundaries apparently played a minor role in providing fast pathways for 39Ar loss, if any. Importantly, unambiguous evidence for a reduction of the diffusion length during step-heating was only observed in experiments with large fragments (≥1 mm in the shortest dimension). The Arrhenius trajectory obtained from the smaller fragments (≤0.5 mm in the shortest dimension) appears to be consistent with the presence of variably-sized non-interacting intra-grain diffusion domains that existed prior to mineral separation. Heating-induced fracturing must be taken into account when interpreting 40Ar/39Ar dates of Alkali Feldspar, with particular focus on the following two points. First, this process is clearly inconsistent with the underlying assumptions of commonly utilised multi-diffusion domain (MDD) theory, although it can result in 39Ar release patterns that appear consistent with them. Therefore, it is important to identify on a case by case basis the dominant fast pathways for 39Ar loss from Alkali Feldspar during step-heating before applying MDD theory. Second, if heating-induced fracturing is ubiquitous in Alkali Feldspar and affects even gem-quality samples, then presently available diffusion parameters of Ar in Alkali Feldspar structure may be inaccurate to some degree, since all of them were obtained without considering the presence of cracks.
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diffusion vs fluid alteration in Alkali Feldspar 40ar 39ar thermochronology does cross correlation of log r r0 and age spectra validate thermal histories
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan SpikingsAbstract:Abstract For six decades geoscientists have been trying to quantitatively understand the nature of radiogenic Ar loss from Alkali Feldspar. Some researchers suggest that volume diffusion is the dominant mechanism, and they use conventional step-heating 40Ar/39Ar data from Alkali Feldspar to recover the thermal histories of rocks. They argue that a high degree of correlation between log(r/r0) and 40Ar/39Ar age spectra, which is observed in a number of natural examples, justifies this hypothesis. In contrast, other investigators suggest that fluid-mediated recrystallisation and alteration control the radiogenic Ar redistribution, hence rendering Alkali Feldspar useless as a thermochronometer. By means of numerical modelling, we found that the latter mechanism as well is able to produce samples with highly correlated log(r/r0) and 40Ar/39Ar age spectra. In addition, we show that apparent thermal histories recovered for altered Alkali Feldspar crystals by interpreting step-heating 40Ar/39Ar data may be grossly inaccurate, and yet seemingly fit the prevailing understanding of regional geology. Such inaccurate apparent thermal histories can be obtained even from Alkali Feldspar crystals that underwent volumetrically low degrees of alteration. Therefore, we conclude that conventional step-heating 40Ar/39Ar data are insufficient to support the assumption that radiogenic Ar loss from Alkali Feldspar occurred solely by volume diffusion and validate the constrained thermal histories, even if upheld by a priori knowledge of regional tectonics. We further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of Alkali Feldspar.
Richard Alan Spikings - One of the best experts on this subject based on the ideXlab platform.
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diffusion and fluid interaction in itrongay pegmatite madagascar evidence from in situ 40ar 39ar dating of gem quality Alkali Feldspar and u pb dating of protogenetic apatite inclusions
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan Spikings, Stephane Scaillet, Gary Osullivan, David Chew, Eszter Badenszki, Stephen J Daly, Theodore Razakamanana, Joshua H F L DaviesAbstract:Alkali Feldspar 40Ar/39Ar and apatite UPb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40Ar and 206-208Pb. Some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures; others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration. These two end-member interpretations have fundamentally different implications for tectonic models derived from geochronological data, and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes. Here, we revisit the mechanisms of 40Ar loss in the famous gem-quality Alkali Feldspar from the Itrongay pegmatite in Madagascar. Previous studies have suggested that volume diffusion is the dominant mechanism of 40Ar loss, providing key evidence to support the use of 40Ar/39Ar dating of Alkali Feldspar for thermochronology. We attempted to verify these results by obtaining time-temperature paths from petrologically characterised cogenetic Feldspar and apatite from the Itrongay pegmatite and comparing them with each other. However, our results suggest that only a minor component in the variability of 40Ar/39Ar dates of Itrongay Feldspar is related to the diffusive loss of 40Ar, and that this loss was not compatible with the majority of previously proposed models, which hinders quantitative interpretations. The crystal studied here grew in five episodes related to the influx initially of co-existing dense SiO2-rich solution and CO2-dominated fluid (the first and supposedly the following two episodes) and subsequently of H2O-rich fluid (supposedly the final two episodes). Much greater component in the variability of the acquired 40Ar/39Ar dates is interpreted to reflect the differences in the ages of these growth episodes, which we estimate to span from 477 Ma to 176 Ma (the first four episodes). Apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the Itrongay pegmatite. These yield older UPb dates than the estimated age of their host Feldspar and have apparently experienced diffusive loss of 206,207Pb prior to entrapment. Our 40Ar/39Ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40Ar from Alkali Feldspar that can be readily interpreted for thermochronological purposes. However, in situ 40Ar/39Ar dating of Alkali Feldspar appears to be a promising tool for tracking fluid-flow events in the Earth's crust whose applicability is not restricted to sedimentary rocks. Our UPb results corroborate previous suggestions that UPb dating of apatite can be used for thermochronology.
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pathways for 39ar loss during step heating of Alkali Feldspar megacrysts from the shap granite uk combined evidence from diffusion experiments and characterisation of heating induced texture modifications
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan Spikings, Kalin KouzmanovAbstract:Abstract This study aims to identify the dominant fast pathways for 39Ar loss during step-heating 40Ar/39Ar analysis of Alkali Feldspar from the Shap granite (UK). In our analysis we combined step-heating 40Ar/39Ar data acquired from variably sized Feldspar fragments (0.4 to 2.2 mm in the shortest dimension) and optical and electron microscopy observations of microtextural modifications caused by heating in a muffle furnace in air. Our optical and electron microscopy results suggest that laboratory heating of the Shap Feldspar causes it to fracture. The resulting cracks form an interconnected network, which may link with some of previously documented nanoscale defects such as nanotunnels, slots, pull aparts and bubbles. Our step-heating 40Ar/39Ar analyses yielded non-linear Arrhenius trajectories of 39Ar release. The non-linearity is best explained by heating-induced reduction of the effective diffusion length due to fracturing, likely with some contribution from nanotunnels, pull-aparts, slots and bubbles. Incoherent grain boundaries apparently played a minor role in providing fast pathways for 39Ar loss, if any. Importantly, unambiguous evidence for a reduction of the diffusion length during step-heating was only observed in experiments with large fragments (≥1 mm in the shortest dimension). The Arrhenius trajectory obtained from the smaller fragments (≤0.5 mm in the shortest dimension) appears to be consistent with the presence of variably-sized non-interacting intra-grain diffusion domains that existed prior to mineral separation. Heating-induced fracturing must be taken into account when interpreting 40Ar/39Ar dates of Alkali Feldspar, with particular focus on the following two points. First, this process is clearly inconsistent with the underlying assumptions of commonly utilised multi-diffusion domain (MDD) theory, although it can result in 39Ar release patterns that appear consistent with them. Therefore, it is important to identify on a case by case basis the dominant fast pathways for 39Ar loss from Alkali Feldspar during step-heating before applying MDD theory. Second, if heating-induced fracturing is ubiquitous in Alkali Feldspar and affects even gem-quality samples, then presently available diffusion parameters of Ar in Alkali Feldspar structure may be inaccurate to some degree, since all of them were obtained without considering the presence of cracks.
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diffusion vs fluid alteration in Alkali Feldspar 40ar 39ar thermochronology does cross correlation of log r r0 and age spectra validate thermal histories
Chemical Geology, 2020Co-Authors: Daniil Popov, Richard Alan SpikingsAbstract:Abstract For six decades geoscientists have been trying to quantitatively understand the nature of radiogenic Ar loss from Alkali Feldspar. Some researchers suggest that volume diffusion is the dominant mechanism, and they use conventional step-heating 40Ar/39Ar data from Alkali Feldspar to recover the thermal histories of rocks. They argue that a high degree of correlation between log(r/r0) and 40Ar/39Ar age spectra, which is observed in a number of natural examples, justifies this hypothesis. In contrast, other investigators suggest that fluid-mediated recrystallisation and alteration control the radiogenic Ar redistribution, hence rendering Alkali Feldspar useless as a thermochronometer. By means of numerical modelling, we found that the latter mechanism as well is able to produce samples with highly correlated log(r/r0) and 40Ar/39Ar age spectra. In addition, we show that apparent thermal histories recovered for altered Alkali Feldspar crystals by interpreting step-heating 40Ar/39Ar data may be grossly inaccurate, and yet seemingly fit the prevailing understanding of regional geology. Such inaccurate apparent thermal histories can be obtained even from Alkali Feldspar crystals that underwent volumetrically low degrees of alteration. Therefore, we conclude that conventional step-heating 40Ar/39Ar data are insufficient to support the assumption that radiogenic Ar loss from Alkali Feldspar occurred solely by volume diffusion and validate the constrained thermal histories, even if upheld by a priori knowledge of regional tectonics. We further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of Alkali Feldspar.
E. Petrishcheva - One of the best experts on this subject based on the ideXlab platform.
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effect of chemically induced fracturing on the ice nucleation activity of Alkali Feldspar
Atmospheric Chemistry and Physics, 2021Co-Authors: Alexei Kiselev, E. Petrishcheva, Alice Keinert, Tilia Gaedeke, T Leisner, Christoph Sutter, Rainer AbartAbstract:Abstract. Feldspar is an important constituent of airborne mineral dust. Some Alkali Feldspars exhibit particularly high ice nucleation (IN) activity. This has been related to structural similarities of the ice (10 1 ‾ 0) prism planes and the (100) planes of Alkali Feldspar. Here the effect of generating Feldspar surfaces with close to (100) orientation by means of chemically induced fracturing on the IN activity of Alkali Feldspar was investigated experimentally. To this end, gem-quality K-rich Alkali Feldspar was shifted towards more Na-rich compositions by cation exchange with an NaCl–KCl salt melt at 850 ∘ C. By this procedure, a system of parallel cracks with an orientation close to the (100) plane of the Feldspar was induced. Droplet-freezing assay experiments performed on grain mounts of the cation-exchanged Alkali Feldspars revealed an increase in the overall density of ice-nucleating active site (INAS) density with respect to the untreated Feldspar. In addition, annealing at 550 ∘ C subsequent to primary cation exchange further enhanced the INAS density and led to IN activity at exceptionally high temperatures. Although very efficient in experiment, fracturing by cation exchange with an Alkali halide salt is unlikely to be of relevance in the conditioning of Alkali Feldspars in nature. However, parting planes with similar orientation as the chemically induced cracks may be generated in lamellar microstructures resulting from the exsolution of initially homogeneous Alkali Feldspar, a widespread phenomenon in natural Alkali Feldspar known as perthite formation. Perthitic Alkali Feldspars indeed show the highest IN activity. We tentatively ascribe this phenomenon to the preferential exposure of Feldspar crystal surfaces oriented sub-parallel to (100).
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Multicomponent diffusion in ionic crystals: theoretical model and application to combined tracer- and interdiffusion in Alkali Feldspar
Physics and Chemistry of Minerals, 2020Co-Authors: E. Petrishcheva, L. Tiede, D. Heuser, H. Hutter, G. Giester, R. AbartAbstract:We present a model for multicomponent diffusion in ionic crystals. The model accounts for vacancy-mediated diffusion on a sub-lattice and for diffusion due to binary exchange of different ionic species without involvement of vacancies on the same sub-lattice. The diffusive flux of a specific ionic species depends on the self-diffusion coefficients, on the diffusion coefficients related to the binary exchanges, and on the site fractions of all ionic species. The model delivers explicit expressions for these dependencies, which lead to a set of coupled non-linear diffusion equations. We applied the model to diffusion of $$^{23}$$ 23 Na, $$^{39}$$ 39 K, and $$^{41}$$ 41 K in Alkali Feldspar. To this end, gem-quality crystals of Alkali Feldspar were used together with $$^{41}$$ 41 K doped KCl salt as diffusion couples, which were annealed at temperatures between 800 $$^\circ$$ ∘ and 950 $$^\circ$$ ∘ C. Concentration-distance data for $$^{23}$$ 23 Na, $$^{39}$$ 39 K, and $$^{41}$$ 41 K were obtained by Time of Flight Secondary Ion Mass Spectrometry. Over the entire investigated temperature range the Na self-diffusion coefficient is by a factor of $$\ge 500$$ ≥ 500 higher than the K self-diffusion coefficient. Diffusion mediated by binary $$^{39}$$ 39 K– $$^{41}$$ 41 K exchange is required for obtaining satisfactory fits of the model curves to the experimental data, and the respective kinetic coefficient is well constrained.
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Spinodal decomposition in Alkali Feldspar studied by atom probe tomography
Physics and Chemistry of Minerals, 2020Co-Authors: E. Petrishcheva, Gerlinde Habler, Lisa Tiede, Kevin Schweinar, Baptiste Gault, Rainer AbartAbstract:We used atom probe tomography to complement electron microscopy for the investigation of spinodal decomposition in Alkali Feldspar. To this end, gem-quality Alkali Feldspar of intermediate composition with a mole fraction of $$a_{\text {K}}=0.43$$ a K = 0.43 of the K end-member was prepared from Madagascar orthoclase by ion-exchange with (NaK)Cl molten salt. During subsequent annealing at $$550\,^\circ \hbox {C}$$ 550 ∘ C and close to ambient pressure the ion-exchanged orthoclase unmixed producing a coherent lamellar intergrowth of Na-rich and K-rich lamellae. The chemical separation was completed, and equilibrium Na–K partitioning between the different lamellae was attained within four days, which was followed by microstructural coarsening. After annealing for 4 days, the wavelength of the lamellar microstructure was $$\approx 17\,\hbox {nm}$$ ≈ 17 nm and it increased to $$\approx 30\,\hbox {nm}$$ ≈ 30 nm after annealing for 16 days. The observed equilibrium compositions of the Na-rich and K-rich lamellae are in reasonable agreement with an earlier experimental determination of the coherent solvus. The excess energy associated with compositional gradients at the lamellar interfaces was quantified from the initial wavelength of the lamellar microstructure and the lamellar compositions as obtained from atom probe tomography using the Cahn–Hilliard theory. The capability of atom probe tomography to deliver quantitative chemical compositions at nm resolution opens new perspectives for studying the early stages of exsolution. In particular, it helps to shed light on the phase relations in nm scaled coherent intergrowth.
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Diffusion-controlled crack propagation in Alkali Feldspar
Physics and Chemistry of Minerals, 2019Co-Authors: E. Petrishcheva, Markus Rieder, Jožef Predan, F D Fischer, Gerald Giester, Rainer AbartAbstract:The chemically driven propagation of interacting parallel cracks in monoclinic Alkali Feldspar was studied experimentally. Single crystals of potassium-rich gem-quality sanidine were shifted towards more sodium-rich compositions by cation exchange with a NaCl–KCl salt melt at a temperature of $$850\,^{\circ }\hbox {C}$$ 850 ∘ C and close to ambient pressure. Initially, a zone with elevated sodium content formed at the crystal surfaces due to the simultaneous in-diffusion of sodium and out-diffusion of potassium, where the rate of cation exchange was controlled by sodium–potassium interdiffusion within the Feldspar. A chemical shift of potassium-rich Alkali Feldspar towards more sodium-rich compositions produces highly anisotropic contraction of the crystal lattice. This induced a tensile stress state in the sodium-rich surface layer of the crystals, which triggered the formation of a system of nearly equi-spaced parallel cracks oriented approximately perpendicular to the direction of maximum shortening. Crack propagation following their nucleation was driven by cation exchange occurring along the crack flanks and was controlled by the intimate coupling of the diffusion-mediated build-up of a tensile stress state around the crack tips and stress release by successive crack propagation. The critical energy release rate of fracturing was determined as 1.8–2.2 $$~ \hbox {J}\,\hbox {m}^{-2}$$ J m - 2 from evaluation of the near-tip J-integral. The mechanism of diffusion-controlled crack propagation is discussed in the context of high-temperature Feldspar alteration.
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sodium potassium interdiffusion in potassium rich Alkali Feldspar i full diffusivity tensor at 850 c
American Journal of Science, 2014Co-Authors: E. Petrishcheva, Gerlinde Habler, Rainer Abart, Anne Kathrin Schaffer, Dieter RhedeAbstract:Anisotropic diffusion is described by a tensor of diffusivities, Dαβ, which may be composition dependent leading to nonlinear anisotropic diffusion. In this work the practical problem of reconstructing such a tensor for Na-K interdiffusion in potassium-rich Alkali Feldspar in the composition range 0.85 ≤ XOr ≤ 1.00 is addressed. Gem quality sanidine with an initial composition of XOr = 0.85 was exchanged with KCl salt melt at 850 °C and ≈1 bar. The diffusivity tensor Dαβ(XOr) and its composition dependence was reconstructed from composition profiles produced by cation exchange in six different crystallographic directions using a generalization of the Boltzmann approach. Na-K interdiffusion in potassium-rich Alkali Feldspar is considerably anisotropic and composition dependent. The principal axes of the diffusivity tensor representing the directions of highest and lowest diffusivity lie in the a-c plane with highest diffusivity parallel to the [101] direction, lowest diffusivity perpendicular to the (101) plane, and the direction with intermediate diffusivity parallel to the crystallographic b-axis. All diffusivities Dαβ(XOr) increase as XOr tends to unity. Our main result is given in the form of numerical values for all components of the Dαβ(XOr) tensor for 0.85 ≤ XOr ≤ 1.