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J Wallinga - One of the best experts on this subject based on the ideXlab platform.
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electrons in Feldspar ii a consideration of the influence of conduction band tail states on luminescence processes
Pacific Rim Conference on Multimedia, 2002Co-Authors: N R J Poolto, A S Murray, K Ozanya, J Wallinga, L OtterjenseAbstract:Most natural Feldspars contain many charged impurities, and display a range of bond angles, distributed about the ideal. These effects can lead to complications in the structure of the conduction band, giving rise to a tail of energy states (below the high-mobility conduction band) through which electrons can travel, but with reduced mobility: transport through these states is expected to be thermally activated. The purpose of this article is twofold. Firstly, we consider what kind of lattice perturbations could give rise to both localized and extended conduction band-tail states. Secondly, we consider what influence the band tails have on the luminescence properties of Feldspar, where electrons travel through the sample prior to recombination. The work highlights the dominant role that 0.04–0.05-eV phonons play in both the luminescence excitation and emission processes of these materials. It also has relevance in the dating of Feldspar sediments at elevated temperatures.
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testing optically stimulated luminescence dating of sand sized quartz and Feldspar from fluvial deposits
Earth and Planetary Science Letters, 2001Co-Authors: A S Murray, J Wallinga, G A T Dulle, Torbjo E TornqvisAbstract:Abstract We apply single-aliquot optically stimulated luminescence (OSL) dating to quartz- and Feldspar-rich extracts from fluvial channel deposits of the Rhine–Meuse system in The Netherlands. The time of deposition of these deposits is tightly constrained by radiocarbon dating or historical sources. This allows us to compare OSL ages obtained on quartz and infrared OSL (IR-OSL) ages obtained on potassium-rich Feldspar with independent ages over the range of 0.3–13 ka. We show that the quartz OSL ages are in good agreement with the expected age. Using IR-OSL dating of Feldspar, we find a slight age overestimate for the youngest sample, whereas for older samples the age is significantly underestimated. We also apply OSL dating to older fluvial and estuarine channel deposits with limited independent chronological constraints. Comparison of Feldspar IR-OSL ages with the quartz OSL ages up to ∼200 ka shows a clear trend, where the former severely underestimates the latter. This trend is similar to that found for the samples with independent age control, indicating that the Feldspar IR-OSL ages are erroneously young for the entire age range. In the youngest samples, incomplete resetting of the IR-OSL signal prior to deposition probably masks the age underestimation. We show that the IR-OSL age underestimation is partly caused by changes in trapping probability due to preheating. Correction for this phenomenon improves the IR-OSL ages slightly, but does not provide a complete solution to the discrepancy. We suggest that, in the light of the problems encountered in the IR-OSL dating of Feldspar, quartz is the mineral of choice for OSL dating of these deposits. However, Feldspar dating should continue to be investigated, because it has potential application to longer time scales.
Michael A Velbel - One of the best experts on this subject based on the ideXlab platform.
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influence of temperature and mineral surface characteristics on Feldspar weathering rates in natural and artificial systems a first approximation
Water Resources Research, 1990Co-Authors: Michael A VelbelAbstract:Rates of alkali Feldspar hydrolysis in the near-neutral pH range are up to 3 orders of magnitude slower in natural systems than in laboratory experiments. Correcting for differences in temperature between natural weathering and laboratory systems reduces the disparity by as much as a factor of 5. Any remaining disparity can be accounted for by differences in the ratio of effective surface area to total surface area; the ratio of effective-to- total surface area in natural systems is generally considerably smaller than in laboratory systems. This may be related, at least in part, to experimental preparation artifacts and to the fact that naturally weathered Feldspars have lost much of their most reactive surface to the formation of etch pits. Hydrological factors such as inhomogeneous access of percolating fluids to mineral surfaces may also reduce the proportion of mineral surface area reacting in natural systems.
N Elmehalawy - One of the best experts on this subject based on the ideXlab platform.
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effect of replacing weathered Feldspar for potash Feldspar in the production of stoneware tiles containing fish bone ash
Ceramics International, 2015Co-Authors: M Awaad, Salma M. Naga, N ElmehalawyAbstract:Abstract In the present study, the use of naturally occurring weathered Feldspar for partial or total substitution of conventional potash Feldspar in the batches containing fish bone ash was evaluated. Weathered Feldspar, Hafafit feldfpasr and El-Tieh kaolin from Sinai, Egypt; together with fish bone ash were characterized and employed to obtain stoneware tiles. Technological properties such as bulk density, apparent porosity, bending strength and thermal expansion behavior were measured. Phase composition and microstructure of the fired tiles were accomplished in order to determine the relationship between them. The results show that the produced tiles fulfill the requirement of ISO 10545-4 and 13006.
A S Murray - One of the best experts on this subject based on the ideXlab platform.
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identifying well bleached quartz using the different bleaching rates of quartz and Feldspar luminescence signals
Radiation Measurements, 2012Co-Authors: A S Murray, Kristina Jorkov Thomse, N Masuda, Janpiete Uylae, Mayank JaiAbstract:Abstract When dating older sedimentary deposits using quartz, there are no unambiguous methods for identifying the presence of incomplete bleaching. Current statistical analysis of dose distributions depends entirely on the assumption that incomplete bleaching and mixing are the main causes of any excess dispersion in the distribution; the only existing way to test this assumption is using independent age control. Here we suggest a new approach to this question, based on the differential bleaching rates of quartz and Feldspar luminescence signals. We first present data that confirm the differences in relative bleaching rates of quartz optically stimulated luminescence (OSL) and Feldspar luminescence stimulated at 50 °C by infrared light (IR50) and Feldspar luminescence stimulated at 290 °C by infrared light after a stimulation at 50 °C (pIRIR290), and use recently deposited samples to determine the likely significance of the difficult-to-bleach residual Feldspar signals in non-aeolian samples. For a set of mainly Late Pleistocene non-aeolian sediments, large aliquot quartz doses are then used to predict Feldspar doses (based on a knowledge of the sample dose rates). The differences between observed and predicted Feldspar doses as a function of the quartz dose, combined with a conservative assumption concerning the relative Feldspar and quartz residual signals after natural bleaching prior to deposition, are used to identify those samples for which the quartz is very likely to be well bleached (20 out of 24). Two of these apparently well-bleached samples are then examined using single-grain quartz dose distributions; one of these is consistent with the well-bleached hypothesis, and one indicates poor bleaching or a multi-component mixture. However, independent age control makes it clear that the large aliquot data are more likely to be correct. We conclude that a comparison of quartz and Feldspar doses provides a useful independent method for identifying well-bleached quartz samples, and that it is unwise to apply statistical models to dose distributions without clear evidence for the physical origins of the distributions.
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laboratory fading rates of various luminescence signals from Feldspar rich sediment extracts
Radiation Measurements, 2008Co-Authors: Kristina Jorkov Thomse, A S Murray, Mayank Jai, L OtterjenseAbstract:Abstract Feldspar as a retrospective dosimeter is receiving more and more attention because of its useful luminescence properties; in particular the dose response curve extends to significantly higher doses than quartz. However, Feldspars have one major disadvantage; both the thermoluminescence (TL) and optically stimulated luminescence (OSL) signals exhibit anomalous (athermal) fading. Much of the OSL work carried out on Feldspars in recent years has focussed on determining fading rates and correcting for them. Almost all work has been carried out using IR stimulation at 50 °C detected in the blue region of the spectrum. In contrast, we have determined fading rates for various sedimentary Feldspar samples using different stimulation and detection windows. If the initial part of the OSL signal is used the lowest fading rate is observed with post-IR blue stimulation (UV detection), but if a later part of the signal is used the lowest fading rate is obtained for IR stimulation (blue detection). Daylight bleaching experiments show that, unlike quartz, the initial and final parts of the Feldspar OSL signal bleach at approximately the same rate. Stimulation at elevated temperatures significantly reduces the apparent fading rate. This signal appears to bleach more rapidly in daylight than the signal observed when stimulation is at 50 °C (for IR stimulation and blue detection). We conclude that there are OSL signals from sedimentary Feldspars which fade at a significantly lower rate than the signal used conventionally, and that further testing of these signals is warranted to see if they are dosimetrically useful.
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minimizing Feldspar osl contamination in quartz uv osl using pulsed blue stimulation
Radiation Measurements, 2008Co-Authors: Kristina Jorkov Thomse, A S Murray, Mayank Jai, Phil M Denby, N Roy, L OtterjenseAbstract:Natural sediments usually contain a mix of quartz and Feldspar. Because these two minerals have different luminescence properties and record different natural doses it is normally necessary to physically separate them before measurement. This paper investigates the potential of using pulsed stimulation to separate the optically stimulated luminescence signals from quartz and Feldspar. Doses are measured from 11 natural (unseparated) fluvial samples from Denmark and Russia, and it is concluded that, by using post-IR pulsed blue stimulation, the quartz dose can be accurately measured without any prior chemical separation. This technique has considerable potential for both field dose measurements and laboratory measurement of separated quartz samples containing Feldspar contamination.
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application of pulsed osl to the separation of the luminescence components from a mixed quartz Feldspar sample
Radiation Measurements, 2006Co-Authors: Phil M Denby, Kristina Jorkov Thomse, A S Murray, L Otterjense, P MoskaAbstract:Abstract It is known that the pulsed optically stimulated luminescence (OSL) characteristics of quartz and Feldspars are very different. These differences can be used to preferentially discriminate against the Feldspar signal in mixed quartz ⧹ Feldspar mineral assemblages, or in separated quartz contaminated with a Feldspar signal. We have developed instrumentation for the study of high-speed pulse stimulated OSL. Our system uses the standard blue/IR LED stimulation unit of a Riso reader (allowing stimulation pulses down to 1 – 2 μ s duration) and can thus be applied to the routine analysis of samples. Using this stimulation source, and high-speed photon timing, the OSL yield can be monitored throughout the pulsing cycle and subsequent OSL decay. It is found that the total photon yield per unit stimulation power in pulsed mode is, for quartz, twice and, for Feldspar, nearly four times, that in continuous wave mode. Observation of this OSL signal, between stimulation pulses, is seen to be characteristic of the mineral being examined, and has been used to preferentially discriminate against Feldspar contamination in a mixed quartz/Feldspar sample. Simple implementation of this technique by gating the counting period, so that counts are only accumulated during a windowed period, reduces the Feldspar signal to 1.6% of its original value relative to that of the quartz.
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combining infrared and green laser stimulation sources in single grain luminescence measurements of Feldspar and quartz
Radiation Measurements, 2003Co-Authors: G A T Duller, L Botterjensen, A S MurrayAbstract:Abstract A system designed for measurement of the optically stimulated luminescence (OSL) from individual sand-sized mineral grains has been constructed. Previously, this system was equipped only with a green laser emitting at 532 nm , but now an infrared (IR) laser at 830 nm has been added. It is now possible to interchangeably use the two laser sources for optical stimulation. This is especially valuable for the measurement of Feldspars. The power density using the IR laser at the grain is ∼500 W cm −2 , and stimulation for 1 s reduces the OSL signal to near background level. Initial results on grains from two Feldspar samples are described. These show that the ratio of the OSL signal under green and IR stimulation is similar for all the Feldspar grains. As expected, the ratio for quartz grains is very different, with a much weaker signal when using IR stimulation. For the two Feldspars, the distribution of OSL between the grains and the saturation of the dose response curve shows much less variability than is typically seen for quartz grains.
Katsuyoshi Michibayashi - One of the best experts on this subject based on the ideXlab platform.
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the role of intragranular fracturing on grain size reduction in Feldspar during mylonitization
Journal of Structural Geology, 1996Co-Authors: Katsuyoshi MichibayashiAbstract:Abstract Measurements of the size, shape and orientation of K-Feldspar and plagioclase grains within a granitic mylonite in the Kashio shear zone, central Honshu, Japan, revealed that grain size was the most effective parameter controlling development of intragranular fracturing during mylonitization. Intragranular fractures are dominantly located near the centre of each grain and form preferentially along the (001) cleavage. The grain size distribution of both Feldspars is log-normal with an average of 760 μm. The proportion of fractured grains expands dramatically to 100% as the grain size increases, whereas few Feldspar grains smaller than 300 μm have been fractured during the mylonitization, indicating that the fractures have preferentially developed in coarser grains. Furthermore, the shape and shape-preferred orientation of Feldspar grains also affects the development of fractures. The proportion of fractured grains not only increases up to 20% towards the extension direction, but also rises slightly as the aspect ratio of Feldspar grains increases. It is, therefore, suggested that Feldspar grains lying perpendicular to the extension direction may resist fracturing regardless of grain size, unless their shape-preferred orientations became sub-parallel to the extension direction due to rotation or modification of their grain shapes during mylonitization. The development of intragranular fractures could be statistically explained in terms of a fibre-loading model, although individual fractures may result from various fracture mechanisms.
