The studies on the high temperature in situ behaviour of plagioclase feldspars are scarce, in spite of the relevance as rock forming minerals. For instance in thermodynamic data bases thermal expansion of plagioclase is modeled using experimental data dating in the '70s. A systematic study was therefore undertaken on nine plagioclase samples, well characterized in previous literature. The sample composition of the stud14 samples is, in terms of albite (NaAlSi3O8, Ab) and anorthite (CaAl&208, An): Abl00, An27Ab73, An35Ab65, An46Ab54, An60Ab40, An78Ab22, An89Ab11 , An96Ab4 Anl00. The study was performed using powder e c t i o n in situ at high and low temperature and synchrotron radiation at ESRF (ID31 beamline). Each sample was measured between T = 90 and 950 K at steps of 3-5 K at lower and 10 K at higher temperature. Cell parameters were obtained after sequential Rietveld analysis of the diffraction patterns. An independent measurement was done on the Ab100 sample by single crystal X-ray diffraction to calibrate the powder diffraction results. Volume and axial cell parameters were fitted according to the equation y = yo + yt theta coth (theta/T), that allows to model the cell parameter saturation at low temperature. The saturation temperature theta varies between 250 and 300 K for the volume; for cell axes it is on average about 250 K along a, but as high as 350 and 450 K along the b and c axes respectively. To note in albite at T less than 300 K the c axis has a negative thermal expansion. The higher expansion is observed along the a axis, while the b and c axes show similar changes with temperature. The thermal expansion coefficient a was fit for the higher temperature IT > 450 K for all but the two An richer samples, for which it was T > 550 K) according to the equation V = Vo exp [a IT - To)], assuming that at higher temperature that thermal expansion does not change with temperature. The thermal expansion coefficient varies from 3.03(1) 10^-5 K-1 in albite to 1.53(1) 10^-5 K-1 in anorthite, and decreases continuously between the two end members. A linear inverse relation is observed between the thermal expansion and the volume, like it was observed in feldspars with Al:Si 1 : 3. The P-1 to I-1 phase transition is detected in An100 and An96Ab4 by the presence of critical low temperature h+k even, l odd (c-type) reflections, and by changes in cell parameters, mostly alpha, beta eand gamma. In anorthite preliminary analysis of the evolution of the spontaneous strain according to a 2-4-6 Landau fit shows a tricritical behaviour with Tc = 515(1) K; critical reflections disappear at temperature slightly lower than Tc, i.e. between 503 and 513 K. In An96Ab4 the same transition shows a second order trend, with T, = 47115) K; the critical reflections disappear at much lower temperature, at about 440 K. In the sample An80Ab11, in which TEM analysis at room temperature showed only diffuse c-type reflections, the critical reflections are lacking in the powder patterns even at the lowest temperature.

High-low temperature thermal expansion and phase transition in plagioclase

Tribaudino M;CAMARA ARTIGAS, Fernando;
2009-01-01

Abstract

The studies on the high temperature in situ behaviour of plagioclase feldspars are scarce, in spite of the relevance as rock forming minerals. For instance in thermodynamic data bases thermal expansion of plagioclase is modeled using experimental data dating in the '70s. A systematic study was therefore undertaken on nine plagioclase samples, well characterized in previous literature. The sample composition of the stud14 samples is, in terms of albite (NaAlSi3O8, Ab) and anorthite (CaAl&208, An): Abl00, An27Ab73, An35Ab65, An46Ab54, An60Ab40, An78Ab22, An89Ab11 , An96Ab4 Anl00. The study was performed using powder e c t i o n in situ at high and low temperature and synchrotron radiation at ESRF (ID31 beamline). Each sample was measured between T = 90 and 950 K at steps of 3-5 K at lower and 10 K at higher temperature. Cell parameters were obtained after sequential Rietveld analysis of the diffraction patterns. An independent measurement was done on the Ab100 sample by single crystal X-ray diffraction to calibrate the powder diffraction results. Volume and axial cell parameters were fitted according to the equation y = yo + yt theta coth (theta/T), that allows to model the cell parameter saturation at low temperature. The saturation temperature theta varies between 250 and 300 K for the volume; for cell axes it is on average about 250 K along a, but as high as 350 and 450 K along the b and c axes respectively. To note in albite at T less than 300 K the c axis has a negative thermal expansion. The higher expansion is observed along the a axis, while the b and c axes show similar changes with temperature. The thermal expansion coefficient a was fit for the higher temperature IT > 450 K for all but the two An richer samples, for which it was T > 550 K) according to the equation V = Vo exp [a IT - To)], assuming that at higher temperature that thermal expansion does not change with temperature. The thermal expansion coefficient varies from 3.03(1) 10^-5 K-1 in albite to 1.53(1) 10^-5 K-1 in anorthite, and decreases continuously between the two end members. A linear inverse relation is observed between the thermal expansion and the volume, like it was observed in feldspars with Al:Si 1 : 3. The P-1 to I-1 phase transition is detected in An100 and An96Ab4 by the presence of critical low temperature h+k even, l odd (c-type) reflections, and by changes in cell parameters, mostly alpha, beta eand gamma. In anorthite preliminary analysis of the evolution of the spontaneous strain according to a 2-4-6 Landau fit shows a tricritical behaviour with Tc = 515(1) K; critical reflections disappear at temperature slightly lower than Tc, i.e. between 503 and 513 K. In An96Ab4 the same transition shows a second order trend, with T, = 47115) K; the critical reflections disappear at much lower temperature, at about 440 K. In the sample An80Ab11, in which TEM analysis at room temperature showed only diffuse c-type reflections, the critical reflections are lacking in the powder patterns even at the lowest temperature.
2009
GEOITALIA 2009. Settimo Forum Italiano di Scienze della Terra
Rimini, Italy
9-11 settembre 2009
35
603
603
plagioclase; thermal expansion; phase transition; single crystal X-ray diffraction; powder XRD; synchrotron radiation
Tribaudino M; Angel R.J; Cámara F; Nestola F; Pasqual D
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/79189
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