S. M. Kireenkova, and G. A. Efimova
United Institute of Physics of the Earth RAS
First, we have made a set of tests on the calcite containing rock - marble of different graininess, with the purpose of assessing capability of the neutron diffraction method for investigation of processes in the rocks. It is well known that reversible polymorphous transitions take place in the marble, and it should be noted that these are observed under certain loading conditions, while they are not observed under other loading conditions. Therefore, the marble samples were tested under different loading conditions. The tests were made on a solid phase cylinder-piston machine up to 1.6 GPa and on a high-pressure hydraulic machine at a constant hydrostatic pressure 10 MPa with an additional uniaxial compression at a speed of 1.8 10-6 mm/s [Efimova et al., 1998]. At the beginning reading was made of the neutron diffraction spectra on each sample and complete pole figures were obtained showing orientation of grains in the sample under investigation in corpore and not in one plane, as a conventional microscopy allows to have. Afterwards, the samples were subjected to different loading conditions and new spectra were recorded after the tests and new pole figures were obtained. The neutron diffraction measurements were made in the UINI (Dubna), on the NSVR spectrometer, sheaf no. 7 of the IBR-2 reactor. A time-of-flight neutron diffraction method was employed for the textural measurements. Two transitions at 0.4 MPa and at 1.6 MPa were found by the impulse ultrasonic method during deformation of marble under a quasihydrostatic pressure.
Two cylindrical marble samples (Sn3d - Sn4d) of 16 mm in diameter and 20 mm long were considered. A prevailing orientation of the calcite grains was measured in the beginning of the tests at a room temperature, i.e. texture of the samples was measured. Afterwards the sample was placed in a high-temperature machine, which was installed in its turn in a SKAT texture diffractometer. The machine is secured in a ring of the SKAT and rotates together with it. The SKAT is essentially a ring 2 m in diameter with 19 detectors mounted thereon. The detector system is arranged axially symmetric to the neutron beam and the angle of deflection for all detectors is equal to 90o [Ivankina et al., 2001]. Thereafter the samples were subjected to a uniaxial compression at 13 and 20 MPa accordingly. The mechanical stress was increasing by 100% and 60% along with the increase of temperature.
In the course of tests temperature first increased from the room temperature up to 50o C at the rate of 2o C per minute and then temperature was stabilizing for 10 minutes and after this the neutron diffraction was recorded by all the detectors of the SKAT spectrometer for an hour. Similar cycles were repeated several times at 30-40o C temperature intervals. The time of the ultrasonic pulses going through the samples and parameters of the sample were recorded continuously.
In case of sample Sn3d spectra were recorded for six temperature points (temperature rise, stabilization of temperature, measurement of diffraction at a constant temperature) and in the case of sample Sn4d spectra were recorded for five temperature values.
A prevailing orientation of grains (texture) of the samples was measured at the beginning of the test at a room temperature. Afterwards measurement of texture in sample Sn3d was made at 220o C and in sample Sn4d at 250o C. After the measurement of the sample Sn4d texture made at 250o C, the uniaxial stress at the ends of the sample was increased up to 110 MPa by pumping a press and with this mechanical stress the diffraction spectra were also recorded. At a reverse temperature reduction the diffraction was measured at 50o C. The texture measurements of sample Sn3d have taken of 3400 minutes of net time, the exposure time for sample Sn4d during the texture measurement was increased and the whole experiment lasted for 7800 minutes. The diffraction spectra were recorded during one hour at the temperature and stress values reached. No changes were observed whatsoever. This suggests that a fast increase of the uniaxial compression did not cause any change of the texture. Subsequently the texture of sample Sn4d was measured once again after it was kept under load for a long time (140 days) in a chamber (TKOS). The stress has reduced down to 10 MPa during this period and this can be attributed to the stress relaxation and to the creep flow in the marble and also to relaxation of the pressure transferring medium. The diffraction spectra of sample Sn3d were also repeatedly measured, but in this case only subject to the temperature impact (at the same temperature points) and without any mechanical compression.
Influence of temperature for both samples became apparent in displacement of the peaks, most characteristic for the theoretical diffraction spectrum of calcite. However the displacement is not great for all the peaks. The maximum displacement is observed in the peak (0006). It amounts to 1.79 10-2 Å in the temperature interval 20-190oC, that corresponds to a linear relative deformation ( Dd/d0 = 6.20 10-3 ). Thus, the change of position of the time-of-flight diffraction spectra peaks may help to determine the change of the interatomic distances in the calcite crystal lattice as the temperature increases [Ivankina et al., 2001].
Comparison of the microdeformation values and thermal characteristics of calcite at different temperatures, obtained from the neutron diffraction data and calculated on the basis of the structural characteristics of calcite defined by an X-ray structure method [Markgraf and Reeder, 1985] has shown that the displacement values and relative deformations in our experiments exceed similar values obtained from data of the X-ray structure analysis on the calcite monocrystal. Different conditions of the experiment may be one of the reasons of this discrepancy. The neutron diffraction experiments were conducted at a simultaneous impact of both temperature and the uniaxial compression, while in the work of [Markgraf and Reeder, 1985] measurements were made subject to temperature only.
The temperature experiment on recording the neutron diffraction spectra under the same temperatures with sample Sn3d, where no texture changes were observed, as was mentioned earlier, was conducted just with the aim of comparison. The results of this experiment, however, have proved that deformations in the plane (0006) as a result of the thermal expansion under all temperature values were higher than in the case of a simultaneous action of the uniaxial compression and temperature. The deformation values of plane (11-20) in two experiments at 120o C and 220o C (Tables 1 and 2) are very close to each other. This speaks about a very weak sensitivity of the lattice in this direction to a mechanical effect. As can be seen from Tables 1 and 2, the calcite thermal expansion coefficient has different values and signs in different planes, i.e. anisotropy of the calcite thermal expansion coefficient is observed. This fact belongs also to the carbonate-otavite, while other carbonates have only positive thermal expansion coefficients. Changes of the thermal expansion components for different crystallographic directions (11-20) (0006) are just of an opposite nature. Thus, the temperature dependence of the thermal expansion components for plane (11-20) has a tendency to a reduction with the rise of temperature, whereas it increases for plane (0006) along with the rise of temperature. A type of the temperature dependence of the thermal expansion coefficient has been also found for other directions in calcite.
As a result of the first experiments with the use of a complex of physical methods, ultrasonic pulse method and the neutron diffraction method the physical, structural and texture characteristics of the rock were recorded and a quantitative assessment was made in the course of its deformation at high pressure and temperature parameters. This suggests that the new line in investigation of the physical properties of the rocks and minerals at high pressures and temperatures is promising for the study of the deformation processes in lithosphere.
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