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Majorite formation from enstatite by experimental shock-loading
Physics of the Earth and Planetary Interiors, 27(1981) 95—99 Elsevier Scientific Publishing Company, Amsterdam — Printed in The Netherlands
95
Majorite formation from enstatite by experimental shock-loading Manuel Jakubith and Ulrich Hornemann2 /
Institutfur Physikalische Chemie, Westfalische Wilhelms- Universität, Schlossplatz 4, D-4400 Münster (Federal Republic of Germany) Fraunhofer Inst itUt fur Kurzzeitdynamik, Ernst Mach Institut, Aht. Ballistik, D- 7858 Weil/Rhein (Federal Republic of Germany)
2
(Received February 6, 1981; revision accepted April 16, 1981)
Jakubith, M. and Hornemann, U., 1981. Majorite formation from enstatite by experimental shock-loading. Phys. Earth Planet. Inter., 27: 95—99. Majorite has been observed for the first time after experimental shock-loading of enstatite (Bamle, Norway). The identification of this high pressure garnet has been carried out by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction experiments, majorite formation was indicated by these techniques for the pressure range between Ca. 35 and 50 GPa. This also proves formation of majorite by impact metamorphosis in the Coorara meteorite as well as terrestrial formation of that phase by the same process.
1. Introduction Recent application of the X-ray photoelectron spectroscopy (XPS) for identification of high pres-
-
sure phases in the quartz—stishovite system was described by Jakubith and Lehmann2p(1981). and 0Deis pending on the shock pressure the Si binding energies showed a typical pattern related to the low and high pressure phase in that system, each phase is therefore represented by pressure independent binding energies for the components. These experiments were continued by the investigation of eclogite, one of the main constituents of the Earth’s mantle. The XPS spectra of the recovered shock-loaded eclogites showed very complicated patterns and necessitated investigation of the rock-forming minerals, enstatite and grossularite. In this paper the experiments are focused on shock-loaded enstatite, the experiments on edogite will be published at a later date (Jakubith and Seidel, 1982). Previous pressure experiments on enstatite were carried out by Ringwood and Major (1968), Ito et
a!. (1972) as well as Ming and Bassett (1975). After undergoing a transformation to the garnet structure the mineral breaks down into a mixture of (Mg, Fe) 2 Si04 (spinel structure) and stishovite then recombines to form a hexagonal (Mg, Fe)Si206 -phasephase (ilmenite structure) and finally the orthorhombic (perovskite structure). These and other results about the pressure-dependent behaviour of olivines and garnets are important for the understanding of the physical and chemical properties of the Earth’s mantle. The stepwise phase transitions are widely correlated with the stepwise increase of seismic wave velocity and lead to the layered structure of the mantle (e.g., Liu, 1980). In addition to these static experiments, dynamic shock-loading has also been carried out. These experiments are important on the one hand for interpreting the effects discussed above, and on the other hand they are essential for understanding impact metamorphosis in terrestrial meteorites and in the evolution in the lunar surface. Smith and Mason (1970) reported the existence of a new
003 l-920l/8l/0000—0000/$02.50 © 1981 Elsevier Scientific Publishing Company
96
garnet in the Coorara meteorite and concluded that this garnet (named “majorite” by these authors) could only be formed from pyroxene and had been synthesized under intense shock pressures generated by meteorite collisions. Subsequently, Ahrens and Gaffney (1971) reported their shock-loading experiments on enstatite. Although these authors did not identify majorite in their recovered samples, the interpretation of the Hugoniot curve suggested its formation at 13.5 GPa, which was roughly the same pressure range in which majorite was synthesized by Ringwood and Major (1971). Physical and structural data for this mineral were recently reported by Jeanloz (1980).
2. Experimental The enstatite samples of composition (Mg3 72Fe028)[Si206] plus 2% Al203 and 1% CaO originate at Bamle, Norway. Platelets of 18 mm diameter and 1 mm thickness were shocked for times of 1 p~swith pressures of 10—50 GPa in arrangements described in detail by Muller and Hornemann (1969). The samples were quenched to room temperature as quickly as possible, normally within 30 s after the shock-loading. The treatment for removing the metal contamination and further details about the XPS measurements are described by Jakubith and Lehmann (1981). The XPS measurements were performed with an ESCA 3 photoelectron spectrometer of Vacuum Generators Ltd. equipped with a VG 3030 data system in which the spectra were stored for data processing. The smooth and sufficiently largesignalsam2 are essential for a good pies (about 40 mm to-noise ratio) were sputtered with argon ions for 30 s at 5 kV and 40 ptA, higher energies generated additional shifts and peak broadening. Binding energies were determined for both sides of the plates and were corrected for charging effects using the difference of the C is signal of the carbon contaminations on the sample and on the sample holder. The deconvolution of the composite spectra were carried out by a computer program according to the method of Stokes (1948) and were performed’ only in cases where presence of more
than one component was suggested by shoulders or inflection points. The X-ray powder photographs were taken with a Guinier camera with high resolution monochromator according to Jagodzinski. The calibration of the reflections was resolved by mixing the sample with a mixture of silicium and quartz.
3. Results and discussion A typical XPS-spectrum of the 0 is binding energy is shown in Fig. 1 for a shock pressure of 50 GPa, spectra of the Si 2p region are similar. Two well-separated components were obtained and after deconvolution plotted in Fig. 2 as a function of the shock pressure (for the evaluation details, see Jakubith and Lehmann, 1981). Well above the elastic limit, up to ca. 20 GPa, the initial binding energies of 532.2 eV for 0 ls and 103.1 eV for Si 2p are unchanged. Beyond this pressure both signais split into two components: the 0 is signal into one band at 532.8 eV and a second one, slightly decreasing binding energy from 532.2 to 531.8 eV. The signal at 532.8 eV may be assigned to a
(a)
(b) 534 536 530 532 534 eV Fig. I. (a) Experimental 0 Is XPS signal (corrected for charging effects) for enstatite shock-loaded at 50 GPa; (b) deconvolution of that signal. 53Q
~
97
0 is binding energy 1eV
533 (b)
(~) 532-
(c) ____________________________________ 10
20
50
30
60 p/GPo
Fig. 2. Changes of 0 Is binding energies in enstatite with shock pressure: (a) elastic region; (b) majorite region; (c) mixture of enstatite and clinoenstatite. (GR) 0 Is binding energy in grossularite (originated from Transvaal).
high pressure phase of that system on account of the shock independent 0 is (and Si 2p) binding energies up to 50 GPa. The results of Ringwood and Major (1968) as well as Ahrens and Gaffney (1971) suggest the high pressure garnet majorite.
For clarification of this point X-ray diffraction photographs have been taken for the samples shocked at 26, 40 and 45 GPa. A complete list of reflections for the samples shocked at 40 and 45 GPa is given in Table I. It is seen that nearly all reflections can be assigned to enstatite (ASTM card 22-7i4), clinoenstatite (ASTM card 19-769) and majorite (ASTM card 25-843). Only a few weak reflections at high angles remain unidentified. The identification of majorite in the sample shocked at 26 GPa remains uncertain. For enstatite only the two reflections at the d-values 2.239 and 1.929 A with intensities less than 3% are lacking. The main reflection of pure majorite is at d = 2.575 A with an intensity of 100%, recovered samples showed an intensity of about 10% in relation to enstatite, so reflections with relative intensities lower than 10% for pure .
.
.
majorite are not observed in all cases. The missing reflections of majorite are therefore at d-values of 4.7, 4.07, 3.08, 1.82, 1.63, 1.463, 1.439, 1.376 and 1.358 A. Some reflections of majorite overlapped with those of the other parts and are uncertain. Fortunately the main reflections at d= 2.575, 2.308, 1.865, i.665, 1.599, 1.542 and 1.260 A are certain.
TABLE I X-ray diffraction data for enstatite shocked at 40 and 45 GPa (Cu K,,) Observed I/b’ 5 5 5 50 10 100 10 20 60 10 10 10 30 40 20 5 5 10
Enstatite
Clinoenstatite
d [AJ
d [A]
6.3 4.43 4.03
6.33 4.43 4.03
210 III 211
2 10 2
333b
3.31 121 not identified 3.18 221
6
3.21 3.185 3.17 2.955 2.88 2.84 2.72 2.575 2.545 2.500” 2.465 2.39 2.365 2.28 b
hkl
I/Is
d [A]
hkl
J/J~
3.17
220
30
321 610 511 421
16 55 10 10
2.54 2.497 2.47
131 202 521
25 18 18 2.38
331 800
d
[A]
hkl
100
2.95 2.878 2.832 2.71
2.364 2.283
Majonte
2 2
231
2.88
400
70
2.575
420
100
2.454
332
45
2.35
422
30
8
98 TABLE I (continued) Observed I/I
a
Enstatite d [A]
5 5 20 20 10 3 2 10
2.255 2.19 2.12 2.10 2.06 2.035 2.02
20
1.96 1.888 1.865 1.840 1.805” 1.790 1.779 1.738 1.720 1.705 1.700 1.683 1.665 1.648 1.610 a 1.603 1.599 1.590 1.570 1.555 1.542” 1.528 1.520 1.503 1.487 1.472 1.420 1.397 1.392 1.363 1.339 1.330 1.307 1.297 1.294 1.268 1.260 1.244 1.230 1.21
5C
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 30 20 5 5 5 5 5 5 5 5 5 5 5 5 5 5
,
99b
a Estimated b
[A]
d
Clinoenstatite hkl
2.257
711
I/Jo
d [A]
hkl
630 531 512
1.988 1.961 1.888
241 631 821
10 12 4
1.841 1.803 1.788 1.779 1.737
332 830 640 541 921
4 2 4 4 6
1.710
831
6
1.681
142
2
1.652 741 1.610 1021 not identified
2 12
1.73
931
040
402
921
[A]
hkl
I/Ia
2.262
431
35
2.103
521
18
2.038
440
25
1.868
532
25
(1.778
541
2)
(1.699
631
4)
1.663
444
20
1.597
640
40
1.564 1.540
633 642
8 60
(1.418 (1.398
741 644
4) 3)
(1.340
743
I)
(1.305
752
2)
1.258 (1.243 1.228
842 761 664
20 3) 14
4
6
6
not identified 1.529 551 1.522 1200 not identified 1.488 1031 1.473 642
d
6
12 12 6 2.015
1.591
I/Jo
4 2.20
2.116 2.10 2.06
Majorite
6 8 25 18
1.394
523
2
not identified not identified not identified not identified not identified
not identified
visually. Overlapped by the reference reflexes of silicium and quartz or both. This and the following intensity values are five and less than 5%.
99
The Si 2p and 0 ls binding energies of majorite are nearly the same as those of the garnet grossularite (originated from Transvaal) at normal pressure, see point GR in Fig. 2, pure majorite for comparing measurements was not available. The second component of the signal with a slight shift in the range from 532.2 to 531.6 eV and 103.1 to 102.8 eV, respectively, is assigned to varying amounts of residual enstatite and clinoenstatite. The limit of the majorite formation is uncertam: the XPS results favour a limit near 20 GPa, the X-ray diffraction experiments favour a limit near 35 GPa. The latter result is in good agreement with the results of Ahrens and Gaffney (197i) who indicated the formation of majorite from the Hugoniot data but did not find any evidence for this phase in their recovered samples. The authors assumed quick and complete decomposition of majorite during the pressure release. for higher shock-loading pressures this assumption might not be correct. As shown by Ringwood and co-workers (1968, 1969, 1971) as well as Liu (1980) the garnet phase undergoes a further transition to the ilmenite structure. During pressure release after shock-loading (within almost 4 X 10 —6 s) the rearrangement of the atoms might be similarly incomplete as in the system quartz—stishovite observed by Jakubith and Lehmann (1981). In this way majorite may be retained to normal pressures in shock-loading experiments. Terrestrial formation of majorite by impact metamorphosis is confirmed.
Acknowledgement With gratitude the authors thank Dr. Peter Seidel (Institut fur Mineralogie, Universität Münster) for the X-ray diffraction experiments.
References Ahrens, T.J. and Gaffney, E.S., 1971. Dynamic compression of enstatite. J. Geophys. Res., 76: 5504—55 14. Ahrens, T.J., Anderson, DL. and Ringwood, A.E., 1969. Equation-of-state and crystal structure of high pressure phases of shocked silicates and oxides. Rev. Geophys., 7: 667—707. Ito, E., Matsumoto, T., Suito, K. and Kawai, N., 1972. High pressure breakdown of enstatite. Proc. Jpn. Acad. 48: 412— 415 Jakubith, M. and Lehmann, G.. 1981. An X-ray photoelectron spectroscopic study of shock-loaded quartz. Phys. Chem. Mineral., 7 (in press). Jakubith, M. and Seidel, P., 1982. Formation of perovskitestructured Mg Si0 3 at 18 GPa as an evidence for dissipative phenomena during experimental shock-loading. (Submitted for publication). Jeanloz, R., 1980. Majorite: vibrational and compressional properties of a high pressure phase. EOS, Trans. Am. Geophys. Union, 61: 1103 (abstr.). Liu, L., 1980. The pyroxene—garnet transformation and its implication for the 200 km seismic discontinuity. Phys. Earth Planet. Inter., 23: 286—292. Ming, L. and Bassett, W.A., 1975. High-pressure phase transformations in the system of MgSiO3 —FeSiO3. Earth Planet. Sci. Lett., 27: 85—89. MUller, W.E. and Hornemann, U., 1969. Shock-induced planar deformation Structure in experimentally shock - loaded olivines andLett., in olivines from chondritic meteorites. Earth Planet. Sci. 7: 252—264. Ringwood, A.E. and Major, A.. 1968. High-pressure transformations in pyroxenes. II. Earth Planet. Sci. Lett., 5: 76—78. Ringwood, A.E. and Major, A., 1971. Synthesis of majorite and other higb-pressure garnets and perovskites. Earth Planet. Sci. Lett., 12: 411—418. Smith, J.V. and Mason, B., 1970. Pyroxene—garnet transformation in Coorara meteorite. Science, 168: 832—833. Stokes, A.R., 1948. A numerical Fourier-analysis method for the correction of width and shapes of lines on X-ray powder photographs. Proc. Phys. Soc., 61: 383—391.
95
Majorite formation from enstatite by experimental shock-loading Manuel Jakubith and Ulrich Hornemann2 /
Institutfur Physikalische Chemie, Westfalische Wilhelms- Universität, Schlossplatz 4, D-4400 Münster (Federal Republic of Germany) Fraunhofer Inst itUt fur Kurzzeitdynamik, Ernst Mach Institut, Aht. Ballistik, D- 7858 Weil/Rhein (Federal Republic of Germany)
2
(Received February 6, 1981; revision accepted April 16, 1981)
Jakubith, M. and Hornemann, U., 1981. Majorite formation from enstatite by experimental shock-loading. Phys. Earth Planet. Inter., 27: 95—99. Majorite has been observed for the first time after experimental shock-loading of enstatite (Bamle, Norway). The identification of this high pressure garnet has been carried out by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction experiments, majorite formation was indicated by these techniques for the pressure range between Ca. 35 and 50 GPa. This also proves formation of majorite by impact metamorphosis in the Coorara meteorite as well as terrestrial formation of that phase by the same process.
1. Introduction Recent application of the X-ray photoelectron spectroscopy (XPS) for identification of high pres-
-
sure phases in the quartz—stishovite system was described by Jakubith and Lehmann2p(1981). and 0Deis pending on the shock pressure the Si binding energies showed a typical pattern related to the low and high pressure phase in that system, each phase is therefore represented by pressure independent binding energies for the components. These experiments were continued by the investigation of eclogite, one of the main constituents of the Earth’s mantle. The XPS spectra of the recovered shock-loaded eclogites showed very complicated patterns and necessitated investigation of the rock-forming minerals, enstatite and grossularite. In this paper the experiments are focused on shock-loaded enstatite, the experiments on edogite will be published at a later date (Jakubith and Seidel, 1982). Previous pressure experiments on enstatite were carried out by Ringwood and Major (1968), Ito et
a!. (1972) as well as Ming and Bassett (1975). After undergoing a transformation to the garnet structure the mineral breaks down into a mixture of (Mg, Fe) 2 Si04 (spinel structure) and stishovite then recombines to form a hexagonal (Mg, Fe)Si206 -phasephase (ilmenite structure) and finally the orthorhombic (perovskite structure). These and other results about the pressure-dependent behaviour of olivines and garnets are important for the understanding of the physical and chemical properties of the Earth’s mantle. The stepwise phase transitions are widely correlated with the stepwise increase of seismic wave velocity and lead to the layered structure of the mantle (e.g., Liu, 1980). In addition to these static experiments, dynamic shock-loading has also been carried out. These experiments are important on the one hand for interpreting the effects discussed above, and on the other hand they are essential for understanding impact metamorphosis in terrestrial meteorites and in the evolution in the lunar surface. Smith and Mason (1970) reported the existence of a new
003 l-920l/8l/0000—0000/$02.50 © 1981 Elsevier Scientific Publishing Company
96
garnet in the Coorara meteorite and concluded that this garnet (named “majorite” by these authors) could only be formed from pyroxene and had been synthesized under intense shock pressures generated by meteorite collisions. Subsequently, Ahrens and Gaffney (1971) reported their shock-loading experiments on enstatite. Although these authors did not identify majorite in their recovered samples, the interpretation of the Hugoniot curve suggested its formation at 13.5 GPa, which was roughly the same pressure range in which majorite was synthesized by Ringwood and Major (1971). Physical and structural data for this mineral were recently reported by Jeanloz (1980).
2. Experimental The enstatite samples of composition (Mg3 72Fe028)[Si206] plus 2% Al203 and 1% CaO originate at Bamle, Norway. Platelets of 18 mm diameter and 1 mm thickness were shocked for times of 1 p~swith pressures of 10—50 GPa in arrangements described in detail by Muller and Hornemann (1969). The samples were quenched to room temperature as quickly as possible, normally within 30 s after the shock-loading. The treatment for removing the metal contamination and further details about the XPS measurements are described by Jakubith and Lehmann (1981). The XPS measurements were performed with an ESCA 3 photoelectron spectrometer of Vacuum Generators Ltd. equipped with a VG 3030 data system in which the spectra were stored for data processing. The smooth and sufficiently largesignalsam2 are essential for a good pies (about 40 mm to-noise ratio) were sputtered with argon ions for 30 s at 5 kV and 40 ptA, higher energies generated additional shifts and peak broadening. Binding energies were determined for both sides of the plates and were corrected for charging effects using the difference of the C is signal of the carbon contaminations on the sample and on the sample holder. The deconvolution of the composite spectra were carried out by a computer program according to the method of Stokes (1948) and were performed’ only in cases where presence of more
than one component was suggested by shoulders or inflection points. The X-ray powder photographs were taken with a Guinier camera with high resolution monochromator according to Jagodzinski. The calibration of the reflections was resolved by mixing the sample with a mixture of silicium and quartz.
3. Results and discussion A typical XPS-spectrum of the 0 is binding energy is shown in Fig. 1 for a shock pressure of 50 GPa, spectra of the Si 2p region are similar. Two well-separated components were obtained and after deconvolution plotted in Fig. 2 as a function of the shock pressure (for the evaluation details, see Jakubith and Lehmann, 1981). Well above the elastic limit, up to ca. 20 GPa, the initial binding energies of 532.2 eV for 0 ls and 103.1 eV for Si 2p are unchanged. Beyond this pressure both signais split into two components: the 0 is signal into one band at 532.8 eV and a second one, slightly decreasing binding energy from 532.2 to 531.8 eV. The signal at 532.8 eV may be assigned to a
(a)
(b) 534 536 530 532 534 eV Fig. I. (a) Experimental 0 Is XPS signal (corrected for charging effects) for enstatite shock-loaded at 50 GPa; (b) deconvolution of that signal. 53Q
~
97
0 is binding energy 1eV
533 (b)
(~) 532-
(c) ____________________________________ 10
20
50
30
60 p/GPo
Fig. 2. Changes of 0 Is binding energies in enstatite with shock pressure: (a) elastic region; (b) majorite region; (c) mixture of enstatite and clinoenstatite. (GR) 0 Is binding energy in grossularite (originated from Transvaal).
high pressure phase of that system on account of the shock independent 0 is (and Si 2p) binding energies up to 50 GPa. The results of Ringwood and Major (1968) as well as Ahrens and Gaffney (1971) suggest the high pressure garnet majorite.
For clarification of this point X-ray diffraction photographs have been taken for the samples shocked at 26, 40 and 45 GPa. A complete list of reflections for the samples shocked at 40 and 45 GPa is given in Table I. It is seen that nearly all reflections can be assigned to enstatite (ASTM card 22-7i4), clinoenstatite (ASTM card 19-769) and majorite (ASTM card 25-843). Only a few weak reflections at high angles remain unidentified. The identification of majorite in the sample shocked at 26 GPa remains uncertain. For enstatite only the two reflections at the d-values 2.239 and 1.929 A with intensities less than 3% are lacking. The main reflection of pure majorite is at d = 2.575 A with an intensity of 100%, recovered samples showed an intensity of about 10% in relation to enstatite, so reflections with relative intensities lower than 10% for pure .
.
.
majorite are not observed in all cases. The missing reflections of majorite are therefore at d-values of 4.7, 4.07, 3.08, 1.82, 1.63, 1.463, 1.439, 1.376 and 1.358 A. Some reflections of majorite overlapped with those of the other parts and are uncertain. Fortunately the main reflections at d= 2.575, 2.308, 1.865, i.665, 1.599, 1.542 and 1.260 A are certain.
TABLE I X-ray diffraction data for enstatite shocked at 40 and 45 GPa (Cu K,,) Observed I/b’ 5 5 5 50 10 100 10 20 60 10 10 10 30 40 20 5 5 10
Enstatite
Clinoenstatite
d [AJ
d [A]
6.3 4.43 4.03
6.33 4.43 4.03
210 III 211
2 10 2
333b
3.31 121 not identified 3.18 221
6
3.21 3.185 3.17 2.955 2.88 2.84 2.72 2.575 2.545 2.500” 2.465 2.39 2.365 2.28 b
hkl
I/Is
d [A]
hkl
J/J~
3.17
220
30
321 610 511 421
16 55 10 10
2.54 2.497 2.47
131 202 521
25 18 18 2.38
331 800
d
[A]
hkl
100
2.95 2.878 2.832 2.71
2.364 2.283
Majonte
2 2
231
2.88
400
70
2.575
420
100
2.454
332
45
2.35
422
30
8
98 TABLE I (continued) Observed I/I
a
Enstatite d [A]
5 5 20 20 10 3 2 10
2.255 2.19 2.12 2.10 2.06 2.035 2.02
20
1.96 1.888 1.865 1.840 1.805” 1.790 1.779 1.738 1.720 1.705 1.700 1.683 1.665 1.648 1.610 a 1.603 1.599 1.590 1.570 1.555 1.542” 1.528 1.520 1.503 1.487 1.472 1.420 1.397 1.392 1.363 1.339 1.330 1.307 1.297 1.294 1.268 1.260 1.244 1.230 1.21
5C
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 30 20 5 5 5 5 5 5 5 5 5 5 5 5 5 5
,
99b
a Estimated b
[A]
d
Clinoenstatite hkl
2.257
711
I/Jo
d [A]
hkl
630 531 512
1.988 1.961 1.888
241 631 821
10 12 4
1.841 1.803 1.788 1.779 1.737
332 830 640 541 921
4 2 4 4 6
1.710
831
6
1.681
142
2
1.652 741 1.610 1021 not identified
2 12
1.73
931
040
402
921
[A]
hkl
I/Ia
2.262
431
35
2.103
521
18
2.038
440
25
1.868
532
25
(1.778
541
2)
(1.699
631
4)
1.663
444
20
1.597
640
40
1.564 1.540
633 642
8 60
(1.418 (1.398
741 644
4) 3)
(1.340
743
I)
(1.305
752
2)
1.258 (1.243 1.228
842 761 664
20 3) 14
4
6
6
not identified 1.529 551 1.522 1200 not identified 1.488 1031 1.473 642
d
6
12 12 6 2.015
1.591
I/Jo
4 2.20
2.116 2.10 2.06
Majorite
6 8 25 18
1.394
523
2
not identified not identified not identified not identified not identified
not identified
visually. Overlapped by the reference reflexes of silicium and quartz or both. This and the following intensity values are five and less than 5%.
99
The Si 2p and 0 ls binding energies of majorite are nearly the same as those of the garnet grossularite (originated from Transvaal) at normal pressure, see point GR in Fig. 2, pure majorite for comparing measurements was not available. The second component of the signal with a slight shift in the range from 532.2 to 531.6 eV and 103.1 to 102.8 eV, respectively, is assigned to varying amounts of residual enstatite and clinoenstatite. The limit of the majorite formation is uncertam: the XPS results favour a limit near 20 GPa, the X-ray diffraction experiments favour a limit near 35 GPa. The latter result is in good agreement with the results of Ahrens and Gaffney (197i) who indicated the formation of majorite from the Hugoniot data but did not find any evidence for this phase in their recovered samples. The authors assumed quick and complete decomposition of majorite during the pressure release. for higher shock-loading pressures this assumption might not be correct. As shown by Ringwood and co-workers (1968, 1969, 1971) as well as Liu (1980) the garnet phase undergoes a further transition to the ilmenite structure. During pressure release after shock-loading (within almost 4 X 10 —6 s) the rearrangement of the atoms might be similarly incomplete as in the system quartz—stishovite observed by Jakubith and Lehmann (1981). In this way majorite may be retained to normal pressures in shock-loading experiments. Terrestrial formation of majorite by impact metamorphosis is confirmed.
Acknowledgement With gratitude the authors thank Dr. Peter Seidel (Institut fur Mineralogie, Universität Münster) for the X-ray diffraction experiments.
References Ahrens, T.J. and Gaffney, E.S., 1971. Dynamic compression of enstatite. J. Geophys. Res., 76: 5504—55 14. Ahrens, T.J., Anderson, DL. and Ringwood, A.E., 1969. Equation-of-state and crystal structure of high pressure phases of shocked silicates and oxides. Rev. Geophys., 7: 667—707. Ito, E., Matsumoto, T., Suito, K. and Kawai, N., 1972. High pressure breakdown of enstatite. Proc. Jpn. Acad. 48: 412— 415 Jakubith, M. and Lehmann, G.. 1981. An X-ray photoelectron spectroscopic study of shock-loaded quartz. Phys. Chem. Mineral., 7 (in press). Jakubith, M. and Seidel, P., 1982. Formation of perovskitestructured Mg Si0 3 at 18 GPa as an evidence for dissipative phenomena during experimental shock-loading. (Submitted for publication). Jeanloz, R., 1980. Majorite: vibrational and compressional properties of a high pressure phase. EOS, Trans. Am. Geophys. Union, 61: 1103 (abstr.). Liu, L., 1980. The pyroxene—garnet transformation and its implication for the 200 km seismic discontinuity. Phys. Earth Planet. Inter., 23: 286—292. Ming, L. and Bassett, W.A., 1975. High-pressure phase transformations in the system of MgSiO3 —FeSiO3. Earth Planet. Sci. Lett., 27: 85—89. MUller, W.E. and Hornemann, U., 1969. Shock-induced planar deformation Structure in experimentally shock - loaded olivines andLett., in olivines from chondritic meteorites. Earth Planet. Sci. 7: 252—264. Ringwood, A.E. and Major, A.. 1968. High-pressure transformations in pyroxenes. II. Earth Planet. Sci. Lett., 5: 76—78. Ringwood, A.E. and Major, A., 1971. Synthesis of majorite and other higb-pressure garnets and perovskites. Earth Planet. Sci. Lett., 12: 411—418. Smith, J.V. and Mason, B., 1970. Pyroxene—garnet transformation in Coorara meteorite. Science, 168: 832—833. Stokes, A.R., 1948. A numerical Fourier-analysis method for the correction of width and shapes of lines on X-ray powder photographs. Proc. Phys. Soc., 61: 383—391.