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High-pressure phase transformations and compressions of ilmenite and rutile, II. Geophysical implications
Physics of the Earth and Planetary Interiors, 10 (1975) 344-347
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
HIGH-PRESSURE PHASE TRANSFORMATIONS AND COMPRESSIONS OF ILMENITE AND RUTILE, II. GEOPHYSICAL IMPLICATIONS LIN-GUN LIU Research School of Earth Sciences, Australian National University, Canberra, A.C.T. (Australia)
(Received January 2, 1975; revised and accepted March 11, 1975)
The hypothesis that the earth's core consists of metallized mantle substances has been recently reanimated. The large volume changes which accompany the phase transformations of rutile-cubic phase and perovskite-mixed oxides for TiO2 and (Fe,Mg)TiO3, respectively, contradict the shock data for similar phase transformations in SiO2 and MgSiO3. In view of the relatively low pressure at the mantle-core boundary, this does not favor the hypothesis of a silicate or oxide core for the earth.
1. Introduction The hypothesis originally proposed by Bernal (1936) and revived by Ramsey (1948, 1949) that the core consists of a metallic form of mantle material has been discarded in the last two decades in favour of that of an iron core in which some light constituents are present. However, the hypothesis of a metallized silicate or oxide core seems to have been reanimated recently by e.g. Kawai and Mochizuki (1971), Dubrovskiy and Pan'kov (1972), Al'tshuler et al. (1973), Simakov et al. (1973), and Kawai and Nishiyama (1974), on the basis of their discoveries of conductive Fe203, FeO, TiO~, and Si02 under static high-pressure conditions, and their observations of high-pressure phase transformations of TiO2, SnO2, CaF2, and BaF2 in shock studies. The phase transformation of ilmenite to the perovskite structure, then to the cubic high-pressure phase of TiO2 plus (Fe,Mg)O (Liu, 1975a), and the.phase transformation of SnO2 reported by Liu (1975b) provide a basis for appraisal of these conclusions.
2. Phase transformations of some MOz compounds Geophysicists and geochemists are interested in the phase transformations of MO2 compounds, main-
ly because of the analogical relationship between SiO2 and the other MO2 compounds. With one exception, coesite, the high-pressure crystal phases of SiO2 are analogous to the dioxides of the elements of Group IV-A in the periodic table. The high-pressure phases for the dioxides of Group IV-A elements, plus TiO2 and MnO2, which naturally occur or have been observed under static high-pressure experiments are summarized in Table 1. Shock compression data for SnO2 are available up to 3.5 Mbar (Simakov et al., 1973) for TiO2 to 3 Mbar (Al'tshuler et al., 1973), for MnO2 to 1.26 Mbar (Birch, 1966), and for SiO2 to 6.5 Mbar (Trunin et al., 1970). The Hugoniot data revealed a phase transformation at about 2.4 Mbar for SnO2 (McQueen and Marsh's (1965) unpublished data, however, indicate a transition at about 1 Mbar) and at about 1 Mbar for TiO2. No apparent transition was observed for MnO2, and no major discontinuity in the Hugoniot data, other than those corresponding to coesite and stishovite, was detectable in SiO2. However, it was pointed out by Liu (1975b) that the major changes found in shock studies of SnO2 and TiO2 correspond, not to the rutile-orthorhombic transition, but to the rutile(orthorhombic)-cubic phase transition as was shown for TiO2 (Liu, 1975a). On the basis of a crystal chemical argument, Liu (1975b) also predicted that MnO2, GeO2, and SiO2 all may crystallize in the orthorhom-
PHASE TRANSFORMATIONS AND COMPRESSIONS OF ILMENITE AND RUTILE, IMPLICATIONS
345
TABLE I Summary of the crystal phases of dioxides of the elements of the Group IV-A plus TiO2 and MnO2 found naturally or under static high-pressure experiments Cation coordination
4
Crystal phase
a-quartz
PbO 2 SnO2 TiO 2 MnO2 GeO 2 SiO2
X X
6
8 fluorite
coesite
rutile
orthorhombic a-PbO2 X X X
X
x X X X X X
12(?) cubic phase
Shock experiment
X X
X X X X
The cation size decreases from PbO2 to SiO2. Crosses denote that the data are available. Sources have been referenced in the text. bic ct-PbO2 structure at elevated pressures and temperatures, although this may not be exposed b y shockexperimental studies because o f the small volume change associated with the r u t i l e - o r t h o r h o m b i c transition. The fluorite phase o f PbO2 has been reported by Syono and Akimoto (1968) and a cubic form o f TiO2 b y Liu (1975a). This cubic phase of TiO2 is probably not the fluorite structure, because of the large discrepancy between its observed lattice parameter and that calculated for fluorite TiO2 and the lack o f resemblance of the relative intensities o f its diffraction pattern to those for other fluorite compounds. Since
the volume change is inversely proportional to the cation radius. Al'tshuler et al. (1973) and Simakov et al. (1973) suggested that stishovite (SiO2) may eventually transform to the fluorite structure as implied by their
the shock compression curve o f SnO2 qualitatively resembles that o f TiO2, it is expected that SnO2 also transforms to the cubic form (neglecting the small volume change for the r u t i l e - o r t h o r h o m b i c transition in SnO2 reported b y Liu (1975b)).
AmO2 CeO2 CmO2 HfO2 NpO2 PaO2 PbO2 PoO2 PrO2 Pu02 TbO2 ThO2 uo2 ZrO2 TiO 2
The hypothetical lattice parameter for the fluorite phase o f TiO2 was calculated to be a0 = 4.775 A (Liu, 1975a). For SnO2 and SIO2, the average value of 1.16 + 0.07 (Table II) for the ratio o f the ionic radii in eight-fold and six-fold coordination, M 4÷(VIII)/M4÷(VI), was used to compute the ionic radii for eight-fold coordinated Sn 4÷ and Si4÷. The lattice parameters for the fluorite phase of SnO2 and SiO2 thus calculated are 5.03 and 4.25 A, respectively. In Table III we list the zero-pressure volume changes for the q u a r t z - r u t i l e , r u t i l e - o r t h o r h o m b i c , r u t i l e fluorite, and r u t i l e - c u b i c phase transformations in MO2 compounds. F o r each o f the transformations,
TABLE II Effective ionic radii (Shannon and Prewitt, 1969 except where noted) and the calculated lattice parameters for various fluorite oxides, MO2 (units are all in A) Oxides
SnO2 SiO2
M4+(VI)*
M4+(VIII)*
0.72 0.605
0.95 0.97 0.95 0.83 0.98 1.~1 0.94 1.10 0.99 0.96 0.88 1.04 1.00 0.78 0.69**
0.690 0.400
0.80 0.46
0.80 0.71 0.775 0.78 0.80 0.76 1.00
M4+(VII1)/ Mao M4+(VI) (VIII)-O 1.21 1.17 1.21 1.27 1.20 1.16 1.04 1.08 1.14! (1.16 ± 0.07)
2.33 2.35 2.33 2,21 2.36 2.39 2.32 2.48 2.37 2.34 2.26 2.42 2.38 2.16 2.068
5.38 5.43 5.38 5.10 5.45 5.52 5.36 5.73 5.47 5.40 5.22 5,59 5.50 5,00 4.775
2.18 1.84
5.03 4.25
* The roman numerals denote the coordination number. **Calculated from M(VIII)-O (see Liu, 1975a).
346
LIN-GUN LIU
TABLE IIl The changes of zero-pressure volume for quartz-rutile, rutile-orthorhombic, rutile-fluorite, and rutile-cubic transitions in some MO2 compounds (-~V/Vo in %)
Quar tz-rutile Rutile-or thorhombic Rutile-fluorite Rutile-cubic
SiO2
GeO2
38.2a
31.8 a
17.6c
TiO2
SnO2
PbO2
2.2b 12.8 c 29.2 e
1.5h 11.1 c
1.3b 7.1 d
The cation size increases from SiO2 to PbO2. a Calculated on data from ASTM Cards. b From listings by Liu (1975b). c From Table II. d Syono and Akimoto (1968). e Liu (1975a).
shock ~tudies of SnO2 and TiO2, and that any fluorite phase of SiO2 may further transform to the metallic structure o f MgCu2-type as implied by their studies of CaF2 and BaF2. These observations have led them to conclude, although qualitatively, that the hypothesis of silicate or oxide core is competitive with that of iron core. 3. Conductive TiO2, SiO2, and FeO The electrical resistance of rutile (TiO2) has been reported by Kawai and Mochizuki (1971) to drop from approximately 105 to 10-1 ~2 at about 2.2 Mbar when it was subjected to static pressures up to 2.5 Mbar at room temperature in a two-stage spheric vessel. The resistance of a shock-recovered orthorhombic phase of TiO2 was measured to be at-out 10 s ~2 by McQueen et al. (1967). Hence, the large resistance change in TiO2 is not likely to be due to the rutileorthorhombic transition. There is some suggestion that the resistance change may be associated with the Mort transition (insulator-metal transition) or related to the crystal transition (Kawai and Mochizuki, 1971). The cubic phase of TiO2 discussed in the previous section may be responsible for the resistance change. This can be clarified in future by measuring the resistance of quenched high-pressure phase of TiO2. However, the quenched fluorite phase of PbO2 found by Syono and Akimoto (1968) is not as conductive as the compressed TiO2 reported by Kawai and Mochizuki (1971 ).
At a pressure higher than that required for TiO2, crystalline SiO~ (what form of the crystal was not specified) displays a similar electric resistance change from the order of 1 0 s - 1 0 -1 ~ accompanying a hysteresis loop on release of pressure (Kawai and Nishiyama, 1974). This again was attributed by them to the Mott transition or to the breakdown of SiO2 into its constituent elements with oxygen in the metallic state. On the other hand, recent shock studies have revealed that SiO2 remains as an insulator at pressures up to 5 Mbar (Hawke et al., 1972) and no drastic velocity or volume changes other than those corresponding to coesite and stishovite in SiO2 up to 6.5 Mbar (Trunin et al., 1970). Feo.9sO was also found by Kawai and Nishiyama (1974) to become conductive at a static pressure lower than that for TiO2 (the pressure was not reported). This, in turn, would support the hypothesis of an FeO outer core proposed by Dubrovskiy and Pan'kov (1972). The discovery of conductive TiO2, SiO2, and FeO has caused Kawai and Mochizuki (1971) and Kawai and Nishiyama (1974) to conclude that the hypothesis of a metallized core is more plausible than ever before.
4. Discussion and conclusion Following the Al'tshuler et al. (1973) suggestion of the sequence of transformations in analogous systems, Simakov et al. (1973) assigned a zero-pressure density
PttASE TRANSFORMATIONS AND COMPRESSIONS OF 1LMENITE AND RUT1LE, IMPLICATIONS o f approximately 6 g/cm 3 for the fluorite phase of SIO2, which corresponds to a volume change o f about 29% for the r u t i l e - f l u o r i t e transition of SIO2. This value seems too large according to the data derived from ionic-radius considerations lited in Table III, but is compatible with the observed volume change for the r u t i l e - c u b i c transition in TiO2. Such a large change in volume (either 18 or 29%), however, was not observed in a shock compression study o f SiO2 up to 6.5 Mbar by Trunin et al. (1970). This pressure (6.5 Mbar) is probably at least equivalent to 2 - 3 Mbar under static conditions for the observation of the same phase transformation, whereas the static pressure at the m a n t l e - c o r e boundary is around 1.4 Mbar. Thus, the hypothesis of a silicate or oxide core with the fluorite, the cubic, or the MgCu2-type structure is not supported b y the results of the shock study o f SiO2 even though at still higher pressures SiO2 might eventually transform to the fluorite, the cubic, or the MgCu2 structure, tn addition, the m a n t l e - c o r e interface represents a s o l i d - l i q u i d boundary as the shearwave velocity distribution indicates. Liu (1975b) has shown that, in the presence o f MgO, SiO2 in the form of either rutile or orthorhombic ct-PbO2 structure is unstable relative to the perovskite phase o f MgSiO3 found by Liu (1974) under pressure and temperature condition comparable to those of the lower mantle. The perovskite phase o f MgSiO3 might disproportionate into a mixture o f oxides as shown in the previous paper (Liu, 1975a) for the perovskite phase o f (Fe,Mg)TiO3. This transition would be accompanied by a change of at least 13% in the zero-pressure volume, as in the case of (Fe,Mg)TiO3. Such a large change in volume was again not observed by Simakov and Trunin (1973) on their remeasurement of shock compressions of natural and artificial ceramic enstatites (MgSiOa) up to 2.5 Mbar. Hence, one may conclude at this stage that the perovskite structure appears to represent the final phase in the evolution o f crystal structure o f minerals in the mantle, and the pressure at the m a n t l e - c o r e boundary is probably too low for the i n s u l a t o r ' m e t a l transition o f FeO and SiO2 found by Kawai and Nishiyama (1974), or for the possible phase transformations of r u t i l e f l u o r i t e - c u b i c - M g C u 2 in SiO2 and o f p e r o v s k i t e mixed oxides in MgSiO3. One may also, however, argue
347
that the time duration might be too short for the substance in the shock wave to transform. A large change in density, which is characteristic of phase transformations o f the r u t i l e - f l u o r i t e - c u b i c and p e r o v s k i t e mixed oxides, is also not suggested by studies of seismic velocities of the lower mantle (e.g., Johnson, 1969; Hales and Roberts, 1970; Whitcomb and Anderson, 1970).
Acknowledgement The author wishes to thank J.R. Cleary, R.C. Lieber. mann and A.E. Ringwood for commenting on this manuscript.
References Al'tshuler, L.V., Podurets, M.A., Simakov, G.V. and Trunin, R.F., 1973. Soy. Phys. Solid State, 15:969 Bernal, J.D., 1936. Discuss., Obs., 59:268 Birch, F., 1966. In: S.P. Clark Jr. (Editor), Handbook of Physical Constants. Geol. Soc. Am., Mem. 97, p. 97. Dubrovskiy, V.A. and Pan'kov, V.L., 1972. Izv. Earth Phys., 7: 48. Hales, A.L. and Roberts, J.L., 1970. Bull. Seismol. Soc. Am., 60: 1427. Hawke, R.S., Duerre, D.E., Huebel, J.G., Keeler, R.N. and Klapper, H., 1972. Phys. Earth Planet. Inter., 6: 44. Johnson, L.R., 1969. Bull. Seismol. Soc. Am., 59: 973. Kawai, N. and Mochizuki, S., 1971. Phys. Lett., 36A: 54. Kawai, N. and Nishiyama, A., 1974. Proc. Jpn. Acad., 50: 72. Liu, L., 1974. Geophys. Res. Lett., 1: 277. Liu, L., 1975a. Phys. Earth Planet. Inter., 10: 167. Liu, L., 1975b. Phys. Earth Planet. Inter., 9: 338. McQueen, R.G., Jamieson, J.C. and Marsh, S.P., 1967. Science, 155: 1401. Ramsey, W.H., 1948. Mon. Not. R. Astron. Soc., 108: 406. Ramsey, W.H., 1949. Mort. Not. R. Astron. Soc., Geophys. Suppl., 5: 409. Shannon, R.D. and Prewitt, C.T., 1969. Acta Crystallogr., Sec. B, 25: 925. Simakov, G.V. and Trunin, R.F., 1973. Izv. Earth Phys., 9: 80. Simakov, G.V., Podurets, M.A. and Trunin, R.F., 1973. Dokl. Akad. Nauk S.S.S.R., 211: 1330. Syono, Y. and Akimoto, S., 1968. Mater. Res. Bull., 3: 153. Trunin, R.F., Simakov, G.V., Podurets, M.A., Moiseyev, B.N. and Popov, L.V., 1970. Izv. Earth Phys., 1: 13. Whitcomb, J.H. and Anderson, D.L., 1970. J. Geophys. Res., 75: 5713.
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
HIGH-PRESSURE PHASE TRANSFORMATIONS AND COMPRESSIONS OF ILMENITE AND RUTILE, II. GEOPHYSICAL IMPLICATIONS LIN-GUN LIU Research School of Earth Sciences, Australian National University, Canberra, A.C.T. (Australia)
(Received January 2, 1975; revised and accepted March 11, 1975)
The hypothesis that the earth's core consists of metallized mantle substances has been recently reanimated. The large volume changes which accompany the phase transformations of rutile-cubic phase and perovskite-mixed oxides for TiO2 and (Fe,Mg)TiO3, respectively, contradict the shock data for similar phase transformations in SiO2 and MgSiO3. In view of the relatively low pressure at the mantle-core boundary, this does not favor the hypothesis of a silicate or oxide core for the earth.
1. Introduction The hypothesis originally proposed by Bernal (1936) and revived by Ramsey (1948, 1949) that the core consists of a metallic form of mantle material has been discarded in the last two decades in favour of that of an iron core in which some light constituents are present. However, the hypothesis of a metallized silicate or oxide core seems to have been reanimated recently by e.g. Kawai and Mochizuki (1971), Dubrovskiy and Pan'kov (1972), Al'tshuler et al. (1973), Simakov et al. (1973), and Kawai and Nishiyama (1974), on the basis of their discoveries of conductive Fe203, FeO, TiO~, and Si02 under static high-pressure conditions, and their observations of high-pressure phase transformations of TiO2, SnO2, CaF2, and BaF2 in shock studies. The phase transformation of ilmenite to the perovskite structure, then to the cubic high-pressure phase of TiO2 plus (Fe,Mg)O (Liu, 1975a), and the.phase transformation of SnO2 reported by Liu (1975b) provide a basis for appraisal of these conclusions.
2. Phase transformations of some MOz compounds Geophysicists and geochemists are interested in the phase transformations of MO2 compounds, main-
ly because of the analogical relationship between SiO2 and the other MO2 compounds. With one exception, coesite, the high-pressure crystal phases of SiO2 are analogous to the dioxides of the elements of Group IV-A in the periodic table. The high-pressure phases for the dioxides of Group IV-A elements, plus TiO2 and MnO2, which naturally occur or have been observed under static high-pressure experiments are summarized in Table 1. Shock compression data for SnO2 are available up to 3.5 Mbar (Simakov et al., 1973) for TiO2 to 3 Mbar (Al'tshuler et al., 1973), for MnO2 to 1.26 Mbar (Birch, 1966), and for SiO2 to 6.5 Mbar (Trunin et al., 1970). The Hugoniot data revealed a phase transformation at about 2.4 Mbar for SnO2 (McQueen and Marsh's (1965) unpublished data, however, indicate a transition at about 1 Mbar) and at about 1 Mbar for TiO2. No apparent transition was observed for MnO2, and no major discontinuity in the Hugoniot data, other than those corresponding to coesite and stishovite, was detectable in SiO2. However, it was pointed out by Liu (1975b) that the major changes found in shock studies of SnO2 and TiO2 correspond, not to the rutile-orthorhombic transition, but to the rutile(orthorhombic)-cubic phase transition as was shown for TiO2 (Liu, 1975a). On the basis of a crystal chemical argument, Liu (1975b) also predicted that MnO2, GeO2, and SiO2 all may crystallize in the orthorhom-
PHASE TRANSFORMATIONS AND COMPRESSIONS OF ILMENITE AND RUTILE, IMPLICATIONS
345
TABLE I Summary of the crystal phases of dioxides of the elements of the Group IV-A plus TiO2 and MnO2 found naturally or under static high-pressure experiments Cation coordination
4
Crystal phase
a-quartz
PbO 2 SnO2 TiO 2 MnO2 GeO 2 SiO2
X X
6
8 fluorite
coesite
rutile
orthorhombic a-PbO2 X X X
X
x X X X X X
12(?) cubic phase
Shock experiment
X X
X X X X
The cation size decreases from PbO2 to SiO2. Crosses denote that the data are available. Sources have been referenced in the text. bic ct-PbO2 structure at elevated pressures and temperatures, although this may not be exposed b y shockexperimental studies because o f the small volume change associated with the r u t i l e - o r t h o r h o m b i c transition. The fluorite phase o f PbO2 has been reported by Syono and Akimoto (1968) and a cubic form o f TiO2 b y Liu (1975a). This cubic phase of TiO2 is probably not the fluorite structure, because of the large discrepancy between its observed lattice parameter and that calculated for fluorite TiO2 and the lack o f resemblance of the relative intensities o f its diffraction pattern to those for other fluorite compounds. Since
the volume change is inversely proportional to the cation radius. Al'tshuler et al. (1973) and Simakov et al. (1973) suggested that stishovite (SiO2) may eventually transform to the fluorite structure as implied by their
the shock compression curve o f SnO2 qualitatively resembles that o f TiO2, it is expected that SnO2 also transforms to the cubic form (neglecting the small volume change for the r u t i l e - o r t h o r h o m b i c transition in SnO2 reported b y Liu (1975b)).
AmO2 CeO2 CmO2 HfO2 NpO2 PaO2 PbO2 PoO2 PrO2 Pu02 TbO2 ThO2 uo2 ZrO2 TiO 2
The hypothetical lattice parameter for the fluorite phase o f TiO2 was calculated to be a0 = 4.775 A (Liu, 1975a). For SnO2 and SIO2, the average value of 1.16 + 0.07 (Table II) for the ratio o f the ionic radii in eight-fold and six-fold coordination, M 4÷(VIII)/M4÷(VI), was used to compute the ionic radii for eight-fold coordinated Sn 4÷ and Si4÷. The lattice parameters for the fluorite phase of SnO2 and SiO2 thus calculated are 5.03 and 4.25 A, respectively. In Table III we list the zero-pressure volume changes for the q u a r t z - r u t i l e , r u t i l e - o r t h o r h o m b i c , r u t i l e fluorite, and r u t i l e - c u b i c phase transformations in MO2 compounds. F o r each o f the transformations,
TABLE II Effective ionic radii (Shannon and Prewitt, 1969 except where noted) and the calculated lattice parameters for various fluorite oxides, MO2 (units are all in A) Oxides
SnO2 SiO2
M4+(VI)*
M4+(VIII)*
0.72 0.605
0.95 0.97 0.95 0.83 0.98 1.~1 0.94 1.10 0.99 0.96 0.88 1.04 1.00 0.78 0.69**
0.690 0.400
0.80 0.46
0.80 0.71 0.775 0.78 0.80 0.76 1.00
M4+(VII1)/ Mao M4+(VI) (VIII)-O 1.21 1.17 1.21 1.27 1.20 1.16 1.04 1.08 1.14! (1.16 ± 0.07)
2.33 2.35 2.33 2,21 2.36 2.39 2.32 2.48 2.37 2.34 2.26 2.42 2.38 2.16 2.068
5.38 5.43 5.38 5.10 5.45 5.52 5.36 5.73 5.47 5.40 5.22 5,59 5.50 5,00 4.775
2.18 1.84
5.03 4.25
* The roman numerals denote the coordination number. **Calculated from M(VIII)-O (see Liu, 1975a).
346
LIN-GUN LIU
TABLE IIl The changes of zero-pressure volume for quartz-rutile, rutile-orthorhombic, rutile-fluorite, and rutile-cubic transitions in some MO2 compounds (-~V/Vo in %)
Quar tz-rutile Rutile-or thorhombic Rutile-fluorite Rutile-cubic
SiO2
GeO2
38.2a
31.8 a
17.6c
TiO2
SnO2
PbO2
2.2b 12.8 c 29.2 e
1.5h 11.1 c
1.3b 7.1 d
The cation size increases from SiO2 to PbO2. a Calculated on data from ASTM Cards. b From listings by Liu (1975b). c From Table II. d Syono and Akimoto (1968). e Liu (1975a).
shock ~tudies of SnO2 and TiO2, and that any fluorite phase of SiO2 may further transform to the metallic structure o f MgCu2-type as implied by their studies of CaF2 and BaF2. These observations have led them to conclude, although qualitatively, that the hypothesis of silicate or oxide core is competitive with that of iron core. 3. Conductive TiO2, SiO2, and FeO The electrical resistance of rutile (TiO2) has been reported by Kawai and Mochizuki (1971) to drop from approximately 105 to 10-1 ~2 at about 2.2 Mbar when it was subjected to static pressures up to 2.5 Mbar at room temperature in a two-stage spheric vessel. The resistance of a shock-recovered orthorhombic phase of TiO2 was measured to be at-out 10 s ~2 by McQueen et al. (1967). Hence, the large resistance change in TiO2 is not likely to be due to the rutileorthorhombic transition. There is some suggestion that the resistance change may be associated with the Mort transition (insulator-metal transition) or related to the crystal transition (Kawai and Mochizuki, 1971). The cubic phase of TiO2 discussed in the previous section may be responsible for the resistance change. This can be clarified in future by measuring the resistance of quenched high-pressure phase of TiO2. However, the quenched fluorite phase of PbO2 found by Syono and Akimoto (1968) is not as conductive as the compressed TiO2 reported by Kawai and Mochizuki (1971 ).
At a pressure higher than that required for TiO2, crystalline SiO~ (what form of the crystal was not specified) displays a similar electric resistance change from the order of 1 0 s - 1 0 -1 ~ accompanying a hysteresis loop on release of pressure (Kawai and Nishiyama, 1974). This again was attributed by them to the Mott transition or to the breakdown of SiO2 into its constituent elements with oxygen in the metallic state. On the other hand, recent shock studies have revealed that SiO2 remains as an insulator at pressures up to 5 Mbar (Hawke et al., 1972) and no drastic velocity or volume changes other than those corresponding to coesite and stishovite in SiO2 up to 6.5 Mbar (Trunin et al., 1970). Feo.9sO was also found by Kawai and Nishiyama (1974) to become conductive at a static pressure lower than that for TiO2 (the pressure was not reported). This, in turn, would support the hypothesis of an FeO outer core proposed by Dubrovskiy and Pan'kov (1972). The discovery of conductive TiO2, SiO2, and FeO has caused Kawai and Mochizuki (1971) and Kawai and Nishiyama (1974) to conclude that the hypothesis of a metallized core is more plausible than ever before.
4. Discussion and conclusion Following the Al'tshuler et al. (1973) suggestion of the sequence of transformations in analogous systems, Simakov et al. (1973) assigned a zero-pressure density
PttASE TRANSFORMATIONS AND COMPRESSIONS OF 1LMENITE AND RUT1LE, IMPLICATIONS o f approximately 6 g/cm 3 for the fluorite phase of SIO2, which corresponds to a volume change o f about 29% for the r u t i l e - f l u o r i t e transition of SIO2. This value seems too large according to the data derived from ionic-radius considerations lited in Table III, but is compatible with the observed volume change for the r u t i l e - c u b i c transition in TiO2. Such a large change in volume (either 18 or 29%), however, was not observed in a shock compression study o f SiO2 up to 6.5 Mbar by Trunin et al. (1970). This pressure (6.5 Mbar) is probably at least equivalent to 2 - 3 Mbar under static conditions for the observation of the same phase transformation, whereas the static pressure at the m a n t l e - c o r e boundary is around 1.4 Mbar. Thus, the hypothesis of a silicate or oxide core with the fluorite, the cubic, or the MgCu2-type structure is not supported b y the results of the shock study o f SiO2 even though at still higher pressures SiO2 might eventually transform to the fluorite, the cubic, or the MgCu2 structure, tn addition, the m a n t l e - c o r e interface represents a s o l i d - l i q u i d boundary as the shearwave velocity distribution indicates. Liu (1975b) has shown that, in the presence o f MgO, SiO2 in the form of either rutile or orthorhombic ct-PbO2 structure is unstable relative to the perovskite phase o f MgSiO3 found by Liu (1974) under pressure and temperature condition comparable to those of the lower mantle. The perovskite phase o f MgSiO3 might disproportionate into a mixture o f oxides as shown in the previous paper (Liu, 1975a) for the perovskite phase o f (Fe,Mg)TiO3. This transition would be accompanied by a change of at least 13% in the zero-pressure volume, as in the case of (Fe,Mg)TiO3. Such a large change in volume was again not observed by Simakov and Trunin (1973) on their remeasurement of shock compressions of natural and artificial ceramic enstatites (MgSiOa) up to 2.5 Mbar. Hence, one may conclude at this stage that the perovskite structure appears to represent the final phase in the evolution o f crystal structure o f minerals in the mantle, and the pressure at the m a n t l e - c o r e boundary is probably too low for the i n s u l a t o r ' m e t a l transition o f FeO and SiO2 found by Kawai and Nishiyama (1974), or for the possible phase transformations of r u t i l e f l u o r i t e - c u b i c - M g C u 2 in SiO2 and o f p e r o v s k i t e mixed oxides in MgSiO3. One may also, however, argue
347
that the time duration might be too short for the substance in the shock wave to transform. A large change in density, which is characteristic of phase transformations o f the r u t i l e - f l u o r i t e - c u b i c and p e r o v s k i t e mixed oxides, is also not suggested by studies of seismic velocities of the lower mantle (e.g., Johnson, 1969; Hales and Roberts, 1970; Whitcomb and Anderson, 1970).
Acknowledgement The author wishes to thank J.R. Cleary, R.C. Lieber. mann and A.E. Ringwood for commenting on this manuscript.
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