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Rutile-like silica and phase transformations in the earth's interior
223
RUTILE-LIKE SILICA AND PHASE TRANSFORMATIONS IN THE EARTH’S INTERIOR S.M. STISHOV Institute of Crystallography, (Received
Academy
of Sciences,
Moscow
(U.S.S.R.)
April 10, 1964)
SUMMARY
A new dense silica modification with a density equal to 4.35 g/cm3 was synthesised by the author under pt conditions corresponding to those of the upper parts of the C-zone of the earth’s mantle. This modification had a structure of rutile type, as was proved by X-ray studies. Ferromagnesium silicates are possibly transformed in the C-zone into a mixture of closely packed oxides with structures of rutile, periclase and others. The transition of olivine into a spine1 structure, favourable under high pressures, is accompanied by a lO-15% decrease in volume. Probably dating from the publication of a well-known paper by Birch (1952) on the elasticity and constitution of the earth’s interior, an inhomogeneous layer in the C-zone of the earth’s mantle (between 200 and 900 km) is commonly considered to be a region in which complex physiochemical changes take place. According to Birch the ratio (T), in the lower mantle (D-zone) is equal to 60 (kmfsecf2, and is too great for common silicates with silicon in tetrahedral coordination. However, this ratio is in good agreement with that of the closely packed oxides such as corundum, rutile, periclase and others. Birch has suggested that the transformation of crystal structures of silicates into closely packed structures of simple and complex oxides occurred in the C-zone. A characteristic feature of Birch’s hypothesis in the necessity of silicon transition from tetrahedral into octahedral coordination, This part of the hypothesis caused most of the objections. The author, together with Popova (1961), succeeded in synthesizing a new dense silica modification with a density equal to 4.35 g/cm3, which is 64% higher than that of quartz. The p-t conditions of synthesis (p = 130 kbar and t = 1,500’ C) correspond to those of the upper parts of the C-zone. X-ray studies (Stishov and Belov, 1962) have proved the new dense silica to possess a structure of rutile type, and hence to contain silicon in octahedral coordination. Recently the author (Stishov, 1963) approximately calculated the equilibrium curve of coesite-rutile-like silica (Fig.1). Tectonophysics,
1 (3) (1964) 223-226
224
S.M. STISHC’r
I......
* 110
130
lx)
170
. p
kbor
.
.
190
Fig.1. The line of equilibrium in the systems: coesite-rutile-like silica; Mg$IiO,-forsterite-M@iO, forsterite-2MgO (periclase)+O, (x-utile-like silica).
spinel; h&SiO,-
With the knowledge of the crystallochemical features of the elements of the mantle, it is reasonable to imagine a sequence of phase transitions in the earth under the action of pressure. The results of studies of Mg,GeO,, Ni&O,, MgGeO,: etc., under high pressure, carried out by Dachile and Roy (1960), Ringwood (1958, 1960), Ringwood and Seabrook (1962) and others, have made the solution of this problem easier. The earth consists mainly of oxygen, silicon, magnesium and iron, with large ions of oxygen occupying the greater part of its volume. Therefore, the problem of obtaining dense crystal structures is reduced to attaining the most dense packing of oxygen ions. Glivines and pyroxenes, the most common minerals occurring in the upper mantle, already possess hexagonal and cubic close packings, respectively. However, the presence of common edges for Si-tetrahedra and Mg, Fe-octahedra in the structure of olivine, and the somewhat greater length of the edges of I@, Feoctahedra as compared with Si-tetrahedra, results in abnormally large Si-0 distances in the olivine structure (Belov, 1959). Therefore, the transition of olivine into a spine1 structure where tetrahedra and octahedra have no common edges is favourable under high pressures and is accompanied by a 10-15% decrease in volume. It is necessary to recall that the normal positions occupied by the small metallic ions are such that they slightly push apart the closely packed oxygen ions, making the structure less dense as compared to that of metals. Hence if small silicon ions usually located in the tetrahedral holes would be placed in large octahedral holes, leaving It@, Fe in their original positions, then the oxygen ions would be more closely packed. Indeed, the average 0-O distance in rutile-like silica equals 2.46 A, which is substantially less than that in the structures of substances with silicon in fourfold coordination, where it amounts to an average of 2.6-2.7 A. In this connection one may expect that the olivine spine1 transition is Tectonophysics,
1 (3) (1964) 223-f-226
RUTILE-LIKE
225
SILICA AND PHASE TRANSFORMATIONS
followed by a series of transformations which will lead to structures with a close packing of oxygen atoms and ions of magnesium, silicon and iron, in octahedral positions in the D-zone. Two extreme cases are possible: (a) As a result of transformations occurring in the C-zone, the ferromagnesium silicates are transformed into a mechanical mixture of closely packed oxides with the structures of rutile, periclase and others. The summary effect of these transformations may be recorded as follows: -
Mg$iO, forsterite
2 MgO
+
rutile-like silica
periclase
(P = 3.22 g/cm3) MgSi0,
-
enstatite
SiO,
(1)
(P = 3.9 g/cm”) MgO
+
rutile-like silica
periclase
(p = 3.1 g/cm3)
SiO,
(2)
(p = 4.0 g/cm”)
An approximate equation for the equilibrium curve of the transformation (1) as composed by the author, and the most probable equilibrium curve for the olivine-spinel transition in forsterite, is also plotted on Fig.1. So the transition to mixture of oxides can be of very complicated character. (b) The ferromagnesian silicate system transforms into an entirely homogeneous substance in the D-zone. As ‘to the crystal structure of this hypothetic substance, it is most probably similar to an imperfect structure of NaCl-type. (Fe,,O, Mg,MnO, are good examples of such structures.) Approximate calculations of the density of such a homogeneous structure give 3.8 g/cm3 and 3.9 g/cm3, which is also close to the zero density of the D-zone. However, the second of the considered variants of the state of substance in the D-zone seems to be more probable to the author. At the present time it is hardly reasonable to consider the suggested variants of crystal-chemical evolution of the substance in the mantle in detail. It is important to emphasize that the available experimental data allow us to explain the density and the elastic properties of the mantle only on the base of physical changes in the state of the substance under the action of the high pressure. REFERENCES Belov, N.V., 1959. Kristallokhimiy’a Osnovnogo protsessa kristallizatsii magmy. Geokhimya redkikh elementov v svyazi s problemoi petrogenezisa. Tr. Geokhim. Simp., 1959, Moscow. Birch, F., 1952. Elasticity and constitution of the earth’s interior. J. Geophys. Res., 57 (2) : 227-286. Dachille, F. and Roy, R., 1960. High pressure studies of the system Mg,GeO,-Mg,SiO, with special reference to the olivinespinel transition. Am,.J. Sci., 258 (4) : 225-246. Tectonophysics,
1 (3) (1964) 223-226
226
5.M. Sl%II( I\
King-wood, A.E.. 195Xa. Constitution ot’ the mantle. 2. Further data on the olivinc-spincl transition. Geochim. Cosmochim. Acta, 15 : 18-29. Ringwood, A.E ., 1958b. Constitution of the mantle. 3. Consequences of the olivinespine1 transition. Geochim. Cosmochim. Acta, 15 : 195-212. Ringwood, A.E., 1960. Silicon in the metal phase of enstatite chondriten. Nature, 1X6 : 465466. Ringwood, A.E. and Seabrook, M., 1962a. High-pressure transition of MgGeO, from pyroxene to corundum structure. J. Geophys. Res., 67 (4) : 1690-1691. Ringwood, A.E. and Seabrook, M., 196213.Olivine-spinel equilibrie at high pressure in the system NizGe04-MgaSiO,. J. Geophys. Res., 67 (5) : 1975-1985. Stishov, S.M., 1963. Ligne d’equilibre entre coesite. et une modification du type rutile de la silice. Dokl. Akad. Nauk S.S.S.R., 143 (4) : 951-954 (in Russian). Stishov, S.M. and Belov, N.V., 1962. Sur la structure cristalline d’une nouvelle modification dense de SiOz. Dokl. Akad. Nauk S.S.S.R., 143 (4) : 951-954 (in Russian). Stishov, S.M. and Popova, S.V., 1961. A new dense modification of silica. Geokhimiya, 10 : 837-839 (in Russian).
Tectonophysics,
1 (3) (1964) 223-226
RUTILE-LIKE SILICA AND PHASE TRANSFORMATIONS IN THE EARTH’S INTERIOR S.M. STISHOV Institute of Crystallography, (Received
Academy
of Sciences,
Moscow
(U.S.S.R.)
April 10, 1964)
SUMMARY
A new dense silica modification with a density equal to 4.35 g/cm3 was synthesised by the author under pt conditions corresponding to those of the upper parts of the C-zone of the earth’s mantle. This modification had a structure of rutile type, as was proved by X-ray studies. Ferromagnesium silicates are possibly transformed in the C-zone into a mixture of closely packed oxides with structures of rutile, periclase and others. The transition of olivine into a spine1 structure, favourable under high pressures, is accompanied by a lO-15% decrease in volume. Probably dating from the publication of a well-known paper by Birch (1952) on the elasticity and constitution of the earth’s interior, an inhomogeneous layer in the C-zone of the earth’s mantle (between 200 and 900 km) is commonly considered to be a region in which complex physiochemical changes take place. According to Birch the ratio (T), in the lower mantle (D-zone) is equal to 60 (kmfsecf2, and is too great for common silicates with silicon in tetrahedral coordination. However, this ratio is in good agreement with that of the closely packed oxides such as corundum, rutile, periclase and others. Birch has suggested that the transformation of crystal structures of silicates into closely packed structures of simple and complex oxides occurred in the C-zone. A characteristic feature of Birch’s hypothesis in the necessity of silicon transition from tetrahedral into octahedral coordination, This part of the hypothesis caused most of the objections. The author, together with Popova (1961), succeeded in synthesizing a new dense silica modification with a density equal to 4.35 g/cm3, which is 64% higher than that of quartz. The p-t conditions of synthesis (p = 130 kbar and t = 1,500’ C) correspond to those of the upper parts of the C-zone. X-ray studies (Stishov and Belov, 1962) have proved the new dense silica to possess a structure of rutile type, and hence to contain silicon in octahedral coordination. Recently the author (Stishov, 1963) approximately calculated the equilibrium curve of coesite-rutile-like silica (Fig.1). Tectonophysics,
1 (3) (1964) 223-226
224
S.M. STISHC’r
I......
* 110
130
lx)
170
. p
kbor
.
.
190
Fig.1. The line of equilibrium in the systems: coesite-rutile-like silica; Mg$IiO,-forsterite-M@iO, forsterite-2MgO (periclase)+O, (x-utile-like silica).
spinel; h&SiO,-
With the knowledge of the crystallochemical features of the elements of the mantle, it is reasonable to imagine a sequence of phase transitions in the earth under the action of pressure. The results of studies of Mg,GeO,, Ni&O,, MgGeO,: etc., under high pressure, carried out by Dachile and Roy (1960), Ringwood (1958, 1960), Ringwood and Seabrook (1962) and others, have made the solution of this problem easier. The earth consists mainly of oxygen, silicon, magnesium and iron, with large ions of oxygen occupying the greater part of its volume. Therefore, the problem of obtaining dense crystal structures is reduced to attaining the most dense packing of oxygen ions. Glivines and pyroxenes, the most common minerals occurring in the upper mantle, already possess hexagonal and cubic close packings, respectively. However, the presence of common edges for Si-tetrahedra and Mg, Fe-octahedra in the structure of olivine, and the somewhat greater length of the edges of I@, Feoctahedra as compared with Si-tetrahedra, results in abnormally large Si-0 distances in the olivine structure (Belov, 1959). Therefore, the transition of olivine into a spine1 structure where tetrahedra and octahedra have no common edges is favourable under high pressures and is accompanied by a 10-15% decrease in volume. It is necessary to recall that the normal positions occupied by the small metallic ions are such that they slightly push apart the closely packed oxygen ions, making the structure less dense as compared to that of metals. Hence if small silicon ions usually located in the tetrahedral holes would be placed in large octahedral holes, leaving It@, Fe in their original positions, then the oxygen ions would be more closely packed. Indeed, the average 0-O distance in rutile-like silica equals 2.46 A, which is substantially less than that in the structures of substances with silicon in fourfold coordination, where it amounts to an average of 2.6-2.7 A. In this connection one may expect that the olivine spine1 transition is Tectonophysics,
1 (3) (1964) 223-f-226
RUTILE-LIKE
225
SILICA AND PHASE TRANSFORMATIONS
followed by a series of transformations which will lead to structures with a close packing of oxygen atoms and ions of magnesium, silicon and iron, in octahedral positions in the D-zone. Two extreme cases are possible: (a) As a result of transformations occurring in the C-zone, the ferromagnesium silicates are transformed into a mechanical mixture of closely packed oxides with the structures of rutile, periclase and others. The summary effect of these transformations may be recorded as follows: -
Mg$iO, forsterite
2 MgO
+
rutile-like silica
periclase
(P = 3.22 g/cm3) MgSi0,
-
enstatite
SiO,
(1)
(P = 3.9 g/cm”) MgO
+
rutile-like silica
periclase
(p = 3.1 g/cm3)
SiO,
(2)
(p = 4.0 g/cm”)
An approximate equation for the equilibrium curve of the transformation (1) as composed by the author, and the most probable equilibrium curve for the olivine-spinel transition in forsterite, is also plotted on Fig.1. So the transition to mixture of oxides can be of very complicated character. (b) The ferromagnesian silicate system transforms into an entirely homogeneous substance in the D-zone. As ‘to the crystal structure of this hypothetic substance, it is most probably similar to an imperfect structure of NaCl-type. (Fe,,O, Mg,MnO, are good examples of such structures.) Approximate calculations of the density of such a homogeneous structure give 3.8 g/cm3 and 3.9 g/cm3, which is also close to the zero density of the D-zone. However, the second of the considered variants of the state of substance in the D-zone seems to be more probable to the author. At the present time it is hardly reasonable to consider the suggested variants of crystal-chemical evolution of the substance in the mantle in detail. It is important to emphasize that the available experimental data allow us to explain the density and the elastic properties of the mantle only on the base of physical changes in the state of the substance under the action of the high pressure. REFERENCES Belov, N.V., 1959. Kristallokhimiy’a Osnovnogo protsessa kristallizatsii magmy. Geokhimya redkikh elementov v svyazi s problemoi petrogenezisa. Tr. Geokhim. Simp., 1959, Moscow. Birch, F., 1952. Elasticity and constitution of the earth’s interior. J. Geophys. Res., 57 (2) : 227-286. Dachille, F. and Roy, R., 1960. High pressure studies of the system Mg,GeO,-Mg,SiO, with special reference to the olivinespinel transition. Am,.J. Sci., 258 (4) : 225-246. Tectonophysics,
1 (3) (1964) 223-226
226
5.M. Sl%II( I\
King-wood, A.E.. 195Xa. Constitution ot’ the mantle. 2. Further data on the olivinc-spincl transition. Geochim. Cosmochim. Acta, 15 : 18-29. Ringwood, A.E ., 1958b. Constitution of the mantle. 3. Consequences of the olivinespine1 transition. Geochim. Cosmochim. Acta, 15 : 195-212. Ringwood, A.E., 1960. Silicon in the metal phase of enstatite chondriten. Nature, 1X6 : 465466. Ringwood, A.E. and Seabrook, M., 1962a. High-pressure transition of MgGeO, from pyroxene to corundum structure. J. Geophys. Res., 67 (4) : 1690-1691. Ringwood, A.E. and Seabrook, M., 196213.Olivine-spinel equilibrie at high pressure in the system NizGe04-MgaSiO,. J. Geophys. Res., 67 (5) : 1975-1985. Stishov, S.M., 1963. Ligne d’equilibre entre coesite. et une modification du type rutile de la silice. Dokl. Akad. Nauk S.S.S.R., 143 (4) : 951-954 (in Russian). Stishov, S.M. and Belov, N.V., 1962. Sur la structure cristalline d’une nouvelle modification dense de SiOz. Dokl. Akad. Nauk S.S.S.R., 143 (4) : 951-954 (in Russian). Stishov, S.M. and Popova, S.V., 1961. A new dense modification of silica. Geokhimiya, 10 : 837-839 (in Russian).
Tectonophysics,
1 (3) (1964) 223-226