Post-ilmenite phases of silicates and germanates

Earth and Planetary Science Letters, 35 (1977) 161-168

161

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands [31

POST-ILMENITE PHASES OF SILICATES AND GERMANATES LIN-GUN LIU t Research School o f Earth Sciences, Australian National University, Canberra, A. C T. (Australia)

Received November 5, 1976 Revised version received February 4, 1977

MgSiO3, ZnSiO3, MgGeO3, MnGeO3, and ZnGeO 3 are the only silicates and germanates known to crystallize in the ilmenite-like structure at high pressures and high temperatures. With the exception of the zinc compounds, the above-mentioned ilmenites have all been found to transform to the orthorhombic modification of the perovskite structure at higher pressures. The ilmenite phase of ZnSiO 3, on the other hand, transforms to its component oxide mixture with the rocksalt and rutile structures, whereas ZnGeO 3 (ilmenite) transforms first to an as yet undetermined orthorhombic phase and then to its component oxide mixture. The direct transformation from the ilmenite to perovskite structures observed in the metasilicates and metagermanates is consistent with all other reported high-pressure post-ilmenite phases (CdTiO 3, CdSnO 3, MnVO3, and (Fe,Mg)TiO3). The observation of the ilmenite-perovskite transformation in MgSiO3 and its solid solutions towards A1203 suggests that MgO (rocksalt) + SiO2 (rutile) + AI203 (corundums is not a stable mineral assemblage for the earth's lower mantle.

1. Introduction The synthesis of the ilmenite phase of MgSiO3 [ 1,2] and its alumina solid solutions up to 25% wt. A120 3 (the pyrope composition) [3] at high pressures and temperatures suggests that these phases might be important to the structure and composition o f the earth's transition zone and the lower mantle. Consequently, post-ilmenite phases o f these compounds at still higher pressures bear significant implications for our understanding o f the lower mantle. The ilmenite phase of MgSiO3 and its alumina solid solutions have been found to transform to the orthorhombic perovskite modification [ 2 - 4 ] with a ca. 7% decrease in the zero-pressure volume. The ilmenite phase o f MgSiO 3 has once been reported to react to its component oxide mixture possessing the rocksalt and rutile structures [5]; the same reaction has also been observed in ZnSiO 3 [6]. The zero-pressure volume for compounds possessing l Present address: Seismological Laboratory, California Institute of Technology, Pasadena, Calif. 91125, U.S.A.

the ilmenite structure is in general 2 - 5 % greater than that o f the isochemical oxide mixture (rocksalt and : rutile structures). Hence, one may postulate that the mixture o f periclase (MgO) plus stishovite (SiO2) might be a stable intermediate phase assemblage between the ilmenite and perovskite phases of MgSiO 3. A general search for the post.ilmenite phases in various silicate and analogue systems provides not only a test of this hypothesis but also information o f crystal-chemical value. MgSi03, ZnSiO3, MgGeO3, MnGeO3, and ZnGe03 are the only silicates and germanates known to crystallize in the ilmenite structure at high pressures and high temperatures. The results of studies concerning the high-pressure phase transformations and especially the post-ilmenite phases o f these metasilicates and metagermanates are presented here.

2. Experimental MgSiO 3 glass, the pyroxenes ZnSiO3, MgGeO3, and MnGeO3, and the ilmenites MgGeO 3 and ZnGeO 3

162 were used as starting materials in the present study and were provided by A. Major (Australian National University). With the exception of MnGeO3, the above-mentioned samples were intimately mixed with a few percent" of graphite, which serves to absorb the laser radiation and thus heats the samples. Very fine powder samples so prepared were compressed in a diamond-anvil press fitted with a lever-and-spring assembly and were irradiated by a continuous YAG laser while the samples were maintained at high pressures. The loading pressures of the sample at the central portion of the anvil were estimated from the length of the spring which was calibrated according to the room temperature NaC1 pressure scale. Pressures thus estimated were probably accurate to +-10%. Temperatures were approximately estimated on the basis of the intensity of incandescent light emitted from the sample. Laser heating causes the real pressure in the sample to be about 50 kbar higher than the loading pressure at a nominal loading pressure of 150 kbar. After quenching and release of pressure, samples were transferred to a modified 57.3-mm Debye-Scherrer camera for X-ray diffraction study using filtered cobalt radiation. Details of the experimental procedure have been described elsewhere [7].

3. Results 3.1. MgSiO 3 MgSiO a crystallizes as protoenstatite, orthoenstatite, and clinoenstatite in the low-pressure region. The last-mentioned phase was reported to persist to pressures substantially exceeding 200 kbar and about 1000°C [8]. The glass starting samples recovered from about 150 and 175 kbar and 1000-1400°C in the present work are mainly clinoenstatite. At 170-180 kbar'loading pressure, clinoenstatite transforms to a mixture of/3-Mg2SiO4 plus stishovite (SiO2). This assemblage transforms to a mixture of Mg2SiO 4 (spinel) plus stishovite at about 200 kbar. These phases then recombine to form the ilmenite phase of MgSiO3 at about 220 kbar. These results confirm the interpretation of earlier work [1,9]. The ilmenite phase of MgSiO 3 has been found to transform directly to the orthorhombic perovskite

modification in the vicinity of 250-260 kbar without the intervention of any other phases or phase assemblages. The experimental details and the X-ray diffraction data have been reported elsewhere [2]. 3.2. MgGeO a The orthopyroxene form of MgGeO3 has previously been found to transform into a clinopyroxene modification at 5 kbar and into an ilmenite modification at 28 kbar and at 700°C [10]. A detailed phase diagram for pressures less than 65 kbar and temperatures less than 1300°C is also available [11 ]. The X-ray diffraction data for MgGeO 3 (pyroxene) quenched from 260 kbar and 1000-1400°C in the present study are listed in Table 1. Only a minor amount of pyroxene and ilmenite can be identified. The remaining lines are attributed to an orthorhombic perovskite phase of MgGeO3, the spinel phase of Mg2GeO4, and the rutile form of GeO2 - these three phases being present in nearly equal amounts. Similar data (no pyroxene, greater proportion of ilmenite) were obtained from a sample recovered from about 250 kbar using the ilmenite form of MgGeO 3 as the starting material. The lattice parameters for the orthorhombic perovskite phase at room temperature and 1 bar pressure are a o = 4.946 +- 0.002, b o = 5.100 + 0.002, c o = 7.233 + 0.003 A, Z = 4; the possible space group is Pmmm or P222. Thus, the zero-pressure molar volume for the perovskite MgGeO a is calculated to be 27.47 + 0.03 cma/mole, which is 5.7% smaller than that for the flmenite phase of MgGeO a. It is noted that the relative intensities for the diffraction pattern of the orthorhombic MgGeO3 observed in this work are unlike those of typical orthorhombic perovskite compounds (e.g. GdFeOa) listed in Wyckoff [12] and those synthesized at high pressures such as MnVO 3 [13], ScAIO3 [ 14], MgSiO 3 [ 15 ], and MnGeO3 ( [ 16], also see the next section). However, a comparison of the volume changes (-5.7%) associated with the ilmeniteorthorhombic MgGeO3 phase transformation with those of the ilmenite-perovskite transitions is a strong indication that the orthorhombic MgGeO3 phase is a perovskite related structure (refer to Table 3). The clinopyroxene form of MgGeO3 has been reported to transform directly into the ilmenite phase at 28 kbar and 700°C [10] and at 36 kbar and 1000°C [17]. The direct transition has also been confirmed

TABLE 1

3.61 3.57 3.35 (px) 3.22 (px) 3.11 2.970 (px) 2.670 2.561 2.533 (px) 2.486 2.402 2.197 2.168 2.053 1.807 1.778 1.684 1.647 1.624 1.593 1.584 1.554 1.531

1.473

1.461 1.437 1.303

10

15 35 10

4.13

5

100 20 15 10 20 15 5 5 90 90 20 5 15 5 30 10 15 15 10 5 5 <5 5

7.22 5.04 4.76 4.58

<5 5 20 <5

1.304

1.588

35

11

1.684

30

1.458

2.063

9

50

2.488

4,76

100

25

I/I100

d (A)

I/I100

d (A)

Mg2GeO4 spinel 2

Observed 1

620

400

511

422

400

311

111

hkl

10 18

18

50

60 16

100

1//100

1.431 1.300

1.555

1.620

2.399 2.199

3.11

d (A)

GeO 2 futile 3

002 112

220

211

101 200

110

hkl

20

40

40 <10 100

80

80

90

50

70

I//1oo

1.306

1.457

1.809 1.775 1.673

2.17

2.46

2.67

3.62

4.55

d (A)

MgGeO 3 ilmenite 4

O, 1, 10

214

024 107 116

113

110

104

012

0~3

hkl

X-ray diffraction data (room temperature and 1 bar pressure) for MgGeO3 quenched from about 260 kbar and 1000 ~ 1400°C (CoKa)

1.533 1.475 1.469 1.460 1.439 1.306

1.594

1.649

2.042 1.808 1.775

2.405 2.180

2.550 2.534

311 024 132 204 312 232

222

300

202 004 220

021 013

020 112

012

111

3.19 2.950

011 101 002 110

001 010 100

hkl

4.17 4.08 3.62 3.55

7.23 5.10 4.95

d (A)

MgGeO3 perovskite 5

LO

1.267 1.248

1.201

1.172

1.155

1.137

1.123

1.081

<5 5

5

<5

<5

5

5

5

1.155

711

hkl

8

2

l/[lO 0

1.122

1.251

d (A)

GeO 2 rutile 3

321

311

hkl

1 1/1100 estimated visually. For abbreviations inside parentheses: px = pyroxene of MgGeO3. 2 From ASTM Card Catalogue (10-180). 3 From ASTM Card Catalogue (9-379). 4 From Ringwood and Seabrook [10]. 5 d-spacings were calculated from a 0 = 4.946, b 0 = 5.100, and c O = 7.233 A.

3

1/1100

d (A)

I/llO 0 d (A)

Mg2GeO4 spinel 2

Observed 1

TABLE 1 (continued)

20

10

10

<10

<10

1/1100

1.086

1.120

1.154

1.178

1.245

d (A)

MgGeO 3 ilmenite 4

226

134

0,2,10

131

217

hkl

(A)

1.140 1.133 1.125 1.120 1.087 1.081

1.267 1.249 1.203 1.202 1.201 1.170 1.169 1.168

d

412 240 332 241 305 242

224 205 042 134, 410 323 402 142 331

hkt

MgGeO 3 perovskite 5

165 b y Kirfel and Neuhaus [1 1 ]. These observations have caused some difficulties in explaining the assemblage of Mg2GeO 4 (spinel) + GeO2 (rutile) observed in this study, since the zero-pressure volume for the ilmenite phase o f MgGeO3 is 1% smaller than that for the spinel plus rutile assemblage. Nevertheless, the P-T phase diagram postulated in Fig. 1 for MgGeO 3 presents a possible solution for the discrepancy b e t w e e n the results reported earlier and those of the present investigation, and also provides the sequence of phase transformations in MgGeO3.

3.3. MnGeO3 The o r t h o p y r o x e n e form of MnGeO3 has b e e n reported to transform to a clinopyroxene form at 15 kbar and to an ilmenite form at 30 kbar and 700°C [ 18]. A nearly pure o r t h o r h o m b i c perovskite phase of MnGeO3,has b e e n synthesized at a b o u t 250 kbar loading pressure and a p p r o x i m a t e l y 1 4 0 0 - 1 8 0 0 ° C and details o f the X-ray diffraction data have been described elsewhere [16]. The zero-pressure volume change associated with the ilmenite-perovskite transition in MnGeO 3 was calculated to be - 6 . 6 % .

TABLE 2 X-ray diffraction data (room temperature and 1 bar pressure) for ZnGeO 3 quenched from about 240 kbar and 1000 ~ 1400 ° C (CoKc~) Orthorhombic ZnGeO 3 **

Observed *

I/I10 0

d (A)

d (A)

hkl

40 <5 15

3.62 2.87 (u) 2.7O2 (i)

3.61

110

100

2.600

1-2.607 ~2.597 1-2.513 ~2.511

112 020 003 200

2.165 2.062 1.968

211 113 122

1.805

220

1.637 1.593 1.530 { 1.525

130 310 302 024

1-1.468 ~ 1.466 1.448 1.447 1.444 1.304 I"1"254 ~1.252 t"1"154 ~1.153 ~1.130 "[1.127

312 223 015 214 105 025 215 304 043 240 420 324 333 422 423 051 341, 150 501 424 512 251 343 433

90 15 <5 30 5 5 5 90 15 30 <5

2.508 1.482 (i) 2.33 (i) 2.170 2.064 1.970 1.834 (i) 1.806 1.694 (i) 1.640 1.60

20

1.525

40

1.467

40

1.448

5 5

1.303 1.250

5

1.153

10

1.128

5

1.084

10

1.030

10 5 10 10

1.018 0.9950 0.9683 0.9528

10

0.9470

3.4. ZnSiO3 ZnSiO3 exists as a m i x t u r e of Zn2SiO 4 (willemite, phenakite structure) plus SiO2 (quartz and coesite) at pressures below 3 0 - 3 5 kbar b u t crystallizes as ZnSiO 3 ( c l i n o p y r o x e n e ) in the pressure range 35

/

I /

MgGeO 3

I

/ / Spinel + G e O 2

I I I

1500

/ /

f~//7 /

/

/ /

1000

/ / / ! /

Cpx Ilmenite

/ Pero~ite

,'

.-~, Opx L

500

! !

/

5'0

~o

1~o

Pressure , kbor

Fig. 1. The postulated temperature-pressure phase diagram for MgGeO 3. Solid lines represent data from Kirfel and Neuhaus [ 11 ].

200

1.1.085 ~1.083 1-1.031 ~1.029 1.017 0.9956 0.9693 ~0.9540 ~0.9522 1-0.9499 L0.9422

* I/I1 O0 estimated visually. For abbreviations inside the parentheses: u = unidentified line; i = ilmenite phase of ZnC-eO3. ** d-spacings were calculated from a 0 = 5.022, b 0 = 5.194, and c o = 7.538 A.

166 < P < 120 kbar [19]. The ilmenite phase o f ZnSiO3 was synthesized in the pressure region between 125 and 320 kbar and found to decompose into a mixture of ZnO plus stishovite betweeen 320 and 350 kbar [6]. In the present study, four experimental runs on ZnSiO 3 have been made between 180 and 300 kbar (inclusive). The sample quenched from 180 kbar and 1000-1400°C comprises mainly the ilmenite phase of ZnSiO3, the pyroxene phase of ZnSiOa (starting material), both the wurtzite and rocksalt phases of ZnO, and stishovite (SiO2) in order of decreasing abundance (deduced from X-ray intensities). The same phase assemblage has been observed in the samples recovered from 230,270, and 300 kbar with systematic changes in their relative abundances. For instance, the wurtzite and rocksalt phases of ZnO and stishovite dominated the X-ray pattern for the sample recovered from 300 kbar with minor amounts of the pyroxene and ilmenite phases of ZnSiO3. The sequence of phase transformation in ZnSiO3 is therefore inferred to be ZnSiO3 (pyroxene) ~ ZnSiO a (ilmenite) ~ ZnO (rocksalt) + SiO2 (stishovite) with increasing pressure. In their studies of both ZnSiO3 and Zn2SiO 4, Ito and Matsui [6] observed only the wurtzite phase of ZnO and suggested that ZnO may have assumed the rocksalt structure under high-pressure conditions but retrogressively transformed to the wurtzite structure

during quench. We have observed partial retrogressive transformation in the rocksalt phase of ZnO in the present work and observed no retrogressive transformation of the rocksalt phase at all in an earlier study of Zn2GeO 4 [20]. Ito and Matsui [6] have also reported an X phase in their study of ZnSiO3 at 350 kbar and 1000°C. We have previously claimed that the 350 kbar pressure reported by them is equivalent to a diamond-anvil press loading pressure no greater than 250 kbar [20]. There is no evidence for the existence of the X phase o f ZnSiO 3 from the present study despite the greater pressure range spanned by the present experiments. 3.5. ZnGeO 3

ZnGeO 3 crystallizes as a mixture of Zn2GeO4 (phenakite structure) plus GeO2 (rutile) at pressures less than 10 kbar (at 1100°C), and at 20 and 35 kbar (1100°C) a mixture of Zn2GeO 4 (tetragonal distorted spinel) plus GeO~ is stable [19]. At 110 kbar and 900°C ZnGeO a appears as a single phase with the ilmenite structure [ 19]. The X-ray diffraction data for ZnGeOa (ilmenite starting material) quenched from 240 kbar and 1000-1400°C of this study are listed in Table 2. It is seen that the ilmenite phase of ZnGeO 3 transforms nearly completely into an orthorhombic modification. The quenched sample has the values ao = 5.022

TABLE 3 Molar volumes (room temperature and 1 bar pressure, cm3/mole) for compounds possessing both the ilmenite and perovskite structures are compared with those for their component oxide mixtures possessing the rocksalt and futile structures Compounds

Ilmenite VI

Mixed oxide VM

Perovskite Vp

( V p - VI)/VI (%)

(Vp- VM)/VM (%)

Reference

(Feo.s Mgo.4)TiO3 *

31.28 38.73 35.41 29.14 26.32 31.28 32.01 29.67 26.93

30.5 37.14 34.39 27.91 25.28 29.88 30.91 ** 28.46 25.83

28.77 36.64 32.96 27.47 24.59 29.22 29.90 29.60 *** -

-8.0 -5.4 -6.9 -5.7 -6.6 -6.6 -6.6 -0.2 -

-5.7 -1.3 -4.2 -1.6 -2.7 -2.2 -3.3 4.0 -

[23] [24] [24] This work [ 15 ] [16] [13] This work [6]

CdSnO3 CdTiO3 MgGeO3 MgSiO3 MnGeO3 MnVO3 ZnGeO3 ZnSiO3

* It is an idealized formula for a naturaUy occurring ilmenite. Molar volume for the mixed oxides was approximately estimated. ** A molar volume of 17.69 cm3/mole for the monoclinic VO2 was used. *** The orthorhombic phase of ZnGeO3 is tentatively listed in the category of perovskite for the convenience of the table.

167 -+ 0.002, bo = 5.194 + 0.002, and c o = 7.538 -+ 0.003 .~ at room temperature and 1 bar pressure and a possible space group of Proton o r ebmn. If the latter is correct, the indices (051) and (015) listed in Table 2 should be deleted. The same starting material (the ilmenite phase of ZnGeO3) quenched from 260 kbar loading pressure comprises the orthorhombic modification of ZnGeO3, both the wurtzite and rocksalt phases of ZnO, and the rutile phase of GeO2. Hence, the sequence of phase transformations of ZnGeO3 is inferred to be ZnGeO3 (ilmenite) ~ ZnGeO3 (orthorhombic modification) ~ ZnO (rocksalt) + GeO2 (rutile) with increasing pressure. Since the zero-pressure molar volume for ZnGeO3 (ilmenite) is 29.67 cm 3 and that for tile mixture ZnO (rocksalt) plus GeO 2 (rutile) is 28.46 cm a, only 4 formula units per unit cell can be assigned to the orthorhombic phase of ZnGeO3. Hence, the zero-pressure volume for the orthorhombic phase is calculated to be 29.60 + 0.04 cm3/mole, which is 0.2% smaller than that for the ilmenite phase of ZnGeO3. Here again the relative intensities for the diffraction pattern of the orthorhombic ZnGeO 3 are unlike those for typical orthorhombic perovskite compounds mentioned in the previous section on MgGeO 3. In addition, the volume change associated with the transition ZnGeO3 (ilmenite) ~ ZnGeO 3 (orthorhombic) is too small to imply that the orthorhombic ZnGeO 3 is closely related to the perovskite structure (see Table 3).

4. Discussion For discussion molar volumes for all ilmenite compounds which are known to possess the post-ilmenite phases or phase assemblages are listed in Table 3 together with the volumes for their post-ilmenite phases and for their component oxide mixtures with the rocksalt and rutile structures. From the new data for the silicate and germanate ilmenites and the existing data for the other ilmenites of Table 3 it may be concluded that, with the exception of the zinc compounds, all known ilmenite-postilmenite phase changes are of the same type and involve direct transformation of the ilmenite to a perovskite-related structure. The ilmenite phase of ZpSiO3, on the other hand, has been found to trans-

form into its component oxide mixture, and ZnGeO 3 (ilmenite) into an orthorhombic modification and ultimately into its component oxide mixture also. The molar volumes for the orthorhombic perovskite phases listed in Table 3 are 1.3-5.7% smaller than those of their component oxide mixtures, whereas that for the orthorhombic ZnGeO 3 is 4.0% greater than its component oxide mixture. No orthorhombic modification of ZnSiO 3 with density greater than the ilmenite phase has been observed. The peculiar behavior of zinc metasilicate and metagermanate at high pressure may be related to the strong tendency of Zn 2+ to adopt lower (oxygen) coordination numbers than divalent ions of comparable size under comparable P-T conditions. This tendency is well demonstrated by the stability of IVZn2SiO4 and IVZn2GeO4 (phenakites) and IVZnO (wurtzite) at atmospheric pressure and by the instability of VIZnSiO3 and VIZnGeO3 (pyroxenes) with respect to the assemblage IVzn2(Si,Ge)O4(phenakite) + (Si,Ge)O 2 at pressures less than 35 kbar. (The Roman numerals denote the coordination of zinc by oxygen.) This strong preference of Zn 2+ for tetrahedral coordination with oxygen is to be contrasted with the octahedral coordination of all other divalent ions of Table 3 in phases such as FeO, CdO, MgO, MnO, Mg2SiO 4 (olivine), MgSiO3 (pyroxene) which are stable under ambient conditions. For the zinc compounds, pressures of 35 and 100 kbar respectively are required to stabilize VIZnSiO3 (pyroxene) and VtZnO (rocksalt) which feature zinc in octahedral coordination [19,20]. Other relevant discussion is also available in the literature [21,22]. We suspect that the orthorhombic modification of ZnGeO3 synthesized in this work may be related to the orthorhombic perovskite structure, although data for both the X-ray diffraction pattern and the zeropressure volume are not in accordance with the typical orthorhombic perovskite structure. Because of the strong tendency of the zinc atom towards lower coordi. nation with oxygen, the eight-coordinated sites in an orthorhombic perovskite structure, which the zinc atoms should occupy, may be strongly distorted to form an octahedron. This would destabilize the orthorhombic perovskite structure and might cause the volume for the orthorhombic ZnGeO 3 to be greater than that of its component oxide mixture with the rocksalt and rutile structures. The same distortion

168

in ZnSiO3 may even be so great that the orthorhombic modification found in ZnGeO3 is no longer stable so that the ilmenite phase of ZnSiOa transforms directly into its component oxide mixture. Alternatively, since the calculated molar volume for the orthorhombic ZnGeO3 phase is so close to that for the ilmenite phase, the orthorhombic phase might be an ilmeniterelated structure. Note that the ilmenite-mixed oxide transition found in ZnSiO3 is the only example so far known to us. It has been suggested that zinc silicates in a high-pressure region provide good models for study of the high pressure phase transformations in magnesium silicates [6]. This suggestion is not supported by the data listed in Table 3. In conclusion, with the exception of zinc compounds, which transform ultimately to their isochemical oxide mixture possessing the rocksalt and rutile structures, all the ilmenite phases investigated in this study are found to transform directly into the orthorhombic perovskite structure without the intervention of their isochemical oxide mixture as an intermediate phase assemblage. On the basis of the volume data of Table 3, we might further conclude that the perovskite phases for the first seven compounds do not break down to their isochemical oxide mixture with the rocksalt and rutile structures at higher pressures. The observed direct transition from the ilmenite to perovskite structures in MgSiO 3 [2] and in its alumina solid solutions [3] suggests that the oxide mixture - periclase (MgO), stishovite (SiO2), and corundum (A1203) - is not a stable mineral assemblage for the earth's lower mantle.

6 7

8

9

10

11

12 13

14

15 16

17

18

Acknowledgements The author is grateful to W.A. Bassett and I. Jackson for commenting on the manuscript.

19

20 References

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21 22

23

24

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