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Some high-pressure transformations of geophysical significance
EARTH AND PLANETARY SCIENCE LETTERS 2 (1967) 106-110. NORTH-HOLLAND PUBL. COMP., AMSTERDAM
SOME HIGH-PRESSURE T R A N S F O R M A T I O N S OF GEOPHYSICAL SIGNIFICANCE A. E. RINGWOOD and Alan MAJOR Department of Geophysics and Geochemistry, Australian National University, Canberra, A.C.T., Australia Received 16 January 1967
1. INTRODUCTION The constitution of the mantle between depths of 400 and 900 k m i s s t r o n g l y influenced by the o c c u r r e n c e of a s e r i e s of m a j o r phase t r a n s f o r m a t i o n s , the n a t u r e of which h a s been i n v e s t i gated in this l a b o r a t o r y f o r s e v e r a l y e a r s [1]. A technique which h as p r o v e d of c o n s i d e r a b l e value ha s been an i n v e s t i g a t i o n of the s t a b i l i t i e s of structural analogues (particularly germanates) of c o m m o n s i l i c a t e m i n e r a l s at high p r e s s u r e s . T h e s e have f r e q u e n t l y been found to t r a n s f o r m to new dense p h a s e s at much l o w e r p r e s s u r e s than the c o r r e s p o n d i n g i s o s t r u c t u r a l s i l i c a t e s . A c o n tinuation of this a p p r o a c h i s r e p o r t e d in the present paper. The p r e p a r a t i o n of s a m p l e s for h i g h - p r e s s u r e i n v e s t i g a t i o n is d e s c r i b e d l a t e r . The p r e p a r e d s a m p l e s have been s u b j e c te d to a range of high p r e s s u r e s v a r y i n g between 100 and 250 kb, at t e m p e r a t u r e s of a p p r o x i m a t e l y 900oc in a h i g h p r e s s u r e , high-temperature apparatus. This device c o n s i s t e d e s s e n t i a l l y of a p a i r of B r i d g m a n a n v i l s with a h e a t e r p l a c e d between t h e m [2]. A f t e r completion of a run the s a m p l e was quenched under p r e s s u r e , r e t r i e v e d , and e x a m i n e d by opt i c a l and X - r a y t ec h n i q u e s . P r e s s u r e on the s a m p l e was obtained f r o m the known p r e s s load and a r e a of a n v i l - f a c e , t o g e t h e r with an e m p i r i c a l l y d e t e r m i n e d c o r r e c t i o n f o r the p r e s s u r e g r a d i e n t between the edge and c e n t e r of the a n vil. T h i s l a t t e r c o r r e c t i o n was d e t e r m i n e d by m e a s u r i n g the load r e q u i r e d to obtain the c o e s i t e - s t i s h o v i t e t r a n s i t i o n at 900oc at the c e n t e r of the a n v i l s . In a c c o r d a n c e with the r e s u l t s of Stishov [3] and the new NaC1 p r e s s u r e s c a le of J e f f e r y et al. [4], the p r e s s u r e r e q u i r e d for this t r a n s i t i o n at 900oc was taken a s 92 kb. Lengths of m o s t r u n s v a r i e d between 3 and 5 minutes. Two p e r c e n t of wat e r was added to s a m p l e s to f a c i l i t a t e r e a c t i o n in t h e s e short t i m e s . The e x -
p e r i m e n t s w e r e of a r e c o n n a i s s a n c e type, a i m e d at d e t e r m i n i n g the n a t u r e and s u c c e s s i o n of p h a s e s stable under high p r e s s u r e , r a t h e r than at d e t e r m i n i n g the p r e c i s e p r e s s u r e s at which transformations occurred.
2. TRANSFORMATIONS IN CaGeO3, Ca (Ge0. 5 Si0.5) O 3 AND CaSiO 3.
CdGeO3,
A s a m p l e of CaGeO 3 was p r e p a r e d by heating a tablet f o r m e d f r o m the i n t i m a t e l y m i x ed oxides at 1400oc for 3 hours. The X - r a y d i f f r act i o n photograph c l o s e l y r e s e m b l e d that of w o l l a s t o n ite and t h e s e two compounds a r e p r o b a b l y i s o m o r p h o u s. Ringwood and Seabrook [5] had p r e v i o u s l y subjected this compound to p r e s s u r e s between 40 kb and 70 kb at 700oc u s i n g a s i m p l e s q u e e z e r a p p a r a t u s and found that it t r a n s f o r m e d c o m p l e t e l y to a new kind of slightly d i s t o r t e d garnet structure, Ca VIII (CaGe) vI GeIV O12 , w h e r e the s u p e r s c r i p t s denote the c o o r d i n a t i o n of the s p e c i f i e d cat i o n s with r e s p e c t to oxygen. The pseudocubic l a t t i c e p a r a m e t e r of the g a r n e t w as 12.43 ,~. In the p r e s e n t s e r i e s of e x p e r i m e n t s , it was decided to subject this compound to even h i g h e r p r e s s u r e s in the hope that CaGeO3 might u l t i m a t e l y t r a n s f o r m to the p e r o v s k i t e (CaTiO3) s t r u c t u r e . Two runs w e r e c a r r i e d out on CaGeO3 at 120 kb and 130 kb and at 900°C. In both c a s e s c o m p l e t e c o n v e r s i o n to a p e r o v s k i t e p o l y m o r p h o c c u r r e d . The CaGeO3 p e r o v s k i t e a p p e a r e d to p o s s e s s an ideal s t r u c t u r e , with no e v i d e n c e of l i n e - s p l i t t i n g . The l a t t i c e p a r a m e t e r of CaGeO 3 ( p e r o v s k i t e ) was found to be 3.723 and i t s c a l c u l a t e d density 5.17 g / c m 3 . T h i s c o m p a r e s with d e n s i t i e s of 3.98 g / c m 3 for CaGeO3 (wollastonite s t r u c t u r e ) , 4.44 g / c m 3 for CaGeO3 (garnet) and 4.81 g / c m 3 for an i s o c h e m i c a l m i x t u r e of CaO + GeO2 (rutile). The p e r o v s k i t e s t r u c t u r e is thus e x t r e m e l y c l o s e - p a c k e d .
SOME HIGH-PRESSURE TRANSFORMATIONS OF GEOPHYSICAL SIGNIFICANCE P r e v i o u s l y , Ringwood and Seabrook [5] had i n v e s t i g a t e d the stability of the compound CdGeO3. T h i s was f o r m e d by calcining the i n t i m a t e l y mixed oxides at 1200°C for 5 hours. It had a complex s t r u c t u r e , but the r e f r a c t i v e index c l e a r l y showed that the g e r m a n i u m was t e t r a h e d r a l l y coordinated. At p r e s s u r e s between 10 kb and 70 kb at 700oc, it was found that CdGeO 3 t r a n s f o r m e d to a slightly d e f o r m e d g a r n e t , a n a l ogous to CaGeO3 with a lattice p a r a m e t e r of 12.4 A. In the p r e s e n t s e r i e s of e x p e r i m e n t s we s u b jected a sample of CdGeO3 to a p r e s s u r e of 130 kb at 900°C. A l m o s t complete c o n v e r s i o n to a slightly d e f o r m e d p e r o v s k i t e s t r u c t u r e (a o = 3.70 /~, d e n s i t y = 7.65 g / c m 3) was observed. In view of the model r e l a t i o n s h i p s between g e r m a n a t e s and s i l i c a t e s [1], these r e s u l t s suggested that wollastonite (CaSiO3) might u l t i m a t e ly t r a n s f o r m to a p e r o v s k i t e s t r u c t u r e at v e r y high p r e s s u r e s and that this s t r u c t u r e might be i m p o r t a n t in the e a r t h ' s m a n t l e , as had been suggested by Ringwood [1]. Accordingly a h o m o geneous sample of Ca(Ge0. 5 Si0. 5) O 3 ( w o l l a s t o n ite s t r u c t u r e ) was p r e p a r e d by s i n t e r i n g the i n t i m a t e l y mixed oxides at 1400oc for 3 h o u r s . This sample was subjected to a p r e s s u r e of 170 kb at 900oc. It was found to c o n s i s t of 2 p h a s e s , the most abundant of which was a slightly d i s t o r t e d p e r o v s k i t e s t r u c t u r e with a p s e u d o - c u b i c lattice p a r a m e t e r of 3.696 A ( c o m p a r e d to 3.723 .~ for pure CaGeO3). It was c l e a r that a s u b s t a n tial a m o u n t of CaSiO3 indeed e n t e r s into solid solution in the p e r o v s k i t e s t r u c t u r e at v e r y high p r e s s u r e s . I n v e s t i g a t i o n s c u r r e n t l y in p r o g r e s s indicate a solid solubility of about 30o/0 of CaSiO3 in the p e r o v s k i t e s t r u c t u r e at 170 kb. T h i s r e s u l t s t r o n g l y supports the suggestion that CaSiO3 may adopt the p e r o v s k i t e s t r u c t u r e at v e r y high p r e s s u r e s . C a l c u l a t i o n s b a s e d upon solid solubility r e l a t i o n s h i p s [6] led to an e s t i m a t e for the t r a n sition p r e s s u r e of between 200 and 250 kb. Accordingly r u n s were c a r r i e d out on w o l l a s tonite and on CaSiO3 g l a s s to check this p r e d i c tion. Woltastonite a p p e a r e d stable up to about 30 kb. Above this p r e s s u r e , a new phase was obs e r v e d . However i t s m e a n r e f r a c t i v e index was well below 1.70 and a c c o r d i n g l y this phase could be no m o r e than a few p e r c e n t d e n s e r than w o l l a s t o n i t e , and its s t r u c t u r e must be b a s e d upon t e t r a h e d r a l l y c o o r d i n a t e d s i l i c o n ions. The diff r a c t i o n p a t t e r n of this phase was c h a r a c t e r i s e d y s t r o n g r e f l e c t i o n s at 3.04 .~, 2.92 ,~ and 2.61 T h i s phase a p p e a r e d to be stable at p r e s s u r e s well above 100 kb. F i n a l l y , a r u n at 250 kb was c a r r i e d out on a sample of CaSiO3 g l a s s . T r a n s f o r m a t i o n to a
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new phase was obtained, but t h i s w a s not a p e r o v s k i t e . The new phase was b i r e f r i n g e n t and had a mean r e f r a c t i v e index of about 1.745, i n dicating a d e n s i t y of about 3.5 g / c m 3, i.e. s i m i l a r to g r o s s u l a r i t e . Only a s m a l l amount of s a m ple was r e c o v e r e d and the r a t h e r complex X - r a y diffraction p i c t u r e was of poor quality. I n t e r p l a n a r "d" s p a c i n g s a r e given in table 1A. Thus it a p p e a r s that CaSiO3 does not i n v e r t d i r e c t l y to p e r o v s k i t e , but t r a n s f o r m s f i r s t into a new phase of i n t e r m e d i a t e density. The packing d e n s i t y of this phase i s s i m i l a r to CaGeO3 g a r net, but its s t r u c t u r e does not r e s e m b l e a g a r net. F r o m the o b s e r v e d solid solubility of CaSiO3 in CaGeO3, u l t i m a t e t r a n s f o r m a t i o n of CaSiO3 i s expected at even higher p r e s s u r e s . A p o s s i b i l i t y which cannot be e n t i r e l y d i s m i s s e d is that the new phase i s a r e t r o g r a d e t r a n s f o r m a tion product f o r m e d f r o m a d e n s e r phase ( p e r haps p e r o v s k i t e ? ) during r e l e a s e of p r e s s u r e . The new phase p o s s e s s e s a p e c u l i a r u n d u l a t o r y extinction unlike most of the stable h i g h - p r e s sure p h a s e s which we have p r e v i o u s l y s y n t h e sized. C u r r e n t i n v e s t i g a t i o n s on e q u i l i b r i a in the s y s t e m CaGeO3 -CaSiO3 should settle this point. The p e r o v s k i t e s t r u c t u r e i s of i n t e r e s t b e cause of its e x t r e m e l y high density. P e r o v s k i t e s a r e u s u a l l y between 5 and 10% d e n s e r than the mean density of the oxides, AO ( r o c k - s a l t ) and BO2 ( r u t i l e s t r u c t u r e ) f r o m which they a r e f o r m e d . Thus, CaGeO3 i s about 7.6% d e n s e r than a m i x t u r e of CaO and GeO2 (rutile). S i m i l a r l y , CaSiO3 (perovskite) is likely to be d e n s e r by a c o m p a r a b l e amount than the i s o c h e m i c a l m i x t u r e of (CaO + stishovite). The p o s s i b i l i t y that MgSiO3 might t r a n s f o r m in the mantle f r o m an i l m e n i t e s t r u c t u r e (usually slightly l e s s ' a e n s e than the i s o c h e m i c a l m i x t u r e of constituent oxides) to a d e n s e r p e r o v s k i t e s t r u c t u r e should also be m e n tioned. F u r t h e r work i s c l e a r l y r e q u i r e d on this i m p o r t a n t c l a s s of compounds.
3. STABILITY OF JADEITE In the upper m a n t l e , sodium o c c u r s p r i n c i p a l ly a s the jadeite component (NaA1Si206) of c l i n o pyroxene. At g r e a t e r depths in the t r a n s i t i o n zone, p y r o x e n e s and o l i v i n e s a r e b e l i e v e d to t r a n s f o r m to spinel, g a r n e t , i l m e n i t e and p e r o v s k i t e p h a s e s [1]. It does not appear likely that sodium will be able to e n t e r these p h a s e s r e a d i l y and a c c o r d i n g l y it must occur in some other m i n e r a l . An i n v e s t i g a t i o n of the s t a b i l i t y of jadeite at high p r e s s u r e t h e r e f o r e a p p e a r e d worthwhile.
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A.E. RINGWOOD and A. MAJOR
As in other c a s e s , the i n v e s t i g a t i o n was c o m m e n c e d on the g e r m a n i u m analogue NaA1Ge20 6. A g l a s s of this c o m p o s i t i o n was p r e p a r e d by m e l t i n g the i n t i m a t e l y m ix e d components ( p r e v i o u s l y s i n t e r e d at 800°C) in a c l o s e d platinum tube, and quenching. It had p r e v i o u s l y been shown by Dachille and Roy [7] that a g l a s s of this c o m p o s i t i o n c r y s t a l l i z e d to a j a d e i te s t r u c t u r e at 12 kb and 600oc. In the p r e s e n t i n v e s t i g a t i o n , a s a m p l e of the NaA1Ge206 g l a s s was run at a p r e s s u r e of 110 kb, 900oc. T r a n s f o r m a t i o n into a dense m i x t u r e of p h a s e s (not j a d e i t e ) w a s o b s e r v e d . C o m p a r i s o n of X - r a y d if f r a c t io n photog r a p h s showed that one of t h e s e p h a s e s was GeO 2 (rutile). T h i s s u g g e s te d that the r e a c t i o n NaA1Ge20 6 ~
NaA1GeO4 (new phase)
+ GeO2 (rutile)
had o c c u r r e d . A c c o r d i n g l y a g l a s s of NaA1GeO4 c o m p o s i t i o n was p r e p a r e d a s b e f o r e , and run at 120 kb, 900°C. Complete conversion was a c h i e v e d into a highly b i r e f r i n g e n t , finely c r y s talline phase with a mean r e f r a c t i v e index of 1.815. The X - r a y d i f f r a c t io n p a t t e r n w a s i d e n t i cal with the unidentified l i n e s obtained in the p r e v i o u s run on NaA1Ge206, showing that this compound indeed b r e a k s down a c c o r d i n g to the equation above. The high r e f r a c t i v e index i m p l i e s that the new phase NaA1GeO4 has a r e l a t i v e l y high density, and s t r o n g l y s u g g e s t s that the g e r m a n i u m ions a r e o c t a h e d r a l l y c o o r d i nated. I n t e r p l a n a r d s p a c i n g s of this phase a r e given in table l B . T h e s e r e s u l t s suggest that c o m m o n j a d e i te might b r e a k down at high p r e s s u r e s a n a l o g o u s l y to the c o r r e s p o n d i n g g e r m a n a t e NaA1Si206 (jadeite)
~
NaA1SiO4 (new phase) + SiO2 (stishovite)
To t e s t this, s a m p l e s of j a d e i t e g l a s s w e r e run at p r e s s u r e s up to 180 kb, 900oc. The p r o d u c t s c o n s i s t e d of pure j a d e i t e . A c c o r d i n g l y j a d e ite i s stable at l e a s t to this p r e s s u r e . A s a m p l e of c r y s t a l l i n e NaA1SiO 4 was next p r e p a r e d by s i n t e r i n g the i n t i m a t e l y m ix e d c o m p o n e n t s just below the solidus f o r 3 h o u r s . T h i s s a m p l e was run at p r e s s u r e s of 120 kb, 170 kb and 250 kb (at 900°C). In e a c h c a s e , the s a m p l e t r a n s f o r m e d dominantly into j a d e i t e . X - r a y d i f f r a c tion photographs i n d i c a t e d the p r e s e n c e of new r e f l e c t i o n s at 5.31 A, 2.11 /~ and 1.36 ~. T h e s e w e r e p r e s e n t in e v e r y run, and a l s o in r u n s on a second s a m p l e of NaA1SiO4, independently p r e pared. This might s u g g e s t that nepheline (NaAISiO4) t r a n s f o r m s at high p r e s s u r e into a m i x t u r e of j a d e i t e plus a new phase such a s
Table 1 Interplanar d spacings for some new high-pressure )hases. A
B
CaSiO 3, 250 kb, 900oc d (A) z
NaA1GeO4, 110 kb, 900°C d (_£) z
3.02 2.91 2.78* 2.73* 2.61 2.53 2.35 2.19
4 2 2 1 <1 2 1 2
2.05 1.98 1.77 1.60
I0 1 3 1
5.19 4.45 3.36 2.83 2.73 2.597 2.562 2.390 2.190 2.160 2.125 2.031 2.003 1.900 1.868 1.756 1.741
1 10 1 <1 <1 6 10 10 1 <1 3 1 1 <1 2 5 4
I = Relative intensities (visual estimates) * = Diffuse due to overlap A = This was a poor-quality photograph owing to minute amount of sample which was available.
NaA102. H o w ev er the a p p e a r a n c e of only 3 new l i n e s and t h e i r c o m p a r a t i v e w e a k n e s s r e n d e r s this explanation s o m e w h a t doubtful. One would e x p e c t many m o r e new s t r o n g l i n e s to a p p e a r if a new phase had been f o r m e d in the r e q u i r e d abundance. A l t e r n a t i v e l y , it i s p o s s i b l e that the nepheline has t r a n s f o r m e d c o m p l e t e l y into a d e fect solid solution between NaA1Si206 and NaA102, possessing e s s e n t i a l l y the j a d e i t e s t r u c t u r e . T h i s might account f o r the few e x t r a l i n e s and a l s o f o r the fact that the r e l a t i v e i n t e n s i t i e s of the 2 . 1 7 ~ , 1.66 ,~, 1.61,~ and 1.28 ,~ r e f l e c t i o n s a r e s t r o n g e r in the j a d e i t e f o r m e d f r o m NaA1SiO4 w h e r e a s the 2.07 r e f l e c t i o n i s w e a k e r , in c o m p a r i s o n to the d i f f r a c t i o n p a t t e r n of s t o i chiometric jadeite. R e g a r d l e s s of which explanation is c o r r e c t , .it i s c l e a r that j a d e i t e i s stable at e x t r e m e l y high p r e s s u r e s , although f r o m analogy with the g e r m an at e, it i s e x p e c t e d that it will u l t i m a t e l y t r a n s f o r m into a m i x t u r e of NaA1SiO4 (new phase) + SiO2 (stishovite). Some r u n s will be c a r r i e d out on NaAl(GeSi)O4 solid solutions to e x p l o r e this h y p o t h e s i s f u r t h e r .
SOME HIGH-PRESSURE TRANSFORMATIONSOF GEOPHYSICAL SIGNIFICANCE 4. STABILITY OF Mg2GeO4 SPINEL 4MgO. GeO2 AT HIGH PRESSURE
AND
Ringwood and Seabrook [5] had p r e v i o u s l y i n vestigated the stability of Mg2GeO 4 (spinel) by r e a c t i n g MgO with h y d r o u s g e r m a n i a (in the 2/1 m o l e c u l a r proportion) in a s q u e e z e r a p p a r a t u s at 600oc. At p r e s s u r e s up to 90 kb, complete r e action to Mg2GeO4 spinel was observed. Howe v e r , at p r e s s u r e s above 90 kb, the r e a c t i o n p r o d u c t s were found to be MgGeO 3 (ilmenite) plus a new phase which was b e l i e v e d to have the composition 4MgO. GeO2 or 5MgO. GeO2. A phase 4MgO. GeO 2 had p r e v i o u s l y been s y n t h e sized by Robbins and Levin [8]; however this phase was not i d e n t i c a l with the new phase found by Ringwood and Seabrook. These r e s u l t s were taken to i m p l y that Mg2GeO4 (spinel) was u n s t a b l e at high p r e s s u r e and t r a n s f o r m e d into a d e n s e r m i x t u r e of MgGeO3 i l m e n i t e + 4 (or 5) MgO. GeO2 (new phase). T h i s i n t e r p r e t a t i o n has been checked by subjecting the phase 4MgO. GeO2 to a p r e s s u r e of 170 kb u n d e r dry conditions. The 4 MgO. GeO2 was p r e p a r e d a c c o r d i n g to the p r o c e d u r e of Robb i n s and Levin [8]. It was o b s e r v e d that the 4MgO. GeO2 t r a n s f o r m e d completely into a mixt u r e of Mg2GeO4 ( s p i n e l ) + MgO. This i n d i c a t e s that the new phase f o r m e d by Ringwood and Seabrook above 90 kb was p r o b a b l y hydrated, and that Mg2GeO 4 spinel is stable u n d e r dry condit i o n s at l e a s t to 170 kb. Accordingly the p r e vious suggestion [1, 5] that Mg2SiO 4 spinel might t r a n s f o r m at high p r e s s u r e s into a m i x t u r e of MgSiO3 (ilmenite) + x MgO. SiO 2 (x > 2) must be c o n s i d e r e d speculative and without e x p e r i m e n t a l support at p r e s e n t . C u r r e n t i n v e s t i g a t i o n s on the s y s t e m M g O - S I O 2 - H 2 0 where MgO/SiO2 >/ 2 have d i s c l o s e d the e x i s t e n c e of s e v e r a l new p h a s e s , at l e a s t one of which has a d e n s i t y b e tween 3.5 and 3.8 g / c m 3. It i s not yet c l e a r whether this phase i s hydrated.
5. THE HIGH-PRESSURE TRANSFORMATIONS OF MnSiO 3 High p r e s s u r e r u n s were c a r r i e d out on s a m ples of MnSiO3 g l a s s and on synthetic and n a t u r a l rhodonite. The g l a s s was f o r m e d by m e l t i n g an i n t i m a t e m i x t u r e of Mn30 4 and s i l i c a in the c o r r e c t p r o p o r t i o n s in a p l a t i n u m tube at 1500°C followed by quenching. The rhodonite sample was f o r m e d by c a l c i n i n g a tablet of the i n t i m a t e ly mixed oxides (Mn304 and S i O 2 ) a t about 1200 °C for 4 h o u r s . After this t r e a t m e n t , it was
109
found to be inhomogeneous, accordingly it was crushed, reformed, and heated at 1200oc for another 8 hours, yieldinghomogeneous rhodonite, as shownby opticaland X-ray examination. The refractive index of the synthetic rhodonite indicated that it was essentially pure and contained a negligible amount of Mn2Mn403 in solid solution [9]. Previous (unpublished) results by Ringwood and Seabrook on the MnSiO3 glass and on natural rhodonite using a squeezer apparatus revealed that at 700oc and at pressures between 30 and 60 kb, rhodonite transformed to a new phase possessing a complex X-ray diffraction pattern, characterised by strong doublets at 2.21, 2.23 ~; 3.00, 3.05 ~; and a strong diffuse reflection (probably an overlapping triplet at 2.67 ~). The refractive indices of this phase were only slightly higher than those of rhodonite indicating that it is at the most, only a few percent denser than rhodonite. In the present investigation at pressures between 60 kb and 90 kb, the MnSiO3 glass and also a sample of natural rhodonite (containing 14 percent FeO) were observed to transform to a pyroxene structure. This has not yet been indexed, but is very probably an orthopyroxene. Finally, at pressures between 120 kb and 170 kb, MnSiO3 glass was observed to transform completely to a garnet structure possessing a lattice parameter of 11.765 ± 0.010 ~. This garnet is clearly analogous to the CaGeO3 and CdGeO3 garnets previously mentioned, and evidently possesses a structural formula MnVIII (MnSi)vl SiIVO 12 where the superscripts denote the coordination of the indicated cations with respect to oxygen. The calculated density of the garnet is 4.27 g/cm3 compared to 3.71 g/cm3 for rhodonite. Because of the possibility that a small amount of Mn3+or Mn4+ions might have been present in the glass, and thereby have influencedthe stability of the garnet, attempts were made to transform pure synthetic rhodonite and natural rhodonite into the garnet structure. Several runs were made at pressures between 120 and 170 kb and in each case substantial to extensive transformation to garnet was observed. The best result was ~ run on pure synthetic MnSiO3at 170 kb for 15 minutes which yielded about 70 percent of garnet and 30 percent of pyroxene. Complete transformation w~s apparently prevented by nucleation difficulties. Nevertheless, this result is important since it implies that Mn3+ and Mn4+ ions are not essential components of the garnet structure, and also that the garnet is not a product of metastable crystallization of glass [10].
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We have p r e v i o u s l y d e s c r i b e d [10] the s y n t h e s i s f r o m g l a s s e s at high p r e s s u r e s of g a r n e t s with c o m p o s i t i o n s MgSiO3, 10%A1203, CaSiO3.10 % A120 3 and CaMgSi206.10°/0 A120 3. T h e s e c o m p o s i t i o n s i m p l y that a substantial p r o p o r t i o n of s i l i c o n ions i s in o c t a h e d r a l coordination. It w a s b e l i e v e d that t h e s e g a r n e t s w e r e t h e r m o d y n a m i c a l l y stable in t h e i r s y n t h e s i s f i e l d s , but t h i s could not be c o n c l u s i v e l y e s t a b l i s h e d b e cause the g a r n e t s w e r e c r y s t a l l i z e d f r o m g l a s s e s , and the p o s s i b i l i t y of m e t a s t a b l e c r y s t a l l i z a tion could not be e l i m i n a t e d . A t t e m p t s to c o n v e r t c r y s t a l l i z e d a l u m i n o u s p y r o x e n e s c o m p l e t e l y to garnets were unsuccessful, presumably because of k i n e t i c d i ff i cu l t i es . The p r e s e n t r e s u l t s d e m o n s t r a t i n g the stable s y n t h e s i s of an MnSiO 3 g a r n e t , t o g e t h e r with the p r e v i o u s stable s y n t h e s i s of CaGeO3 and CdGeO3 g a r n e t s , s t r o n g l y support the view that the other g a r n e t s w e r e stable in t h e i r s y n t h e s i s f ie ld s . It is probable that t h e s e g a r n e t s , c h a r a c t e r i s e d by p a r t i a l o c t a h e d r a l c o o r d i n a t i o n of s i li c o n ions, play a m a j o r r o l e in the t r a n s i t i o n zone in the e a r t h ' s mantle between depths of 350 and 500 km. A d e t a i l e d e x a m i n a t i o n of this r o l e will be made when c u r r e n t e x p e r i m e n t s on th e s e g a r n e t s a r e c o m pleted.
ACKNOWLEDGEMENT The authors are grateful to Dr. D.H. Green for critical reading of the manuscript.
REFERENCES [1] A.E.Ringwood, in: Advances in Earth Science, ed. P.MoHurley (M.I.T. Press, 1966} p. 357. [2] A.E.Ringwood and A. Major, Earth Planet. Sci. Letters 1 (1966) 241. [3] S.M.Stishov, Dokl.Akad. Nauk SSR 148 (1963} 1186. [4] R. N. Jeffery, J.D.Barnett, H.B.Vanfleet and H.T. Hall, J. Appl. Phys. 37 (1966) 3172. [5] A.E.Ringwood and M.Seabrook, J. Geophys. Res. 68 {1963) 4601. [6] A.E. Ringwood, Geochim. Cosmochim. Acta 26 (1962) 457. [7] F.Dachille and R.Roy, in: Modern very high pressure techniques, ed. R.H.Wentorf (Butterworth and Co., London, 1962) Chapter 9. [8] L.R.Robbins and E.Levin, Am. J. Sci. 257 (1959) 63. [9] A.Muan, Am. J. Sci. 257 (1959} 297. [10] A.E.Ringwood and A.Major, Earth Planet. Sci. Letters 1 (1966) 351.
SOME HIGH-PRESSURE T R A N S F O R M A T I O N S OF GEOPHYSICAL SIGNIFICANCE A. E. RINGWOOD and Alan MAJOR Department of Geophysics and Geochemistry, Australian National University, Canberra, A.C.T., Australia Received 16 January 1967
1. INTRODUCTION The constitution of the mantle between depths of 400 and 900 k m i s s t r o n g l y influenced by the o c c u r r e n c e of a s e r i e s of m a j o r phase t r a n s f o r m a t i o n s , the n a t u r e of which h a s been i n v e s t i gated in this l a b o r a t o r y f o r s e v e r a l y e a r s [1]. A technique which h as p r o v e d of c o n s i d e r a b l e value ha s been an i n v e s t i g a t i o n of the s t a b i l i t i e s of structural analogues (particularly germanates) of c o m m o n s i l i c a t e m i n e r a l s at high p r e s s u r e s . T h e s e have f r e q u e n t l y been found to t r a n s f o r m to new dense p h a s e s at much l o w e r p r e s s u r e s than the c o r r e s p o n d i n g i s o s t r u c t u r a l s i l i c a t e s . A c o n tinuation of this a p p r o a c h i s r e p o r t e d in the present paper. The p r e p a r a t i o n of s a m p l e s for h i g h - p r e s s u r e i n v e s t i g a t i o n is d e s c r i b e d l a t e r . The p r e p a r e d s a m p l e s have been s u b j e c te d to a range of high p r e s s u r e s v a r y i n g between 100 and 250 kb, at t e m p e r a t u r e s of a p p r o x i m a t e l y 900oc in a h i g h p r e s s u r e , high-temperature apparatus. This device c o n s i s t e d e s s e n t i a l l y of a p a i r of B r i d g m a n a n v i l s with a h e a t e r p l a c e d between t h e m [2]. A f t e r completion of a run the s a m p l e was quenched under p r e s s u r e , r e t r i e v e d , and e x a m i n e d by opt i c a l and X - r a y t ec h n i q u e s . P r e s s u r e on the s a m p l e was obtained f r o m the known p r e s s load and a r e a of a n v i l - f a c e , t o g e t h e r with an e m p i r i c a l l y d e t e r m i n e d c o r r e c t i o n f o r the p r e s s u r e g r a d i e n t between the edge and c e n t e r of the a n vil. T h i s l a t t e r c o r r e c t i o n was d e t e r m i n e d by m e a s u r i n g the load r e q u i r e d to obtain the c o e s i t e - s t i s h o v i t e t r a n s i t i o n at 900oc at the c e n t e r of the a n v i l s . In a c c o r d a n c e with the r e s u l t s of Stishov [3] and the new NaC1 p r e s s u r e s c a le of J e f f e r y et al. [4], the p r e s s u r e r e q u i r e d for this t r a n s i t i o n at 900oc was taken a s 92 kb. Lengths of m o s t r u n s v a r i e d between 3 and 5 minutes. Two p e r c e n t of wat e r was added to s a m p l e s to f a c i l i t a t e r e a c t i o n in t h e s e short t i m e s . The e x -
p e r i m e n t s w e r e of a r e c o n n a i s s a n c e type, a i m e d at d e t e r m i n i n g the n a t u r e and s u c c e s s i o n of p h a s e s stable under high p r e s s u r e , r a t h e r than at d e t e r m i n i n g the p r e c i s e p r e s s u r e s at which transformations occurred.
2. TRANSFORMATIONS IN CaGeO3, Ca (Ge0. 5 Si0.5) O 3 AND CaSiO 3.
CdGeO3,
A s a m p l e of CaGeO 3 was p r e p a r e d by heating a tablet f o r m e d f r o m the i n t i m a t e l y m i x ed oxides at 1400oc for 3 hours. The X - r a y d i f f r act i o n photograph c l o s e l y r e s e m b l e d that of w o l l a s t o n ite and t h e s e two compounds a r e p r o b a b l y i s o m o r p h o u s. Ringwood and Seabrook [5] had p r e v i o u s l y subjected this compound to p r e s s u r e s between 40 kb and 70 kb at 700oc u s i n g a s i m p l e s q u e e z e r a p p a r a t u s and found that it t r a n s f o r m e d c o m p l e t e l y to a new kind of slightly d i s t o r t e d garnet structure, Ca VIII (CaGe) vI GeIV O12 , w h e r e the s u p e r s c r i p t s denote the c o o r d i n a t i o n of the s p e c i f i e d cat i o n s with r e s p e c t to oxygen. The pseudocubic l a t t i c e p a r a m e t e r of the g a r n e t w as 12.43 ,~. In the p r e s e n t s e r i e s of e x p e r i m e n t s , it was decided to subject this compound to even h i g h e r p r e s s u r e s in the hope that CaGeO3 might u l t i m a t e l y t r a n s f o r m to the p e r o v s k i t e (CaTiO3) s t r u c t u r e . Two runs w e r e c a r r i e d out on CaGeO3 at 120 kb and 130 kb and at 900°C. In both c a s e s c o m p l e t e c o n v e r s i o n to a p e r o v s k i t e p o l y m o r p h o c c u r r e d . The CaGeO3 p e r o v s k i t e a p p e a r e d to p o s s e s s an ideal s t r u c t u r e , with no e v i d e n c e of l i n e - s p l i t t i n g . The l a t t i c e p a r a m e t e r of CaGeO 3 ( p e r o v s k i t e ) was found to be 3.723 and i t s c a l c u l a t e d density 5.17 g / c m 3 . T h i s c o m p a r e s with d e n s i t i e s of 3.98 g / c m 3 for CaGeO3 (wollastonite s t r u c t u r e ) , 4.44 g / c m 3 for CaGeO3 (garnet) and 4.81 g / c m 3 for an i s o c h e m i c a l m i x t u r e of CaO + GeO2 (rutile). The p e r o v s k i t e s t r u c t u r e is thus e x t r e m e l y c l o s e - p a c k e d .
SOME HIGH-PRESSURE TRANSFORMATIONS OF GEOPHYSICAL SIGNIFICANCE P r e v i o u s l y , Ringwood and Seabrook [5] had i n v e s t i g a t e d the stability of the compound CdGeO3. T h i s was f o r m e d by calcining the i n t i m a t e l y mixed oxides at 1200°C for 5 hours. It had a complex s t r u c t u r e , but the r e f r a c t i v e index c l e a r l y showed that the g e r m a n i u m was t e t r a h e d r a l l y coordinated. At p r e s s u r e s between 10 kb and 70 kb at 700oc, it was found that CdGeO 3 t r a n s f o r m e d to a slightly d e f o r m e d g a r n e t , a n a l ogous to CaGeO3 with a lattice p a r a m e t e r of 12.4 A. In the p r e s e n t s e r i e s of e x p e r i m e n t s we s u b jected a sample of CdGeO3 to a p r e s s u r e of 130 kb at 900°C. A l m o s t complete c o n v e r s i o n to a slightly d e f o r m e d p e r o v s k i t e s t r u c t u r e (a o = 3.70 /~, d e n s i t y = 7.65 g / c m 3) was observed. In view of the model r e l a t i o n s h i p s between g e r m a n a t e s and s i l i c a t e s [1], these r e s u l t s suggested that wollastonite (CaSiO3) might u l t i m a t e ly t r a n s f o r m to a p e r o v s k i t e s t r u c t u r e at v e r y high p r e s s u r e s and that this s t r u c t u r e might be i m p o r t a n t in the e a r t h ' s m a n t l e , as had been suggested by Ringwood [1]. Accordingly a h o m o geneous sample of Ca(Ge0. 5 Si0. 5) O 3 ( w o l l a s t o n ite s t r u c t u r e ) was p r e p a r e d by s i n t e r i n g the i n t i m a t e l y mixed oxides at 1400oc for 3 h o u r s . This sample was subjected to a p r e s s u r e of 170 kb at 900oc. It was found to c o n s i s t of 2 p h a s e s , the most abundant of which was a slightly d i s t o r t e d p e r o v s k i t e s t r u c t u r e with a p s e u d o - c u b i c lattice p a r a m e t e r of 3.696 A ( c o m p a r e d to 3.723 .~ for pure CaGeO3). It was c l e a r that a s u b s t a n tial a m o u n t of CaSiO3 indeed e n t e r s into solid solution in the p e r o v s k i t e s t r u c t u r e at v e r y high p r e s s u r e s . I n v e s t i g a t i o n s c u r r e n t l y in p r o g r e s s indicate a solid solubility of about 30o/0 of CaSiO3 in the p e r o v s k i t e s t r u c t u r e at 170 kb. T h i s r e s u l t s t r o n g l y supports the suggestion that CaSiO3 may adopt the p e r o v s k i t e s t r u c t u r e at v e r y high p r e s s u r e s . C a l c u l a t i o n s b a s e d upon solid solubility r e l a t i o n s h i p s [6] led to an e s t i m a t e for the t r a n sition p r e s s u r e of between 200 and 250 kb. Accordingly r u n s were c a r r i e d out on w o l l a s tonite and on CaSiO3 g l a s s to check this p r e d i c tion. Woltastonite a p p e a r e d stable up to about 30 kb. Above this p r e s s u r e , a new phase was obs e r v e d . However i t s m e a n r e f r a c t i v e index was well below 1.70 and a c c o r d i n g l y this phase could be no m o r e than a few p e r c e n t d e n s e r than w o l l a s t o n i t e , and its s t r u c t u r e must be b a s e d upon t e t r a h e d r a l l y c o o r d i n a t e d s i l i c o n ions. The diff r a c t i o n p a t t e r n of this phase was c h a r a c t e r i s e d y s t r o n g r e f l e c t i o n s at 3.04 .~, 2.92 ,~ and 2.61 T h i s phase a p p e a r e d to be stable at p r e s s u r e s well above 100 kb. F i n a l l y , a r u n at 250 kb was c a r r i e d out on a sample of CaSiO3 g l a s s . T r a n s f o r m a t i o n to a
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new phase was obtained, but t h i s w a s not a p e r o v s k i t e . The new phase was b i r e f r i n g e n t and had a mean r e f r a c t i v e index of about 1.745, i n dicating a d e n s i t y of about 3.5 g / c m 3, i.e. s i m i l a r to g r o s s u l a r i t e . Only a s m a l l amount of s a m ple was r e c o v e r e d and the r a t h e r complex X - r a y diffraction p i c t u r e was of poor quality. I n t e r p l a n a r "d" s p a c i n g s a r e given in table 1A. Thus it a p p e a r s that CaSiO3 does not i n v e r t d i r e c t l y to p e r o v s k i t e , but t r a n s f o r m s f i r s t into a new phase of i n t e r m e d i a t e density. The packing d e n s i t y of this phase i s s i m i l a r to CaGeO3 g a r net, but its s t r u c t u r e does not r e s e m b l e a g a r net. F r o m the o b s e r v e d solid solubility of CaSiO3 in CaGeO3, u l t i m a t e t r a n s f o r m a t i o n of CaSiO3 i s expected at even higher p r e s s u r e s . A p o s s i b i l i t y which cannot be e n t i r e l y d i s m i s s e d is that the new phase i s a r e t r o g r a d e t r a n s f o r m a tion product f o r m e d f r o m a d e n s e r phase ( p e r haps p e r o v s k i t e ? ) during r e l e a s e of p r e s s u r e . The new phase p o s s e s s e s a p e c u l i a r u n d u l a t o r y extinction unlike most of the stable h i g h - p r e s sure p h a s e s which we have p r e v i o u s l y s y n t h e sized. C u r r e n t i n v e s t i g a t i o n s on e q u i l i b r i a in the s y s t e m CaGeO3 -CaSiO3 should settle this point. The p e r o v s k i t e s t r u c t u r e i s of i n t e r e s t b e cause of its e x t r e m e l y high density. P e r o v s k i t e s a r e u s u a l l y between 5 and 10% d e n s e r than the mean density of the oxides, AO ( r o c k - s a l t ) and BO2 ( r u t i l e s t r u c t u r e ) f r o m which they a r e f o r m e d . Thus, CaGeO3 i s about 7.6% d e n s e r than a m i x t u r e of CaO and GeO2 (rutile). S i m i l a r l y , CaSiO3 (perovskite) is likely to be d e n s e r by a c o m p a r a b l e amount than the i s o c h e m i c a l m i x t u r e of (CaO + stishovite). The p o s s i b i l i t y that MgSiO3 might t r a n s f o r m in the mantle f r o m an i l m e n i t e s t r u c t u r e (usually slightly l e s s ' a e n s e than the i s o c h e m i c a l m i x t u r e of constituent oxides) to a d e n s e r p e r o v s k i t e s t r u c t u r e should also be m e n tioned. F u r t h e r work i s c l e a r l y r e q u i r e d on this i m p o r t a n t c l a s s of compounds.
3. STABILITY OF JADEITE In the upper m a n t l e , sodium o c c u r s p r i n c i p a l ly a s the jadeite component (NaA1Si206) of c l i n o pyroxene. At g r e a t e r depths in the t r a n s i t i o n zone, p y r o x e n e s and o l i v i n e s a r e b e l i e v e d to t r a n s f o r m to spinel, g a r n e t , i l m e n i t e and p e r o v s k i t e p h a s e s [1]. It does not appear likely that sodium will be able to e n t e r these p h a s e s r e a d i l y and a c c o r d i n g l y it must occur in some other m i n e r a l . An i n v e s t i g a t i o n of the s t a b i l i t y of jadeite at high p r e s s u r e t h e r e f o r e a p p e a r e d worthwhile.
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As in other c a s e s , the i n v e s t i g a t i o n was c o m m e n c e d on the g e r m a n i u m analogue NaA1Ge20 6. A g l a s s of this c o m p o s i t i o n was p r e p a r e d by m e l t i n g the i n t i m a t e l y m ix e d components ( p r e v i o u s l y s i n t e r e d at 800°C) in a c l o s e d platinum tube, and quenching. It had p r e v i o u s l y been shown by Dachille and Roy [7] that a g l a s s of this c o m p o s i t i o n c r y s t a l l i z e d to a j a d e i te s t r u c t u r e at 12 kb and 600oc. In the p r e s e n t i n v e s t i g a t i o n , a s a m p l e of the NaA1Ge206 g l a s s was run at a p r e s s u r e of 110 kb, 900oc. T r a n s f o r m a t i o n into a dense m i x t u r e of p h a s e s (not j a d e i t e ) w a s o b s e r v e d . C o m p a r i s o n of X - r a y d if f r a c t io n photog r a p h s showed that one of t h e s e p h a s e s was GeO 2 (rutile). T h i s s u g g e s te d that the r e a c t i o n NaA1Ge20 6 ~
NaA1GeO4 (new phase)
+ GeO2 (rutile)
had o c c u r r e d . A c c o r d i n g l y a g l a s s of NaA1GeO4 c o m p o s i t i o n was p r e p a r e d a s b e f o r e , and run at 120 kb, 900°C. Complete conversion was a c h i e v e d into a highly b i r e f r i n g e n t , finely c r y s talline phase with a mean r e f r a c t i v e index of 1.815. The X - r a y d i f f r a c t io n p a t t e r n w a s i d e n t i cal with the unidentified l i n e s obtained in the p r e v i o u s run on NaA1Ge206, showing that this compound indeed b r e a k s down a c c o r d i n g to the equation above. The high r e f r a c t i v e index i m p l i e s that the new phase NaA1GeO4 has a r e l a t i v e l y high density, and s t r o n g l y s u g g e s t s that the g e r m a n i u m ions a r e o c t a h e d r a l l y c o o r d i nated. I n t e r p l a n a r d s p a c i n g s of this phase a r e given in table l B . T h e s e r e s u l t s suggest that c o m m o n j a d e i te might b r e a k down at high p r e s s u r e s a n a l o g o u s l y to the c o r r e s p o n d i n g g e r m a n a t e NaA1Si206 (jadeite)
~
NaA1SiO4 (new phase) + SiO2 (stishovite)
To t e s t this, s a m p l e s of j a d e i t e g l a s s w e r e run at p r e s s u r e s up to 180 kb, 900oc. The p r o d u c t s c o n s i s t e d of pure j a d e i t e . A c c o r d i n g l y j a d e ite i s stable at l e a s t to this p r e s s u r e . A s a m p l e of c r y s t a l l i n e NaA1SiO 4 was next p r e p a r e d by s i n t e r i n g the i n t i m a t e l y m ix e d c o m p o n e n t s just below the solidus f o r 3 h o u r s . T h i s s a m p l e was run at p r e s s u r e s of 120 kb, 170 kb and 250 kb (at 900°C). In e a c h c a s e , the s a m p l e t r a n s f o r m e d dominantly into j a d e i t e . X - r a y d i f f r a c tion photographs i n d i c a t e d the p r e s e n c e of new r e f l e c t i o n s at 5.31 A, 2.11 /~ and 1.36 ~. T h e s e w e r e p r e s e n t in e v e r y run, and a l s o in r u n s on a second s a m p l e of NaA1SiO4, independently p r e pared. This might s u g g e s t that nepheline (NaAISiO4) t r a n s f o r m s at high p r e s s u r e into a m i x t u r e of j a d e i t e plus a new phase such a s
Table 1 Interplanar d spacings for some new high-pressure )hases. A
B
CaSiO 3, 250 kb, 900oc d (A) z
NaA1GeO4, 110 kb, 900°C d (_£) z
3.02 2.91 2.78* 2.73* 2.61 2.53 2.35 2.19
4 2 2 1 <1 2 1 2
2.05 1.98 1.77 1.60
I0 1 3 1
5.19 4.45 3.36 2.83 2.73 2.597 2.562 2.390 2.190 2.160 2.125 2.031 2.003 1.900 1.868 1.756 1.741
1 10 1 <1 <1 6 10 10 1 <1 3 1 1 <1 2 5 4
I = Relative intensities (visual estimates) * = Diffuse due to overlap A = This was a poor-quality photograph owing to minute amount of sample which was available.
NaA102. H o w ev er the a p p e a r a n c e of only 3 new l i n e s and t h e i r c o m p a r a t i v e w e a k n e s s r e n d e r s this explanation s o m e w h a t doubtful. One would e x p e c t many m o r e new s t r o n g l i n e s to a p p e a r if a new phase had been f o r m e d in the r e q u i r e d abundance. A l t e r n a t i v e l y , it i s p o s s i b l e that the nepheline has t r a n s f o r m e d c o m p l e t e l y into a d e fect solid solution between NaA1Si206 and NaA102, possessing e s s e n t i a l l y the j a d e i t e s t r u c t u r e . T h i s might account f o r the few e x t r a l i n e s and a l s o f o r the fact that the r e l a t i v e i n t e n s i t i e s of the 2 . 1 7 ~ , 1.66 ,~, 1.61,~ and 1.28 ,~ r e f l e c t i o n s a r e s t r o n g e r in the j a d e i t e f o r m e d f r o m NaA1SiO4 w h e r e a s the 2.07 r e f l e c t i o n i s w e a k e r , in c o m p a r i s o n to the d i f f r a c t i o n p a t t e r n of s t o i chiometric jadeite. R e g a r d l e s s of which explanation is c o r r e c t , .it i s c l e a r that j a d e i t e i s stable at e x t r e m e l y high p r e s s u r e s , although f r o m analogy with the g e r m an at e, it i s e x p e c t e d that it will u l t i m a t e l y t r a n s f o r m into a m i x t u r e of NaA1SiO4 (new phase) + SiO2 (stishovite). Some r u n s will be c a r r i e d out on NaAl(GeSi)O4 solid solutions to e x p l o r e this h y p o t h e s i s f u r t h e r .
SOME HIGH-PRESSURE TRANSFORMATIONSOF GEOPHYSICAL SIGNIFICANCE 4. STABILITY OF Mg2GeO4 SPINEL 4MgO. GeO2 AT HIGH PRESSURE
AND
Ringwood and Seabrook [5] had p r e v i o u s l y i n vestigated the stability of Mg2GeO 4 (spinel) by r e a c t i n g MgO with h y d r o u s g e r m a n i a (in the 2/1 m o l e c u l a r proportion) in a s q u e e z e r a p p a r a t u s at 600oc. At p r e s s u r e s up to 90 kb, complete r e action to Mg2GeO4 spinel was observed. Howe v e r , at p r e s s u r e s above 90 kb, the r e a c t i o n p r o d u c t s were found to be MgGeO 3 (ilmenite) plus a new phase which was b e l i e v e d to have the composition 4MgO. GeO2 or 5MgO. GeO2. A phase 4MgO. GeO 2 had p r e v i o u s l y been s y n t h e sized by Robbins and Levin [8]; however this phase was not i d e n t i c a l with the new phase found by Ringwood and Seabrook. These r e s u l t s were taken to i m p l y that Mg2GeO4 (spinel) was u n s t a b l e at high p r e s s u r e and t r a n s f o r m e d into a d e n s e r m i x t u r e of MgGeO3 i l m e n i t e + 4 (or 5) MgO. GeO2 (new phase). T h i s i n t e r p r e t a t i o n has been checked by subjecting the phase 4MgO. GeO2 to a p r e s s u r e of 170 kb u n d e r dry conditions. The 4 MgO. GeO2 was p r e p a r e d a c c o r d i n g to the p r o c e d u r e of Robb i n s and Levin [8]. It was o b s e r v e d that the 4MgO. GeO2 t r a n s f o r m e d completely into a mixt u r e of Mg2GeO4 ( s p i n e l ) + MgO. This i n d i c a t e s that the new phase f o r m e d by Ringwood and Seabrook above 90 kb was p r o b a b l y hydrated, and that Mg2GeO 4 spinel is stable u n d e r dry condit i o n s at l e a s t to 170 kb. Accordingly the p r e vious suggestion [1, 5] that Mg2SiO 4 spinel might t r a n s f o r m at high p r e s s u r e s into a m i x t u r e of MgSiO3 (ilmenite) + x MgO. SiO 2 (x > 2) must be c o n s i d e r e d speculative and without e x p e r i m e n t a l support at p r e s e n t . C u r r e n t i n v e s t i g a t i o n s on the s y s t e m M g O - S I O 2 - H 2 0 where MgO/SiO2 >/ 2 have d i s c l o s e d the e x i s t e n c e of s e v e r a l new p h a s e s , at l e a s t one of which has a d e n s i t y b e tween 3.5 and 3.8 g / c m 3. It i s not yet c l e a r whether this phase i s hydrated.
5. THE HIGH-PRESSURE TRANSFORMATIONS OF MnSiO 3 High p r e s s u r e r u n s were c a r r i e d out on s a m ples of MnSiO3 g l a s s and on synthetic and n a t u r a l rhodonite. The g l a s s was f o r m e d by m e l t i n g an i n t i m a t e m i x t u r e of Mn30 4 and s i l i c a in the c o r r e c t p r o p o r t i o n s in a p l a t i n u m tube at 1500°C followed by quenching. The rhodonite sample was f o r m e d by c a l c i n i n g a tablet of the i n t i m a t e ly mixed oxides (Mn304 and S i O 2 ) a t about 1200 °C for 4 h o u r s . After this t r e a t m e n t , it was
109
found to be inhomogeneous, accordingly it was crushed, reformed, and heated at 1200oc for another 8 hours, yieldinghomogeneous rhodonite, as shownby opticaland X-ray examination. The refractive index of the synthetic rhodonite indicated that it was essentially pure and contained a negligible amount of Mn2Mn403 in solid solution [9]. Previous (unpublished) results by Ringwood and Seabrook on the MnSiO3 glass and on natural rhodonite using a squeezer apparatus revealed that at 700oc and at pressures between 30 and 60 kb, rhodonite transformed to a new phase possessing a complex X-ray diffraction pattern, characterised by strong doublets at 2.21, 2.23 ~; 3.00, 3.05 ~; and a strong diffuse reflection (probably an overlapping triplet at 2.67 ~). The refractive indices of this phase were only slightly higher than those of rhodonite indicating that it is at the most, only a few percent denser than rhodonite. In the present investigation at pressures between 60 kb and 90 kb, the MnSiO3 glass and also a sample of natural rhodonite (containing 14 percent FeO) were observed to transform to a pyroxene structure. This has not yet been indexed, but is very probably an orthopyroxene. Finally, at pressures between 120 kb and 170 kb, MnSiO3 glass was observed to transform completely to a garnet structure possessing a lattice parameter of 11.765 ± 0.010 ~. This garnet is clearly analogous to the CaGeO3 and CdGeO3 garnets previously mentioned, and evidently possesses a structural formula MnVIII (MnSi)vl SiIVO 12 where the superscripts denote the coordination of the indicated cations with respect to oxygen. The calculated density of the garnet is 4.27 g/cm3 compared to 3.71 g/cm3 for rhodonite. Because of the possibility that a small amount of Mn3+or Mn4+ions might have been present in the glass, and thereby have influencedthe stability of the garnet, attempts were made to transform pure synthetic rhodonite and natural rhodonite into the garnet structure. Several runs were made at pressures between 120 and 170 kb and in each case substantial to extensive transformation to garnet was observed. The best result was ~ run on pure synthetic MnSiO3at 170 kb for 15 minutes which yielded about 70 percent of garnet and 30 percent of pyroxene. Complete transformation w~s apparently prevented by nucleation difficulties. Nevertheless, this result is important since it implies that Mn3+ and Mn4+ ions are not essential components of the garnet structure, and also that the garnet is not a product of metastable crystallization of glass [10].
110
A.E. RINGWOOD and A. MAJOR
We have p r e v i o u s l y d e s c r i b e d [10] the s y n t h e s i s f r o m g l a s s e s at high p r e s s u r e s of g a r n e t s with c o m p o s i t i o n s MgSiO3, 10%A1203, CaSiO3.10 % A120 3 and CaMgSi206.10°/0 A120 3. T h e s e c o m p o s i t i o n s i m p l y that a substantial p r o p o r t i o n of s i l i c o n ions i s in o c t a h e d r a l coordination. It w a s b e l i e v e d that t h e s e g a r n e t s w e r e t h e r m o d y n a m i c a l l y stable in t h e i r s y n t h e s i s f i e l d s , but t h i s could not be c o n c l u s i v e l y e s t a b l i s h e d b e cause the g a r n e t s w e r e c r y s t a l l i z e d f r o m g l a s s e s , and the p o s s i b i l i t y of m e t a s t a b l e c r y s t a l l i z a tion could not be e l i m i n a t e d . A t t e m p t s to c o n v e r t c r y s t a l l i z e d a l u m i n o u s p y r o x e n e s c o m p l e t e l y to garnets were unsuccessful, presumably because of k i n e t i c d i ff i cu l t i es . The p r e s e n t r e s u l t s d e m o n s t r a t i n g the stable s y n t h e s i s of an MnSiO 3 g a r n e t , t o g e t h e r with the p r e v i o u s stable s y n t h e s i s of CaGeO3 and CdGeO3 g a r n e t s , s t r o n g l y support the view that the other g a r n e t s w e r e stable in t h e i r s y n t h e s i s f ie ld s . It is probable that t h e s e g a r n e t s , c h a r a c t e r i s e d by p a r t i a l o c t a h e d r a l c o o r d i n a t i o n of s i li c o n ions, play a m a j o r r o l e in the t r a n s i t i o n zone in the e a r t h ' s mantle between depths of 350 and 500 km. A d e t a i l e d e x a m i n a t i o n of this r o l e will be made when c u r r e n t e x p e r i m e n t s on th e s e g a r n e t s a r e c o m pleted.
ACKNOWLEDGEMENT The authors are grateful to Dr. D.H. Green for critical reading of the manuscript.
REFERENCES [1] A.E.Ringwood, in: Advances in Earth Science, ed. P.MoHurley (M.I.T. Press, 1966} p. 357. [2] A.E.Ringwood and A. Major, Earth Planet. Sci. Letters 1 (1966) 241. [3] S.M.Stishov, Dokl.Akad. Nauk SSR 148 (1963} 1186. [4] R. N. Jeffery, J.D.Barnett, H.B.Vanfleet and H.T. Hall, J. Appl. Phys. 37 (1966) 3172. [5] A.E.Ringwood and M.Seabrook, J. Geophys. Res. 68 {1963) 4601. [6] A.E. Ringwood, Geochim. Cosmochim. Acta 26 (1962) 457. [7] F.Dachille and R.Roy, in: Modern very high pressure techniques, ed. R.H.Wentorf (Butterworth and Co., London, 1962) Chapter 9. [8] L.R.Robbins and E.Levin, Am. J. Sci. 257 (1959) 63. [9] A.Muan, Am. J. Sci. 257 (1959} 297. [10] A.E.Ringwood and A.Major, Earth Planet. Sci. Letters 1 (1966) 351.