;

EARTH AND PLANETARY SCIENCE LETTERS 12 (1971) 411 - 418. NORTH-HOLLAND PUBLISHING COMPANY

SYNTHESIS OF MAJORITE AND OTHER HIGH PRESSURE GARNETS AND PEROVSKITES

A.E. RINGWOOD and Alan MAJOR Department of Geophysics and Geochemistry.

Australian National University

Received 30 August 1971

The synthesis is described at a pressure of 250-300 kb of the garnet, majorite, previously found to occur as a shock produced phase in a chondri tic meteorite. A series of new high pressure garnets containing sodium and/or titanium is also described. One of these has 2/5 of its silicon atoms in octahedral coordination. Garnets of these types are likely hosts for sodium in the mantle at depths of 350-650 km. Perovskite type solid solutions along the join CaTi03 - CaSi03 ranging up to a composition Ca(Sio. 83 TiO•17)03 have been synthesized. Experimen tal evidence leading to the conclusion that pure CaSi03 most probably transforms to a perovskite-type structure at 100 kb and 10000 C is described. CaSi03 perovskite is 8 percent dense!' than an isochemical mixture of CaO + Si02 (as stisho­vile).

1. Introduction

A few years ago, we demonstrated that enstatite dissolved extensively in pyrope garnet at pressures above 100 kb and at elevated temperatures [1,2] . The existence of a solid solution series between Mg3Al'2ShOl'2 and an MgSi03 garnet with the struc­tural formula Mg3(MgSi)Sh012 was inferred. At the highest pressure employed (- 200 kb), a garnet with the approximate composition [Mg3(MgSi)Si 30 12ho [Mg3Al2Si3012ho was synthesized corresponding to the composition MgSi03. 7% Al20 3 [3]. It was .in­ferred that at still higher pressures, a pure MgSi03 garnet might become stable. Analogous behaviour was also discovered for the compositions FeSi03.10% Al20 3, CaSi03 .10% Al20 3 and CaMgSi20 6 .10% Al20 3. Each bf these could be crystallized entirely to garnets at very high pressure. Moreover, it was found that the pyroxenoid MnSi03 transformed with in­creasing pressure first to a clinopyroxene structure, and then, at higher pressures, to a slightly distorted garnet structure [4]. These results demonstrated that the pyroxene to garnet transformation was a fairly general phenomenon and supported the conclusion that it might be anticipated in (MgFe )Si03 pyroxenes

at extremely high pressures [1-3]. Confirmation of this conclusion was provided by

discoveries of garnets in heavily shocked regions of the Coorara [5,6] and Tenham (7,8] chondrites. Binns and coworkers (7,8] concluded that the garnet in the Tenham chondrite might represent the predict­ed high pressure polymorph of (MgFe)Si03 and had been synthesized under intense shock pressures gener­ated by extraterrestrial meteorite collisions. However they were unable to obtain a chemical analysis of the phase. Working on the Coorara chondrite, Smith and Mason [6] obtained an analysis of the garnet and showed that it consisted dominantly of (Mgo.7sFeo.2s)Si03 together with some minor com­ponents. They named the new mineral majorite.

We now report the synthesis of this phase. A study of the entry of sodium and titanium into the garnet structure has also been carried out and syntheses of several new high pressure garnets containing these elements as major components is also described. Finally, we describe some results of high pressure experimentation on the system CaTi03 -CaSi03, leading to the synthesis of a series of high pressure perovskites.

412 A. E. Ringwood and A. Major, Synthesis ofmajorite

Table 1 Compositions of natural and synthetic majorite.

Si02 Al203 Cr203 Fe203 FeO MgO Na20

Naturali occurrence

52.0 2.6 0.7

16.9 27.5

0.7

100.4

Synthetic2

52.43 2.46 0.62 0.65

17 .14 26.07

0.63

100.00

I Smith and Mason [6], average of "best" analyses. All iron calculated as Fe2 +.

2 This paper.

2. Preparation of starting materials

The composition and structural formula of major­ite obtained by Smith and Mason [6] are given in

tables 1 and 2. These authors were somewhat uncer­tain regarding the state of oxidation of iron. However present knowledge of redox relations in the primary mineral assemblages of chondrites permits little doubt that nearly all the iron present was in the ferrous state. We prepared a glass with a composition similar to majorite as follows: Oxide components were tho­roughly mixed and then sintered in air at 1 100°C. Sufficient powdered iron to reduce all Fe3+ to Fe2+ was then mixed in and the material was pelletized. The pellet was wrapped in platinum foil, placed in an evacuated silica tube and heated at IOOO°C for 24 hr. The product was reground and melted together with 2 percent of excess iron powder in an iron-saturated crucible in a non-oxiding atmosphere, and the liquid was subsequently quenched to a glass. Excess metallic iron was removed with a magnet after crushing, and the glass was analyzed for FeO and Fe203 by Mr. E. Kiss. The composition of the glass so produced, in­cluding the analytical data for iron, are given in table 1. It is seen to be very similar to that of natural majorite. The differences are well within the experimental un­certainties in measuring the composition of majorite as given by Smith and Mason [6].

Two other glasses were prepared by similar methods to those used above. These had compositions equiva- . lent to (MgO.7S Feo.2s)Si03 plus 5 and 2 weight percent Al20 3 respectively.

Table 2

Analyses of natural and synthetic majorite recast to garnet formulae, A3B2C3012 (cations per 12 oxygen anions) .

Natural majorite I Synthetic majorite2

C group Si 3.00 Si 3.00

B group Si 0.78 ) Si 0788) Al 0.23 2.06 Al 0.210 Fe2+ 1.02 Cr 0.036 2.000 Cr 0.03 Fe3+ Om5

Mg 0.931

A group Mg 2.98} 3.08 Mg 1.877 } Na 0.10 Fe2+ 1.036 3.001

Na 0.088

Mg = 0.75 Mg + Fe2+

Mg = 0.73 Mg + Fe2+

I Smith and Mason [6]. 2 Ref. table 1.

A further series of glasses having the compositions NaCa2 (AITi)Si3 0 12 , (Na2 Ca)Ti2 Si3 0 12 , NaCa2 (AlSi)Si3 0 12 , Ca3 (MgTi)Si3 0 12 and Mg3(MgTi)Si30 12 was also prepared. Oxide compo­nents were thoroughly mixed and sintered in air for 4 hr at about 1 100°C. They were reground and remixed, and then fused to homogeneous glasses on an iridium strip heater. A glass of composition Na2 CaSis 0 12 had a relatively low melting point and was prepared by direct fusion in a platinum crucible followed by quench­ing. Finally, a series of glasses with compositions lying on the join CaTi03 (CT)- CaSi03 (CS) was prepared using similar methods to those described above. Com­positions so prepared were CS7s CT25 , CS90 CTlo and CSI 00. It was not possible to quench the composition CSsoCT 50 to a glass.

A sample of Fe2+Mn2+Si03 pyroxene was also pre­pared as follows. A stoichiometric mixture of Fe2 0 3, Mn304 metallic iron powder and Si02 was first heated at 1000°C in an evacuated silica tube for 24 hr. The product was then subjected to a pressure of 35 kb at 1200°C for 2 hr in a piston-cylinder high pressure apparatus. Complete conversion to FeMnSi03 clinopyroxene was obtained. Chemical analysis by Mr. Kiss confirmed that nearly all the iron was present in the ferrous state (only 0.45% Fe2 0 3).

A. E. Ringwood and A. Major, Synthesis ofmajorite 413

3. Results

3.1. Majorite The glass of majorite composition (table 1) was

subjected to pressures of 150 and 180 kb for 3 min at about 1000°C in a Bridgman anvil device equipped with an internal heater as previously described by us [9] . After the runs the samples were quenched by turning off the power, pressure was released, and the samples recovered and examined by optical micro­scopy and X-ray diffraction. The glass was found to have crystallized to mixtures of garnet (~ 30- 40%) and clinopyroxene (60-70%). Evidently the pres­sures employed were insufficient to synthesize major­ite of the composition observed in the meteorite.

We have recently developed an improved high pres­sure apparatus, capable of developing pressures sub­stantially higher than previously attained. One run was carried out in this apparatus. The pressure was not well known but was believed to be between 250 and 300 kbar at a temperature of 1000°C.

Complete conversion to garnet was achieved. The back diffraction lines of the garnet were slightly dif­fuse and doublets were not resolved. The lattice parameter of the garnet was 11.529 ± 0.005 A com­pared to 11.524 ± 0.002 A observed for natural majorite. The refractive index of the synthetic garnet was 1.771 ± 0.005 . There is thus no doubt that the new phase is synthetic majorite.

High pressure experiments were also carried out upon the related glasses (Mgo.7sFeo .zs)Si03 . 5% AIz 0 3 and (MgO.7S Feo.zs)Si03 .2% AIz 0 3, A run at 180 kb, 1000°C upon the former produced a mixture of about 50 percent each of garnet and clinopyroxene. Using the new apparatus, a run at 250-300 kb pro­duced complet6 conversion to garnet with a lattice parameter of 11.502 A. In the case of the glass con­taining 2 percent Alz 0 3, runs at 120-180 kb yielded between 40 and 60 percent of garnet, the remainder being clinopyroxene. However a run at 250- 300 kb produced only 20 percent of garnet. We suspect that the small yield in this case was due to retrogressive transformation of garnet to pyroxene which occurred on release of pressure. We have previously encountered examples of this behaviour in other systems [3] .

3.2. Sodium-bearing garnets Ringwood and Lovering [10] described the high

pressure transformation of pyroxene-ilmenite inter­growths found in some diamond pipes into homo­geneous garnet solid solutions. The starting material contained 1.2 percent of Naz 0 and they suggested that the sodium might be accommodated in the gar­net as (CaNaz) Tiz Si30 lz and (CazNa)(AITi)Si3 0IZ components. These components imply the coupled substitutions of (Na + Ti) for (Ca + AI) in the grossu­larite structure. The natural occurrence of majorite contained about 0.7% ofNa20, but insufficient Ti was present to account for the presence of sodium via the Na-Ti substitution. This suggested the possibility of an Na-Si substitution, and of the existence of garnets of the type (NaCa2)viii(AISi)viSi3iv012 and (Naz Ca)viii(Si)z viSi3 ivOIZ , where the superscripts in­dicate the coordination of the designated cations with respect to oxygen. It is seen that the entry of each sodium atom is accompanied by the substitution of a silicon atom in the octahedrally coordinated site.

A similar suggestion has been made independently by Sobolev and Lavrentev [11] who found that gar­nets from diamond-bearing eclogites and garnets oc­curring as inclusions in natural diamonds commonly contained significant amounts of sodium (up to 0.26% Na20) whereas garnets from ordinary eclogites con­tained only very small amounts of sodium. Sobolev and Lavrentev concluded that the sodium entered the garnets via an Na-Si rather than Na-Ti substitution and that the presence of sodium was indicative of very high pressure origin.

We found that a glass of composition (CaNa2) Ti2 Si3 0 12 crystallized completely to garnet in runs at 50, 60, 80 and 120 kb. The back reflexions were sharply resolved and the lattice parameter was 11.968 A. However, a run at 35 kb, 1200°C failed to produce garnet. Similarly, the glass of composition (Ca2 Na) (AITi)Si3 0 lz crystallized completely to garnet at 120 kb, 1000°C. The lattice parameter of the garnet was 11.943 A. A run at 35 kb, 1200°C produced a mixture of garnet and another phase. The garnet had a lattice parameter of 11.920 A indicating a com­position intermediate between (Ca2 Na) (AITi)Si3 0 12 andgrossularite,Ca3AIzSi3012 (ao = 11.851 A).Evi­dently, in the presence of titanium, a substantial amount of sodium may enter the garnet structure at rather modest pressures.

Runs were carried out on the Na2 CaSis 0IZ and NaCa2(AISi)Si3012 glasses at 180kb, 1000°C. The

414 A. E. Ringwood and A. Major, Synthesis of majorite

latter glass was completely transformed to garnet with a lattice parameter of 11.715 A.. The Na2 CaSis 0 12 composition crystallized dominantly (> 90%) to a garnet with a lattice parameter of 11.542 A.. It is likely that the non-garnet component was accidentally introduced. These experiments con­firm the existence of the Na-Si substitution for Ca-Al in garnets at very high pressures [11]. Relations be­tween lattice parameters are shown in fig. 1.

These results have a bearing on the occurrence of sodium in the earth's mantle. Previously, we found that jadeite was stable at pressures up to at least 200 kb [4] and suggested that in the depth interval be­tween 350 and 650 km, sodium might be accom­modated in jadeite [3] . The present syntheses of NaCa2 (AlSi)Si3 0 12 and Na2 CaSis 0 12 garnets strongly indicate however, that sodium would enter the complex garnet solid solutions which are believed to be major constituents of the mantle in this depth interval [3] , and jadeite is probably not present.

3.3 . Ca3(MgTi)Si3012 and Mg3(MgTi)Si3012 Ringwood and Lovering [10] demonstrated the

occurrence of another kind of coupled substitution in­volving the replacement of 2 AI3+ atoms by Mg2+ + Ti4+. This substitution was involved in the formation of a homogeneous garnet at high pressure from the diopside-ilmenite intergrowths found in some dia, mond pipes. The two principal end-member garnet molecules involved were Ca3 (MgTi)Sh 0 12 and Mg3 (MgTi)Si3 ° 12 .

The Ca3(MgTi)Si3012 glass crystallized to a mix­ture of clinopyroxene and CaTi03 perovskite in runs at 35 kb, 1200°C and 60 kb, 1000°C. In runs at 70,90, and 120 kb, the glass crystallized to a mixture of 70-80 percent garnet plus some clinopyroxene. Finally, in a run at 150 kb, complete conversion to garnet with sharply resolved back reflexions and a lattice parameter of 12.085 A. was observed.

In two runs at 120 and 180 kb on this glass, dispro­portionation to diopside plus a perovskite-type Ca(TiSi)03 solid solution was observed. The lattice parameter of the perovskite was 3.706 A., which im­plies a composition in the vicinity of Ca(Tio.sSio.s)03 - see fig. 2. The following dispro­portionation is inferred to have occurred.

12 ' 0 Na2CaTi2Si30'2

° A

Na2CaSip" 11 ·5'----------'~--------'

NaCa2(AI xl Sip" Na,GaX2Si

30

12

Fig. 1. Relations between lattice parameters of some high pressure sodium garnets.

diopside perovskite

The two runs in which this disproportionation was ob­served took place at rather higher temperatures than the runs in which garnet was produced. This might ac­count for the difference in results.

We were unsuccessful in transforming the Mg3 (MgTi)Si30 12 glass into garnet in runs at pres­sures up to 180 kb .

3.4. FeMnSi20 6

Ringwood and Major [4] had previously demon­strated that MnSi03 transformed to a garnet, Mn3 (MnSi)Si3 °12 , at high pressure. The X-ray diffrac­tion pattern obtained was rather diffuse in the high angle region and a lattice parameter of 11.765 ± 0.010 A. was obtained. Subsequent runs have yielded slightly better diffraction photographs and have shown the existence of line splitting, indicating a symmetry low­er than cubic, perhaps tetragonal as in CdGe03 [12]. Further studies are in progress.

Our sample of FeMnSi03 (clinopyroxene) dis­played about 80 percent transformation to garnet in a run at 140 kb, the remaining material consisting of un­transformed clinopyroxene. The back diffraction lines were somewhat diffuse, and a l a2 doublets were not resolved. No evidence of line splitting indicative of a

0<

'" w .. w

" < '" < ...

A. E. Ringwood and A. Major, Synthesis ofmajorite 415

ColiO;, PEROVSKITE - TYPE SOLID SOLUTIONS

3·80

3·70

3.60 L-_L--''----'_---L_---'-_-'-_--'-_-'-_-'----' o 20 40

MOL % 60 80 100

CoSiO,

Fig. 2. Lattice parameters of CaSi03-CaTi03 (rectangles) and CaSi03-CaGe03 (circles) perovskite solid solutions synthesized at high pressures. Solid symbols represent homo­geneous perovskite phases. Open symbols indicate presence of other phases. Note the anomalously high lattice parame­ters in the two phase field of the CG-CS system, probably due to ex solution of CaSi03 from perovskite solid solution on re-

lease of pressure.

lower symmetry than cubic was seen. The lattice parameter was 11.713 ± 0.010 A.

3.5. Perovskite solid solutions in the system CaSi03 -

CaTi03

Ringwood and Major [4] demonstrated that CaGe03 (wollastonite str.) transformed through an intermediate garnet modification to a perovskite struc­ture at high pressure. A study of the system CaGe03-CaSi03 revealed that at least 35 percent of CaSi03 dissolved in the perovskite at high pressure. Almost certainly, the amount of CaSi03 dissolved substantial­ly exceeded 35%. However on releasing pressure, ex­solution of the solid solution apparently occurred and proceeded nearly to completion in some cases [3,4] . In fig. 2, it is seen that in the two phase region, the CaSi03 content of the perovskite solid solution is much smaller than in the single phase homogeneous region at saturation.

The extensive solid solubility of CaSi03 in the perovskite structure implies a small free energy dif­ference between the low pressure form of CaSi03 and the perovskite form [3, 16] . Calculations [3] demon­strated that CaSi03 should transform to the perovskite structure at high pressures.

In the present experiments on the system CaSi03(CS)-CaTi03(CT) we found at 125 kb that a glass of composition CS7s CT2S crystallized com­pletely to a perovskite structure with a lattice parame­ter of 3.678 ± 0.005 A. Furthermore, in some runs on the composition Ca3 (MgTi)Si3 0 12 as discussed in sect. 3.3, the synthesis of perovskite solid solution with the approximate composition CSsoCTso was described.

A series of runs was carried out on CS90 CTlO

glass at pressures of 120,130,150,180 and 200 kb. The principal product recovered in all runs was glass, sometimes with a peculiar striated structure and con­taining variable amounts of an extremely finely crystalline phase possessing a complex X-ray diffrac­tion pattern. In subsequent discussion, this phase will be referred to as €-CaSi03 • The proportion of glass was estimated to lie in the range 60- 90%. In a run at 125 kb a small amount of perovskite solid solution was found to be present. The perovskite back diffrac­tion lines were rather diffuse , but yielded a lattice parameter of 3.662 ± 0.005 A. From the relationship between composition and lattice parameter (fig. 2) the composition of the perovskite solid solution was found to CSS3 CT 17' Optical examination revealed that the predominant glass recovered contained small, sometimes embayed inclusions of a high refractive in­dex phase - presumably perovskite solid solution. The results thus indicate the existence of a continu­ous series of perovskite solid solutions between CT 100 and CSs3 CT17 (fig. 2).

3.6. Transformations in CaSi03

In view of the fact that the perovskite composition range extended so close to pure CaSi03 , we carried out further runs on a glass of this composition. In runs at 60, 90 and 97 kb, the glass transformed com­pletely to a well crystallized modification of CaSi03

possessing a relatively low refractive index and den­sity. This phase had previously been synthesized by us [4] and also by Trojer [13] who determined its crys­tal structure. In sharp contrast, the material recovered from runs at 105, 120 and 200 kb consisted dominant­ly (up to 95%) of glass (refractive index 1.63), which sometimes possessed a striated structure, caused by sub-parallel strings of high-refractive inclusions. X-ray powder patterns revealed the presence of small amounts of €-CaSi03 , or of a phase very similar to

416 A. E. Ringwood and A. Major, Synthesis of majorite

€-CaSi03 . This phase presumably corresponded to the high refractive index inclusions.

The origin of the glass found in all runs at pres­sures above 100 kb in both CS90 CT 10 and CS 100

compositions merits discussion. We are confident that this glass does not represent glass starting mate­rial which had failed to devitrify. We have carried out more than 700 high pressure runs on starting ma­terials consisting of glasses in the present apparatus. Occasionally, when the heater has failed and insuffi­cient power was delivered, the glasses have not devitri­fied and have been recovered in their original states. However in all cases where the heater has functioned properly and the normal amount of power and heat delivered to the sample, complete crystallization of the glasses has occurred. Nearly all the present series of runs received normal amoun ts of power, and dis­played every sign of being heated to an average tem­perature of 1000°C. Moreover, CaSi03 glass readily devitrifies at temperatures as low as 650°C in appara­tus of this kind [17] . All of the runs carried out at pressures below 100 kb were indeed well crystallized. It seems inescapable that the runs carried out at pres­sures above 100 kb also caused the glass to devitrify to a crystalline phase. The fact that the material re­covered consisted mainly of glass would therefore imply that this high pressure crystalline phase must have retrogressively transformed to glass upon release of pressure.

To check this conclusion we carried out a series of runs on crystalline wollastonite. In runs at 80, 85, 90, 95 and 100 kb, the wollastonite transformed to coarse, well-crystallized samples of the familiar inter­mediate pressure polymorph of CaSi03 [4, 13]. Dif­fering behaviour was found in eleven runs carried out at pressures between 100 and 150 kb.In no case was the intermediate pressure polymorph found. The samples recovered consisted of mixtures of glass and €-CaSi03 . In runs carried out at relatively low tem­peratures (700- 900°C), glass was the principal pro­duct, whereas at higher temperatures (900- 1100° C), €-CaSi03 was more abundant, constituting up to 95% of the sample. This type of occurrence was sometimes observed in the hot and cool regions of samples which had been non-uniformly heated during the same run.

€-CaSi03 has a refractive index of about 1.745. Application of the Gladstone- Dale rule indicates a density of about 3.46 gJcm3 which is 19 percent

greater than wollastonite (2.91 g/cm3). The density

of €-CaSi03 is similar to that which might be ex­pected for a pyroxene modification of CaSi03 and it is possible that its structure is related to that of pyro­xene. The density and refractive index of €-CaSi03

imply that the silicon atoms are tetrahedrally coordi­nated. The density and refractive index of a CaSi03

polymorph in which silicon atoms were octahedrally coordinated would be much higher than observed. €­CaSi03 is birefrigent and exhibits an extremely small crystal size. Sometimes it displays a peculiar undula­tory extinction. Its appearance is unlike that of nor­mal well-crystallized high pressure polymorphs which have been synthesized in the present apparatus under similar conditions. X-ray powder patterns of €-CaSi03

are often poorly resolved and reflexions sometimes show variable relative intensities. In these respects also , they are unlike the patterns of most other high pressure phases previously synthesized in the present apparatus. We synthesized this phase earlier [4] and suggested that it might represent a metastable retro­grade transformation product formed during release of pressure. Our present observations have reinforced this impression. The interplanar "d" spacing of €­CaSi03 are given in table 4. This supplants the set previously published [4] which were obtained from a very poorly crystallized specimen. The 2.05 A re­flexion in the previous pattern does not occur in the present patterns and was presumably caused by an impurity.

The experimental data, both on CaSi03 glass and on wollastonite, demonstrate that a transformation from the intermediate pressure polymorph [13] of CaSi03 into a denser structure occurs at a pressure of about 100 kb. The glass recovered at higher pressures than 100 kb is clearly a retrogressive transformation product from this phase and was formed on release of pressure.

What was the nature of the primary high pressure phase which was stable above 100 kb? Two possibili­ties appear. The stable high pressure phase might have been €-CaSi03 which underwent varying degrees of retrogressive transformation to glass on release of pressure. The second possibility is that both the glass and €-CaSi03 represent retrogressive transformation products from a third phase which was stable under pressure. The unusual optical appearance and X-ray diffraction patterns of €-CaSi0 3 incline us towards

11

•

A. E. Ringwood and A. Major, Synthesis of majorite 417

Lattice parameters of new high pressure garnets.

Composition

Majorite (synthetic) Majorite natural' (MgO.7S FeO.2S)Si03. 5% A120 3

NaCa2 (AlSi)Si30 12 Na2CaSis012 NaCa2(A1Ti)Si3012 Na2CaTi2Si3012

Ca3(MgTi)Si30 12

FeMnSi03 MnSi032

, Smith and Mason [6]. 2 Ringwood and Major [4] .

A

11.529 ± 0.005 11.524 ± 0.002 11.502 ± 0.003

11.715 ± 0.005 11.542 ± 0.002 11.943 ± 0.002 11.968 ± 0.002

12.085 ± 0.002

11.713 ± 0.010 11.765 ± 0.010 3

3 Pseudo-cubic unit cell. Actual symmetry lower than cubic.

the second interpretation although this is necessarily somewhat subjective.

Further evidence points in the same direction and is based upon the recognition of glass as the main retrogressive transformation product in most runs. To the best of our knowledge, glass has never been ob­served·as a product of retrogressive transformation at low temperatures from any metastable crystalline phase in which the silicon atoms are tetrahedrally co­ordinated. On the other hand, it has been observed in the only two previously known high pressure phases in which all the silicon atoms are octahedrally coor­dinated. Skinner and Fahey [I 4] observed that stisho­vite transformed to glass when heated at temperatures between 300°C and 800°C. It also transformed com­pletely to glass when ground in a mortar at room tem­perature. We have heated a sample of KAlSh 0 8 hol­landite [15] at 780°C for 3 hr and observed complete transformation to glass. (Temperature required for complete melting of KAlSi3 Os is about lSS0°C at atmospheric pressure) . These results strongly suggest that the parent phase to the CaSi03 glass contained silicon in octahedral coordination. We have previously pointed out that the silicon in €-CaSi03 is tetrahedral­ly coordinated. Accordingly it seems unlikely that €-CaSi03 was the parent phase.

Confirmation of this conclusion was provided by grinding experiments upon samples of previously synthesized CS 75 CT 25 perovskite, €-CaSi03 and €-

Interplanar d spacings and relative intensities I for e-CaSi03.

I dA

10 3.02 3 2.91 8 2.78 2 2.73(5)

<1 2.68 4 2.61 3 2.52 2 2.44

2.397 1 2.279 6 2.189 1 2.045 6 1.981 1 1.631 1 1.595

CS90 CT 10 . After five minutes of grinding in an agate mortar, the CS7s CT2s perovskite solid solution was completely converted to glass. (Another sample was completely converted to glass by heating for 3 days at 600°C). In contrast, the samples of €-CaSi03 and €-CS90 CT 10 were unaltered by grinding under similar conditions.

On general crystal chemical grounds, a structure of the parent phase related to that of perovskite appears to be the only likely candidate among AB03 type crystal structures in which the B cations are octahe­drally coordinated. The fact that we have synthesized perovskite solid solutions containing up to 83 percent of CaSi03 , and that these silica-rich perovskites have been demonstrated to transform readily to glass , strongly suggests that the high pressure pure CaSi03

phase was a member of the perovskite family of struc­tures. The free energy difference between pure CaSi03 perovskite and the perovskite solid solution containing 83 percent of CaSi03 is very small - on the order of 500 calories per mole of CaSi03 . If Pis the minimum pressure needed to synthesize CSS3 CT 17 perovskite, the approximate additional pressure t:.P necessary to stabilize pure CS100 perovskite is

RT t:.P <: t:.v In a ,

where t:.v is the molar volume difference between the

418 A. E. Ringwood and A. Major, Synthesis of mojo rite

low pressure and perovskite forms of CaSi03 and a is the activity of CaSi03 in the CSS3 CT 17 perovskite solid solution [3,16]. Taking a as 0.83 and llv as 8 cm 3 [3] the additional pressure is only 2.5 kb. The CSS3 CT 17 solid solution is stable at about 100 kb and we have squeezed pure CaSi03 at pressures exceeding 200 kb which is far more than is necessary to provide the small additional stabilization necessary to synthe­size pure CaSi03 perovskite.

The extrapolated lattice parameter of CaSi03

perovskite is 3.62 ± 0.01 A (fig. 2) corresponding to a density of 4.07 g/cm3

. This is 8% higher than that of an isochemical mixture of CaO (rocksalt str.) and Si02 (stishovite str.), indicating the extremely high intrinsic density of the perovskite structure. As sug­gested earlier by Ringwood [3] it appears likely that CaSi03 perovskite may be a significant constituent of the mantle. At depths shallower than 350 km, CaSi03 occurs as a component of pyroxene minerals, whereas below 350 km, CaSi03 occurs as a component of complex garnet solid solutions [3] . This mode of oc­currence would have the effect of increasing the pres­sure needed for the stability of CaSi03 perovskite in the mantle, as compared with the pressure required to transform pure CaSi03 to the perovskite structure. In the light of these considerations, the possibility is suggested that an inferred seismic discontinuity at 520 km [18] might be caused by exsolution of the CaSi03 component of complex garnet solid solution to form denser CaSi03 perovskite.

Acknowledgements

The authors are indebted to Dr. D. H. Green and Dr. B. Mason for the benefit of helpful discussion and constructive criticism.

References

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mations in pyroxenes, Earth Planet. Sci. Letters 1 (1966) 351.

(2) A. E. Ringwood, The pyroxene-garnet transformation in the earth's mantle, Earth Planet. Sci. Letters 2 (1967) 255.

[3) A. E. Ringwood, Phase transformations and the con­stitution of the mantle, Phys. Earth Planet. Interiors 3 (1970) 109.

(4) A. E. Ringwood and A. Major, Some high pressure trans­formations of geophysical interest, Earth Planet Sci. Letters 2 (1967) 106.

(5) B. Mason, J. Nelen and J. White, Olivine-garnet transfor­formation in a meteorite, Science 160 (1968) 66.

(6) J. Smith and B. Mason, PyrQxene-garnet transformation in Coorara meteorite, Science 168 (1970) 832.

(7) R. Binns, R. Davis and S. Reed, Ringwoodite, natural (MgFehSi04 spinel in the Tenham meteorite, Nature 221 (1969) 943.

(8) R. Binns, (MgFehSi04 spinel in a meteorite, Earth Planet. Int. 3 (1970) 156.

(9) A. E. Ringwood and A. Major, The system Mg2Si04-Fe2Si04 at high pressures and temperatures, Phys. Earth Planet. Int. 3 (1970) 89.

(10) A. E. Ringwood and J. F. Lovering, Significance of pyroxene-ilmenite intergrowths among kimberlite xenoliths, Earth Planet. Sci. Letters 7 (1970) 371.

(11) V. Sobolev and J. Lavrentev, Isomorphic sodium admix­ture in garnets formed at high pressures, Contrib. Mineral. Petrol. 31 (1971) 1.

(12) C. Prewitt and A. Sleight, Garnet-like structures of high pressure CdGe03 and CaGe03, Science 163 (1969) 386.

(13) F. Trojer, Crystal structure of high pressure CaSi03, Z. Krist. 13 0 (1969) 185.

(14) B. Skinner and J. Fahey, Observations on the inversion of stishovite to silica glass, J. Geophys. Res. 68 (1963) 5595.

(15) A. E. Ri!}gwood, A. F. Reid and A. D. Wadsley, High pressure KAlSi30 S , an aluminosilicate with 6-fold coordination, Acta Cry st. 23 (1967) 1093.

(16) A. E. Ringwood, Prediction and confirmation of olivine­spinel transition in Ni2Si04' Geochim. Cosmochim. Acta 26 (1962) 457.

(17) A. E. Ringwood and Merren Seabrook, unpublished ob­servations (1963).

(18) J. H. Whitcomb and D. L. Anderson, Reflection of P'P' seismic waves from discontinuities in the mantle, J. Geophys. Res. 75 (1970) 5713.

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