JOUR�AL OF GEOPHYSICAL RESEARCH VOL. 68. No. 15 AUGUST 1. 1963

High-Pressure Phase Transformations in Gennanate Pyroxenes and Related Compounds

A. E. RINGWOOD AND MERREN SEABROOK

Department of Geophysics Australian National Univenity, Canberra

Abstract. Crystal chemical relationships between germanates and silicates are reviewed. It i� .observed that. germanat�s ten� t� behave as high-pressure models for the corresponding SIlIcates. :lccordm .gly, the mvestlgatlOn of pressure transformations in germanates provides valuable mfo�atlo�.

on the proba�le modes of transformation which would be displayed by the correspondmg silicates under hIgher pressures. We have applied this technique to germ­anate pyroxenes in order to infer the behavior of silicate analogs-particularly enstatite. The following germanate pyroxenes were synthesized: MgGeO., MnGeO., CoGeO.; FeGeO. (Mg.,. Ni.,.)GeO., CaMgGe.O •. The related compounds CaGeO. and CdGeO. were also prepared. At 700°C all these compounds were found to transform to much denser phases at pressures which were mostly less than 40 kb. MgGeO. and MnGe03 pyroxenes transformed to ilmenite structures; CoGeO., FeGeO., and (Mg.15Ni ... ) GeO. broke down into orthogermanate spinels + GeO. (rutile structure). The compounds CaGeO. and CdGeO. transformed to garnet structures, and CaMgGe.O, broke down into a mixture of MgGeO. ilmenite + CaGeO. garnet. In the system MgGeO .... MgSiO., pyroxene solid solutions containing between 10 and 50 per cent of MgSiO. broke down into Mg.(GeSi)O. spinels + (GeSi)O. rutile solid solutions. These results are dis­cussed in detail and the probable behavior of MgSiO. at high pressures is inferred with the aid of thermodynamic data. It is concluded that enstatite will probably break down into forsterite + stishovite at a pressure of approximately 120 kb in the temperature range 500° to 200QoC.

1. INTRODUCTION

It is probable that minerals of the pyroxene family are important constituents of the upper­most mantle . At greater depths in the mantle,

below 300 to 400 km, there is strong evidence that the characteristic upper-mantle minerals transform into denser phases. An adequate understanding of the constitution of the deeper mantle therefore requires knowledge of the types of phase transformations which pyroxenes undergo at very high pressures.

We have searched for major phase transitions in common silicate pyroxenes at pressures up to 100 kb, equivalent to a depth of 300 km. Results have so far been negative . It is clear that appa­ratus capable of higher pressures than those developed in our 'squeezer' apparatus will be required if the inferred transitions are to be studied directly. Nevertheless, valuable results can be obtained by indirect methods, using the present apparatus. In particular, by a syste­matic study of the behavior of germanate pyrox­

enes at high pressure, considerable insight is gained into the crystal chemical and thermo­dynamic factors which govern the stability of

pyroxenes generally . This enables us to make

useful predictions about the probable transfor­mation modes of silicate pyroxenes at high pres­sure. Justification for this approach depends

upon an understanding of the special crystal chemical relationships which exist between ger­manates and silicates .

2. CRYSTAL CHEMICAL RELATIONSHIPS BE­

TWEEN SILICATES AND GERMANATES

These relationships were first elucidated in a

classical paper by Goldschmidt [1931J. Both silicon and germanium readily form tetravalent ions which possess similar outer electronic struc­tures and radii (Si* 0.42 A, GeH 0.49A) [Ahrens, 1952, and personal communicationJ. Accord­

ingly the crystal chemistry of silicates is very closely related to that of germanates, particu­larly for oxy-compounds. Corresponding silicates and germanates are usually isostructural, and in the few cases so far investigated they display complete solid solutions with one another. To illustrate this point, a list of isostructural ger­manates and silicates, exhibiting a wide variety of different structures, has been compiled from the literature (Table 1). From these results, it

4601

4602 RINGWOOD AND SEABROOK

TABLE 1. List of Isostructural Silicates and Germanates*

Si Compound Ge Compound

SiO, Ge02 Si02 Ge02 MgSiOa MgGeOa

MgSiO. MgGeOa

CaMgSi,Os CaMgGe20s Mg,SiO, Mg,GeO, Ca2SiO, Ca2GeO. Mn2SiO, Mn2GeO, CaMgSiO, Cn,MgGeO, Ni2SiO, NiaGeO, Co,SiO, Co2GeO, FeaSiO, Fe2GeO, Zn2SiO, Zn2GeO, Be2SiO, BeaGeO, ThSiO, ThGeO, SrSiOa SrGeOa

Li2SiOa LizGeOa Li,SiO, Li,GeO, Na2SiOa Na2GeOa ScSi207 ScGe207 Bi,SiaOl2 Bi,GeoOI2 BaTiSbOg Bn,TiGeaOg NaAlSiaOs NaAlGeaOs KAlSiaOs KAlGeaOa CaAI2Si,Oa CaAl,Ge20s BaAl2Si20a BaAI.Ge20a CaaAhSiaOl2 CaaAI,Gea012 CaaCr2SiaOl2 CaaCr2GesOl2 CaaFe2SiaOl2 CaaFe2Ge3012 NaAlSiO, NaAlGeO. KAlSbOs KAlGe20s 3NaAlSiO,·NaCI 3Nn,AlGeO.·NaCl MgaSi20s(OH). MgaGe,O,(OH), NiaSiaOs(OH). NiaGe20s(OH). MgaSi.010(OH)2 MgaGe,OIO(OH)2

Structure Type

Quartz Rutile Ortho-

pyroxene Clino-

pyroxene Diopside Olivine Olivine Olivine Olivine Spinel Spinel Spinel Phenacite Phenacite Zircon Pseudowol-

lastonite

Thortveitite Eulytite Benitoite Feldspar Feldspar Feldspar Feldspar Garnet Garnet Garnet Nepheline Leucite Sodalite Serpentine Serpentine Talc

* Many of the examples cited in this table were obtained from a compilation by Strunz [1960J.

follows that if a germanate with a new structure should be synthesized there would be a reason­able probability that under some appropriate P-T conditions a corresponding isostructural sili­cate would be stable. In fact, this statement has been repeatedly verified during recent years.

A second important relationship between ger­manates and silicates has also emerged: it ap­pears that germanates often behave as high­pressure models for the corresponding silicates. If a germanate is found to display a given phase transformation at a particular pressure, the

corresponding silicate usually displays the same transformation but at a much higher pressure (not strictly correct for jadeite, see Table 2). The reverse of this relationship has not been observed. Furthermore, in cases where high. pressure polymorphism is displayed by a sili­cate, the corresponding germanate, if it does not display the same polymorphism, is found to display only the structure of the high-pressure silicate polymorph. Examples of this relation­ship are given in Table 2.

The reason for this behavior has been indi­cated by Bernal [1936] and Wentorf [1962]. The structures of ionic compounds are largely governed by the radius ratios of constituent ions. This applies particularly to oxide compounds in which the critical parameters are usually the radius ratios between the small cations and the large oJ..1'gen anions. When a silicate or germa­nate is subjected to high pressure, the large oxygen ions tend to contract relatively more than the small Si<+ and Ge4+ ions; hence the radius ratios Rs,/Ro and RGe/Ro increase. Trans­formation into a new high-pressure phase occurs when these radius ratios attain some critical value. Since the zero-pressure radius of Ge'+ (0.48 A) is already slightly larger than that of SiH (0.42 A), germanates require smaller pres­sures to achieve the critical radius ratios reo

quired for given transitions than the correspond­ing silicates do. Alternatively, because of their initially higher radius ratios, germanates may crystallize at zero pressure in a structure which is only attained by the silicate at high pressure.

For these reasons, the study of germanates as high-pressure models of silicates offers us the possibility of obtaining useful information about phase transformations which may occur in sili­cates at pressures beyond the range of currently available experimental techniques. This is the justification for the basic approach employed in the present investigation. A preliminary sum·

mary of some of the experimental results to be described in this paper has been published [Ringwood and Seabrook, 1962a].

3. ExPERIMENTAL PROCEDURE

A number of germanates possessing the gen­eral formula MGeO. (where M = Mg, Mn, Co, Fe, Ni, Ca, and Cd) have been synthesized using solid-state reaction or fusion techniques. Details

of the syntheses are given in the separate sec-

PHASE TRANSFORMATIONS IN PYROXENES 4603 TABLE 2. Oomparative Stabilities of Isostructural Germanate and Silicate Phases

Stability Oon- Stability Oon-Structure Germanate ditions Silicate ditions

Type Oompound P, kb Oompound P, kb Referencest

Rutile Ge02 P = 0, �1007°0 Si02 { �120 kb, 1000°0 1,2,3

Oa3A12Ge3012 �105 kb, 530°0

Garnet P = 0, "'1000°0 Oa.AhSiaOI2 �15 kb, 800-1000°0 4,5 , Spinel Ni2GeO, P = 0, 650°0 Ni2SiO. �18 kb, 650°0 6

Spinel 002GeO, P = 0, 700°0 002SiO. �70 kb, 700°0 7 Spinel Fe2GeO. P = 0, 800°0 Fe2SiO. �38 kb, 600°0 8 Spinel Mg2GeO. P = 0, 600°0 Mg2SiO. �130 kb,t 600°0 9, 10 Jadeite* NaAlGe20 6 �12 kb, 600°0 NaAlSi206 �11.2 kb, 600°0 11,12

* The differences between pressures required for stability of germanium and silicon iadeites are much smaller than the experimental uncertainty range.

t Extrapolated. t 1. Robbins and Levin, 1!l5rl; 2. Stishov and Popova, 1961; 3. Ringwood and Seabrook, 1962a; 4. Tauber

et aI., Ifl58; 5. Pistorius and Kennedy, 1960; 6. Ringwood, 1962; 7. Ringwood, 1963; 8. Ringwood, 1958; 9. Dachille and Roy, 1960; 10. Ringwood and Seabrook, 1962b; 11. Dachille and Roy, 1962; 12. Robertson et al., 1957.

tions on results. The starting compounds were

examined by microscope and X-ray diffraction. Samples so prepared were subjected to various desired pressures in a uniaxial pressure appa­

ratus using techniques which we have previously described [Ringwood and Seabrook, 1962b]. Runs usually lasted 1 hour at a temperature of 700°C. Where necessary, mineralizers such as

water and/or NH.Cl were used to facilitate reac­tions. After completion of a run, the sample was quickly cooled and examined by microscopy and X-ray diffraction.

4. RESULTS

(a) MgGeO.. We have previously described results of high-pressure experiments upon this compound [Ringwood and Seabrook, 1962cJ .

The product of complete solid-state reaction of MgO and GeO. in 1: 1 ratio had the ortho­

pyroxene structure . At pressures of 5 kb and above, the orthopyroxene transformed com­pletely into clinopyroxene. At pressures above 28 kb, transformation into a new phase which possessed the ilmenite1 structure was observed.

Unit cell dimensions, referred to hexagonal axes, of the ilmenite phase were a = 4.936 A and c = 13.76 A. These values imply that the ilmenite

1 In the reference previously cited, we called this the corundum structure. Use of the term 'ilmenite structure' is preferable, this structure be­ing essentially an ordered corundum structure.

phase is 15.4 per cent denser than the ortho­pyroxene.

(b) MnGeO.. A sample of MnGeO. was

prepared by intimately mixing Mn.O. and GeO.

in the correct proportions, forming it into a tab­let, and sintering it in air at 1100°C for 4 hours. An X-ray diffraction photograph revealed that MnGeO. so prepared possessed the orthopyrox­ene structure with unit cell dimensions a =

9.268, b = 19.325, and c = 5.478 A. A sample of this material was ground and charged into a platinum tube which was held in an induction furnace at 1600°C (approximately) for 5 min­utes, after which it was rapidly quenched in water. The product had clearly melted but had not quenched to a glass. Optical and X-ray

analyses revealed comparatively large crystals of orthopyroxene.

Samples of MnGeO. orthopyroxene so pre­pared were moistened with water and run in the squeezer at 700°C under pressures of 5, 15, 20, 25,30,40, and 60 kb. In the 5-kb run, the ortho­pyroxene was unchanged. Between 15 and 25 kb (inclusive) complete transformation to clino­pyroxene was observed. In the 30-kb run, traces of a new phase were observed. This new phase became more abundant in the higher-pressure runs. In the 60-kb run, conversion was about 70 per cent. The new phase had the ilmenite struc­ture, with unit cell dimensions (referred to hex­agonal axes ) a = 5.013 and c = 14.32 A. The behavior of MnGeO. orthopyroxene under high

4604 RINGWOOD AND SEABROOK

pressure thus closely resembles that of MgGeO.. (Ni.5Mg.5) GeO. was inhomogeneous, consisting Both orthopyroxenes transform first into a clino- of pyroxene, spinel, and glass. However the com­pyroxene structure, and then, at about 30 kb, position (Ni.,.Mg.75) GeO., at 1200°C, yielded a into an ilmenite structure. The density increase homogeneous pyroxene.

associated with the transition in MnGeO. is 18 This was moistened and run in the squeezer per cent. at 700°C and at pressures of 10, 20, 25, 30, and

(c) CoGeO,. This compound was prepared 40 kb. At 10 and 20 kb, clinopyroxene was ob­by intimately mixing Co.O. and GeO" in the re- served. At 25 kb and above, complete or partial

quired proportions, forming it into a tablet, and decomposition of the pyroxene according to the sintering it in air at 1230°C for 24 hours. reaction

CoGeO. so prepared had the clinopyroxene struc-

ture. Samples were moistened with water and 2(Mg.75Ni.25) GeOa nm in the squeezer at 700°C and at pressures

of 8,13, 18,23, and 50 kb for 1 hour. No change

was observed in the 8-kb run. At higher pres­

sures, however, partial or complete conversion

into Co,GeO. spinel and GeO. (rutile) was ob­

served. It is clear that CoGeO. decomposes under

high pressure according to the reaction

2CoGeO. (pyroxene)

� C02Ge04 (spinel) + Ge02 (rutile)

The breakdown is accompanied by a density in­

crease of approximately 11 per cent .

The behavior of CoGeO. thus differs funda­

mentally from that of MnGeO. and MgGeO •.

(d) FeGeO.. This compound was prepared

by intimately mixing GeO., Fe.O., and Fe (powder) in the required proportions, and forming it into a tablet. The table t was wrapped

in platinum foil, sealed in an evacuated silica

tube, and heated at 850°C for 24 hours . FeGeO.

so prepared had the clinopyroxene structure. Samples were moistened with water and run at 700°C at a variety of pressures. At pressures below 10 kb, no change was observed. Above 10 kb, complete breakdown into FeaGeO. (spinel)

+ GeO. (rutile) occurred.

2 FeGeO. (pyroxene) � Fe2Ge04 (spinel) + GeO. (rutile)

Density increase associated with this breakdown was about 11 per cent .

(e) The system MgGeO,-NiGeO,. Several attempts to prepare a compound v",ith the com­position NiGeO. were unsuccessful. In each case , the assemblage Ni.GeO. (spinel ) + GeO. (quartz or rutile) was observed. Intermediate compositions between NiGeO. and MgGeO. were next investigated. At 1200°C, the composition

pyroxene

� (Mg.nNi.25)2GeO, + Ge02 spinel rutile

was observed. The behavior of the pyroxene forms an interesting contrast to that of pure MgGeO., which transforms directly to the ilme­nite structure under similar conditions. The den­sity increase accompanying the above break­down is approximately 10 per cent .

(f) The system MgGeO.-MgSi03• Having investigated the behavior of MgGeO. pyroxene under high pressure, the next step appeared to be an analogous study of pyroxene solid solu­tions of MgGeO. with MgSi03• We have con­ducted more than 80 runs in this system, but the results have been only partially definitive. The chief difficulty has been the high tempera­ture which was required in order to attain or approach equilibrium. It was found necessary to conduct most experiments at 700°C. At this temperature, the pistons deform at pressures above 60 kb. Although substantially higher pres­sures can be attained, they can be estimated only roughly, and consequently it is not possible t{) present a normal phase diagram. N everthe­less, some important qualitative phase relation­ships have been established.

Starting materials consisted either of homoge­neous pyroxene solid solutions formed by appro­priate solid-state reaction of oxide components, or of glasses formed by melting these pyroxenes in sealed platinum tubes in an induction heater, followed by quenching. It was found possible to produce glasses in the composition range MS,oo to MS215MG75 (MS = MgSi03, MG = MgGeO.). However, in the range MS""MG7• to MG,oo, quenched liquids crystallized directly to py-roxene .

PHASE TRANSFORMATIONS IN PYROXENES 4605

Various mineralizers were mixed with the samples to be run in order to facilitate reaction at the lowest possible temperature. The use of water was precluded because it caused the for­mation of hydrous phases, principally talc. The addition of 5 to 10 per cent of NH.Cl to the sample was often found to be effective. Devitrifi­cation temperatures of glasses were sometimes lowered by seeding them with 0.1 per cent of colloidal platinum .

Principal results obtained in this system may be summarized as follows:

(i) Pyroxene solid solutions in the composi­tion range MG,,,,, to MG.7MS. can be trans­formed directly to the ilmenite structure at 700°C and pressures greater than 60 kb. The small amount of Si<+ appears to be in solid solu­tion in the il menite.

(ii) In the composition range MG.aMS,o to MG",MS50, behavior at high pressures is funda­mentally different. Above 50 kb at 700°C, the following breakdown apparently occurs :

pyroxene or glass

spinel solid solution

+ (GeSi)02 (1) rutile

solid solution

The beginning of this breakdown is indicated by the presence of rutile solid solution in the X-ray diffraction pattern. The amount of rutile formed increases with pressure. In the early stages, the complementary spinel cannot be observed be­cause of line overlap with untransformed pyrox­ene. However, at higher pressures, where the degree of transformation is more complete, co­existing spinel and rutile solid solutions can be identified, and they show clearly that transfor­mation is proceeding according to (1). This transformation has not yet been followed to completion. Even at the highest pressures, some untransforrned clinopyroxene remains. All the phases involved in (1) are solid solutions and it is likely that the residual pyroxene is a silicon­rich variety. We have observed that the pres­sure required to initiate breakdown according to (1) generally increases with increasing silicon content in the starting material.

(iii) At the highest pressures obtainable,

probably about 90 kb, the spinel phase becomes

unstable and is replaced by an ilmenite phase. The following reaction apparently occurs :

Mg2( G eSi)04 + (GeSi)02 � 2Mg( GeSi)Oa

spinel rutile ilmenite

We have observed a related breakdown in Mg,GeO. spinel at 570°C and pressures above 90 kb into MgGeOa (ilmenite) plus a new phase which appears to have the composition 5MgO GeO.. (This phase is definitely not the com­pound 4MgO GeO. described by Robbins and Levin [1959].)

(iv) In the composition range MG,,,,, to MG",MSso, the low-pressure phase is orthopy­roxene. At higher pressures , this transforms to clinopyroxene. In M G,00, this transformation re­quires less than 5 kb at 700°C. However, in the composition range MG.;MSlO to MGISf)MS.o trans­formation of orthopyroxene to clinopyroxene re­quires higher pressures, ranging between 20 and 40 kb. It is interesting that clinopyroxene is the stable high-pressure phase in the germania-rich compositions. The reverse relationship has been observed for pure MgSiO., where orthopyroxene is the stable high-pressure phase [Boyd and England, 1961].

(g) CaGeO,. This compound was prepared by heating a tablet formed from the intimately mixed oxides at 1430°C for 3 hours. The X-ray diffraction photograph resembled that of wol­lastonite, and these two compounds may be isomorphous. The mean refractive index of CaGeO. was 1.72, which implies tetrahedral co­ordination for the Ge'.

Samples were run at 700°C and at pressures of 40 and 70 kb. Complete conversion to a new phase which possessed the garnet structure was observed in both runs. The lattice parameter of the garnet was 12.43 A.

This type of transformation is of interest from a crystal chemical point of view and, to the be�t of our knowledge, has not been pre­viously described. The general formula for a

garnet is A.VIIIB.V'C.'VOu where the superscripts denote the coordination of specified ions with respect to oxygen. To write the compound CaGeO. with a garnet formula, we require four 'molecules' of CaGeO., and we obtain Ca.VIIl (CaGe) V1Ge,xv01.2. Accordingly, the transforma­tion implies that one in four of the Ca ions

4606 RINGWOOD AND SEABROOK

moves from eightfold coordination into sixfold, and similarly one-quarter of the germanium ions move from fourfold to sixfold coordination. These changes in coordination are consistent with the known crystal chemical properties of these ions.

(h) CdGeO,. This compound was prepared by heating a tablet formed from the intimately mixed oxides at 1200°C for 5 hours. The X-ray diffraction pattern was most complex and could not be readily compared with any other ana­logous compound. The mean refractive index of CdGeO. (1.89) was similar to that of MnGe03 orthopyroxene (1.87). This suggests that the Ge in CdGeOa is in tetrahedral coordination. Octa­hedral coordination would bo expected to result in a substantially higher refractive index.

Samples were tested at 700°C and at a variety of pressures from 10 kb upward. In all cases, complete transformation into a garnet-like struc­ture was observed. The reflections were split , in­dicating that the symmetry was lower than that of a truo garnet. The pseudo-cubic lattice pa­

rameter was 12.4 A. The formula of the garnet modification of

CdGe03 would be Cd3vIIJ (CdGe) VIGe3IVO" as in the case of CaGc03 garnet.

(i) CaMgGe,O.. This compound was first synthesized by Goldschmidt [1931]. We pre­pared it by sintering the intimately mixed oxides at 1350°C for 6 hours, then by grinding, melt­ing, and regrinding.

A sample was run at 60 kb and 700°C, but no change was observed. However, at a substan­tinily higher pressure, perhnps in the vicinity of 80 kb,2 and at 700°C, complete brenkdoWIl of the diopside to n mi:..:ture of MgOeO" (ilmenite) and CaOe03 (garnet) wns observed.

CaMgGe20s (diopside)

� l\IgGeOa (ilmenite) + CaGeO, (garnet)

The interplanar spacings of the ilmenite and garnet phases differed slightly from those of the corresponding pure MgGe03 and CaGeO. phases. This wns probably caused by the presence of small amounts of Ca and Mg in solid solutions in the ilmenite and garnet, respectively.

2 The pressure was not accurateJy known be­cause of piston deformation.

5. DISCUSSION

A summary of experimental results is given in Table 3. The first point to note is that all the germanate pyroxenes which we have inves, tignted transform to much denser phases at compnratively low pressures. Bearing in mind the close crystal chemical relationships between germanates and silicates and the fact that germ­anates behave as models for silicates , as dis­cussed in section 2, we might reasonably expect that silicate pyroxenes would undergo analogous transformations nt higher pressures.

All of the transformations in germanate pyrox­enes involve either a complete or partial change in coordinntion of the germnnium ions from fourfold to sixfold. In pure GeO", this change in coordinntion occurs at atmospheric pressure and at a tempemture of 1007°C [Robbins and Levin, 1959J. However , in complex germanium oAy-compounds, germanium almost invariably occurs in tetrahedral coordination at atmos­pheric pressure. Of more than 50 germanates (excluding hydroAyl bearing varieties) which possess kno\vn structures, there is only one3 ex­ample known to the authors in which germanium occurs in octahedral coordination, namely La. MgGeO", which possesses the perovskite struc­ture [Roy, 1954]. It appears that the combina­tion of a bnsic oxide with GeO, to form a

germanate tends to stabilize germanium in tet­rahedml coordiDfltion. There are straightfor­wnrd reasons for this behavior which have been well treated in general terms by Weyl [1951, 1956].

Bccnuse of the relative stabilization of GeO;" groups in germnnates as compared with GeO" higher pressures are required to cause change in Ge coordination from 4 to 6 in germanates than in GeO,. This generalization may reason­ably be applied to silicates. It implies that high­pressure transformations in silicates which lead to octahedral coordination for silicon are likely to require pressures which are higher than those required for the 4 to 6 transformation in pure SiO,. Thus the equilibrium curve for the coesite -stishovite transition will constitute a lower-

3 Dachille and Roy [1960] have claimed that Mg,GeO .. spinel is inverse, with Ge4 in octahedral coordination. However, further studies by P. Tart.e

(personal communication) have not supported thIS claim.

PHASE TRANSFORMATIONS IN PYROXENES 4607 TABLE 3. Results of High-Pressure Experiments upon Germanate Pyroxenes and Related Compounds

at 700° ± 10°C

Trans-

formation Initial Pressure,

I Compound Structure kb

jlgGeO, Orthopyroxene 28 ± 3 IlnGeOs Orthopyroxene 25 ± 5 FeGeO, Clinopyroxene 10 ± 3 eoGeOa Clinopyroxene 10 ± 3 i�lg. 7sNi.2s)GeOa Clinopyroxene 22 ± 3 Ilg(Ge. 97Si. oa)Oa Orthopyroxene ...... 60 )fG9I1MS,O to Orthopyroxene

MG60MS.o or Glass 50 C�GeOa Wollastonite (?) ...... 40 CdGeOs ? 10 CaMgGe20s Diopside ...... 80

pressure limit for 4 to 6 transitions in silicates. This curve runs from approximately 105 kb at 530°0 [Ringwood and Seabrook, 1962a] to 120 kb at 12000e [Stishov and Popova, 1961, re­l�,ged P scale].

Possible transformations in magnesia-rich illicate pyroxenes, e.g. enstatite, may now be considered in the light of these generalizations.

,iVe do not expect enstatite to break down below the stishovite stability region. Indeed, the nega­tive results of our experiments on enstatite sta­bility to approximately 100 kb at 6000e support this expectation. Glasses of enstatite composi­tion invariably crystallized to pyroxene in our experiments.

From Table 3 we observe two principal modes of transformation for germanate pyroxenes at high pressure. They may either transform di­rectly to the ilmenite structure, or they may break down into a denser mixture of ortho­germanate + GeO. rutile. These results suggest

. that enstatite might break down analogously. The transformation which will be followed by Instatite depends largely upon the free energy of the following reaction at zero pressure:

2MgSiOa = Mg2SiO, + Si02 (2)

If the free energy is small, the direction of this ';reaction will be governed by the P'Av term. rhus, if assemblages on the right-hand side are

substantially denser than those on the left, the

In rrease in Density

Due to Phase Trans .

(approx.), Reaction Products %

MgGeOa ilmenite structure 15 MnGeOa ilmenite structure 18 F�Ge04 spinel + Ge02 rutile 11 C02GeO, spinel + Ge02 rutile 11 (Mg. nNi.2s )2GeO. spinel + Ge02 rutile 10 Mg(Ge.97Si.oa)Oa ilmenite 14

Mg2(GeSi)O. spinel + (GeSi)02 rutile CaGeOa garnet

15

CdGe03 garnet MgGeOa ilmenite + CaGeOa garnet

reaction will run to the right at high pressure. On the other hand, if the free energy for (2) is large and positive at low pressure, MgSiO. will resist decomposition and is therefore likely to transform directly to an ilmenite structure with the same formula.

For enstatite, the relevant thermodynamic data are available (Table 4). Enstatite is only slightly more stable at room temperature than the equivalent of forsterite + quartz. Further­more, the relative stability of enstatite decreases with increasing temperature, and above 15000e forsterite + quartz becomes the more stable association. The incongruent melting of enstatite to form forsterite plus a siliceous liquid is a

consequence of these thermodynamic relation­ships.

Accordingly, it is probable that enstatite will

TABLE 4. Free Energies of Formation* of Enstatite from Forsterite and Quartz

T, OK r:..G, cal

298 -2208 1000 - 1 523 1500 -183 2000 2046 2500 5277

* Thermodynamic results from compilation of data by MacDonald [1954].

4608 RINGWOOD AND SEABROOK

break down according to (2) at high pressure. This has previously been pointed out by Ring­wood [1958]. The compound FeSiO. is ap­parently unstable at all temperatures compared with Fe"SiO. and quartz. The effect of iron in solid solution as in natural orthopyroxenes would therefore be to lower the breakdown pressure.

The above inferences on the behavior of en­statite at high pressure are strongly supported by the results of our experiments on Mg (GeSi) O. pyroxene solid solutions (section 4f). Germania­rich pyroxenes containing less than 3 per cent of MgSiO. transform to the ilmenite structure at high pressure. HO"wever, in pyroxenes which contain as little as 10 per cent of the MgSiO.

component, the mode of high-pressure break­down is radically changed to

2Mg( GeSi)Oa --T Mg2( GeSi)04 + (GeSi)02

pyroxene spinel rutiJe

This behavior has been observed in the com­position range MG,oMS,O to MG5f)MS,o and could doubtless be followed into more silica-rich com­positions with apparatus capable of attaining more extreme P-T conditions than the squeezer.

In pure MgSi03, three alternative modes of high-pressure breakdown according to (2) are possible.

{(i) forsterite + stishovite enstatite (ii) Mg2Si04 (spinel) + coesite

(iii) Mg2Si04 (spinel) + stisho\"ite

Which of the first two breakdowns would be preferred depends upon the relutive pressures required for the coesite-stishovite transition compared with the Mg,SiO. olivine-spinel tran­sition. Current evidence [Stishov and Popova, 1961; Ringwood and Seabrook, 1962a, c, and unpublished] rather strongly indicates that sti­shovite will become stable at lower pressures than Mg,SiO. spinel . Accordingly, breakdown in favor of forsterite + stishovite (reaction i) is likely to be preferred to reaction ii. The pres­sure required for this breakdown can be ap­proximately calculated from the zero-pressure thermodynamic data of Table 4 combined with available P-T-V data for quartz-coesite and coesite-stishovite transitions . It was found that enstatite would break down into forsterite + stishovite around 120 kb and that the break-

down pressure is relatively insensitive to tem­peratures in the range 500° to 2000°C. This pressure is probably below that required to pro­duce Mg,SiO. spinel. Consequently, enstatite is likely to break down according to reaction i above, rather than to reaction iii. The various sources of experimental uncertainty are such however, that this inference can hardly be re� garded as final.

At extreme pressures, an ilmenite form of MgSi03 may wel! become stable [RingWood, 1962]. This could be formed by the high-pres­sure breakdown of Mg,SiO, (spinel) according to the equations

Mg2Si04 + Si02 --T 2 MgSi03 D.v = -1.7cc (spinel) (stishovite) (ilmenite)

Mg2Si04 --T MgSi03 + MgO D.v = -2.7cc (spinel) (ilmenite) (periclase)

The experimental results discussed in section 4f(iii) strongly support the possibility of such reactions. We have also observed an analogous breakdown in Mn,GeO. (olivine ) at 700°C and approximately 90 kb:

Mn2 Ge04 --T Mn Ge03 + MnO ( ?)

(olivine) (ilmenite) (manganosite)

The evidence discussed in this paper has strongly favored a particular mode of break­down for enstatite and enstatite-rich pyroxenes. It must be pointed out that the evidence does not finally prove that enstatite will in fact break down in this manner. This can only be estab­lished by direct experiment on enstatite in the required high-pressure range. The evidence pre­sented herein is of such a nature that it sets an

upper limit to the stability of enstatite. This would not prevent enstatite from transforming to an entirely new and unexpected phase at some pressure below this limit. If, however, such an unforeseen transition does not occur, en­statite will break down to forsterite + stishovite. The results of our direct experiments upon germ­anate pyroxenes and in the system MgGeO,­MgSiO. suggest that the probability of an ell­tirely new type of phase transition is rather low.

Acknowledgments. We are indebted to Dr. D. H. Green and Professor S. P. Clark, Jr., for critical review of the manuscript.

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