pyroxenes in meteorites

P Y R O X E N E S I N M E T E O R I T E S

BRIAN M A S O N

~IAsoN, B. 1968: Pyroxenes in meteorites. Lithor 1, 1-11.

Meteoritic pyroxenes, while analogous in many respects to those from terrestrial rocks, show some unique features. Among these are the presence of minerals close to MgSiO~ in composition, the common occurrence of minerals belonging to the clinoenstatite-clinohypersthene series, the rarity of calcium-rich and aluminium-rich pyroxcnes, and the absence of jadeitic varieties. Shock effects are prominent in some meteorites, and have resulted in the inversion of orthopyroxene to clinopyroxene of essentially the same composition. In many chondrites orthopyroxene has been formed from low-calcium clinopyroxene by thermal metamorphism. Coexisting orthopyroxene and diopside in most chondrites and in silicate inclusions in irons indicate equilibration temperatures below 900~

I n t r o d u c t i o n

I'yroxenes are almost ubiquitous in meteorites (Table 1). Among the stones and stony-irons only the pallasites and a few carbonaceous chondrites do not contain pyroxenes, and even the iron meteorites sometimes have pyroxene-bearing silicate inclusions. Meteoritic pyroxenes show less variety in composition than those of terrestrial origin. Apart from ureyite, NaCrSi206, a rare accessory in a few iron meteorites, and titanaugite (containing about 2% TiO2 and 10% A1,O3) in the Angra des Reis meteorite, meteoritic

OH ' OP m ' - - N

.o . . . . . . . A C E F

M~Z $1206 v V V-- V V V "~ FezSiz06

Fig,. I. Pyroxene compositions in meteorites. A: enstatite and clinoenstatite in enstatiie chondrites and enstatite achondrites; B: diopside in the Pena Blanca Spring enstatite aehondrite; C: enstatite in silicate inclusions in irons; D: clinobronzite in the Dingo Pup Donga ureilite; E: orthopyroxene in bronzite chondrites; F: orthopyroxene in hypersthene chondrites; G: diopside in chondrites and in silicate inclusions in irons; t l , I I t : coexisting orthoproxene and diopside in the Shaw chondrite; L: orthopyroxene in achondrites and mesosiderites; M: pigeonite in achondrites and mesosiderites; N: augite in achondrites and mesosiderites; P: diopside in the Nakhla aehondrite.

2 BRIAN MASON

pyroxenes can be plotted within the Mg2SizO6-Fe2Si206-CaMgSi206- CaFeSizO 6 quadrilateral (Fig. 1). Unlike terrestrial pyroxenes, they contain little or no ferric iron, since they crystallized in the presence of free nickel- iron. They contain only small amounts of other elements, such as AI, Na, Cr, and Ti. Meteorites have crystallized at low pressures, as indicated by the presence of tridymite in enstatite chondrites and in many achondrites and stony-irons, and the absence of jadeitic pyroxenes. In effect, meteoritic pyroxenes correspond very well with pyroxenes produced by laboratory synthesis in the CaSiO3-MgSiO:FeSiO3 system. Fairly reliable evidence of their phase relationships should thus be provided by laboratory studies of this system. Most meteorites are low in calcium (the chondrites contain less than 2% CaO, and only the eucrites and howardites, the mesosiderites, and the unique achondrites Nakhla and Angra dos Reis have more than 4% CaO), hence calcium-poor pyroxenes predominate.

Table I. Classification of the meteorites (Figures in parentheses are the numbers of observed falls in each class)

Group Class Principal minerals

Chondrites Enstatite (I l) Enstatite, nickel-iron Olivine-bronzite (227) Olivine, bronzite, nickel-iron Olivine-hypersthene (303) Olivine, hypersthene, nickel-iron Carbonaceous (31) Serpentine, olivine

Achondrites Aubrites (8) Enstatite Diogenitcs (8) Ilypersthene Chasslgnite (I) Olivine Ureilites (3) Olivine, cllnobronzlte, nickel-iron Angrite (1) Augite Nakhlites (I) Diopside, olivine Eucrites and howardites (39) Pyroxene, plagioclase

Stony-irons Pallasites (2) Olivine, nickel-iron Siderophyre (1) # Orthopyroxene, nickel-iron Lodranite (1) Orthopyroxene, olivine, nickel-iron Mesosiderites (6) Pyroxene, plagioclase, nickel-iron

Irons Ilexahedrites (8) Kamacite Octahedrites (30) Kamacite, taenite Ataxites (1) Taenite

* Find.

Although the range of composition of meteoritic pyroxenes is not as great as that of terrestrial pyroxenes, one finds within this range compositions and structures unknown or practically unknown in terrestrial rocks. The principal phase in enstatite chondrites and enstatite aehondrites is enstatite or elinoenstatite, which is more than 98% MgSiO3 (this has proved very useful for laboratory studies of this component, as was shown by Allen et al. (1906) in their pioneer study of the polymorphism of MgSiO3). Another remarkable feature is the frequent occurrence of minerals of the clinoenstatite-elinohypersthene series, contrasted to their almost complete

PYROXENES IN METEORITES 3

absence from terrestrial rocks. Clearly the study of meteorites can extend and complement our understanding of pyroxene phase relations as derived from synthetic studies and petrological experience.

E N S T A T I T E C H O N D R I T E S A N D E N S T A T I T E A C H O N D R I T E S

It is appropriate to begin this review by considering the pyroxenes from the enstatite chondrites and enstatite achondrites, which are close to pure MgSiO 3 in composition, as is shown by the data in Table 2. A significant feature of these data is that the calcium content of clinoenstatite is lower than that of orthoenstatite, a feature also noted in the terrestrial occurrence of clinoenstatite (Dailwitz et al. 1966). Tile phase relations for MgSiO 3 have recently been elucidated by extensive laboratory studies (Fig. 2), and can be applied directly to the phase relations in these meteorites.

Table 2. Composition of pyroxenes from enstatite chondrites and enstatite achondrites

l 2 3

SiO, 59.5 59.4 59.2 MgO 39.3 39.2 39.8 FeO 0.69 0.18 0.02 CaO 0.20 0.74 0.35 MnO 0.14 < 0 . 0 1 0.05 AI20~ 0.28 0.29 0.08 NazO 0.10 < 0.05 n.d.

1. Average of 4 clinoenstatites from enstatite chondrites (Keil 1967) 2. Average of 8 orthoenstatites from enstatite chondrites (Keil 1967) 3. Average of 9 orthoenstatites from enstatite achondrites (Rein & Cohen 1967)

The silicate material in the enstatite chondrites (neglecting a small amount of calcium, prob/tbly less than 1 % CaO) can be plotted directly on the ternary diagram for the system Mg2SiO4-NaAISi3Os-SiOz, the average composition being 55 : 15 : 30 (by weight). When plotted on the phase diagram of Schairer & Yoder (1960) this composition falls almost exactly on the forsterite-protoenstatite cotectic line, with a liquidus temperature of approximately 1500~ The enstatite chondrites comprise a series of meteorites ranging from chondritic types to those in which chondritic structure has completely disappeared, evidently through recrystallization (Mason 1966). In the least recrystailized meteorites the silicate mineral association is clinoenstatite-cristobalite (glass); a little primary forsterite, showing signs of resorption, is present in two of them (Binns 1967 b). This association implies crystallization from a melt (sometimes in the form of spherical droplets) followed by quenching, so that the initially-formed protoenstatite cooled rapidly through the orthoenstatite field without change and then inverted to clinoenstatite a t temperatures below 600~ The formation and crystallization evidently took place at low pressures, judging by the initial formation and survival of cristobalite. The survival of cristobalite and feldspathic glass in these meteorites for nearly five billion years is

4 BRIAN MASON

graphic testimony to the preservation of metastable phases under suitable conditions (in this instance probably a combination of low temperatures and an essentially anhydrous environment).

I i

L ICU ;3

fORSTERITE - + LIqUiD J

1 I

(.3 o

"~ 1o

E �9 le

RHCh!a lC F,.NST,.*~TI T F"

CLINOI~/STATITE

I l I ! ! ' o io 20 ~o . to c,o 6o

P r e s ~ r e , k b

F~g. 2. Phase relations of MgSiO3 (Boyd & England 1965).

In the more recrystallized enstatite chondrites the silicate association is cnstatite-oligoclase-SiO2, the SiO z being usually present as tridymite, rarely as quartz. The most reasonable explanation of the mineralogy and structure of these meteorites is that they are thermally metamorphosed equivalents of the preceding type. The temperature of metamorphism must have been between 600 ~ and 1000~ in order to convert the clinocnstatite to orth0enstatite (Fig. 2). The presence of quartz indicates a temperature of recrystalllzation below 867~ tridymite a higher temperature. Asteroidal- sized bodies are sufficiently large to attain such temperatui'es through .radioactivity~ Their-formation evidently followed closely on the original crystallization of the meteoritic material, and short-lived as well as long- lived radioactMties provided the source of heat.

The silicate composition of ttle enstatite chondrites is similar to that of tile enstatite achondrites (t~xcept that free SiOz is absent in the latter); the structure, however, is very different. All the enstatite achondrites except Shallowater are highly brecciated, consisting of coarse angular fragments of enstatite up to 20-30 mm across occurring in a groundmass of crushed and broken enstatite (Fig. 3); Shallowater is unbrecciated, and contains

I'YROXENES IN METE()P.ITES 5

�9 ~ . a "" " t . . :

~ ' " ,p . "~

. - ~ ~ ~" , . , , ~ ' - . ~

�9 " ' T . t �9

Fig. 3. Porphyroclasts of enstatite, up to 5 mm across, in a matrix of commlnuted enstatite, in the Pena Blanca Spring enstatite achondrite.

Fig. 4. Clinobronzite crystal (0.5 m m long), showing characteristic close-spaced polysynthetic twinning, in the Chainpur chondrite.

Ft~. 5. Clinobronzite gra ins ,untwinned, up to 1 mm across, in the Goalpara ureilite; the groundmass is ~/mosaic of small grains of olivine with interstitial carbonaceous material.

Fig. 6. Clinobronzite, showing coarse polysynthetic twinning, in the Dingo Pup Donga ureilite; the grain in the center is 0.2 m m across.

6 BRIAN MASON

individual enstatite crystals up to 45 mm long. In contrast to the enstatite chondrites, these meteorites contain comparatively little nickel-iron and sulphides. The differences may, however, be merely a matter of degree and intensity of recrystallization; reerystallization at a higher temperature in a somewhat larger asteroid may have been responsible for the large size of the enstatite crystals, and could also have facilitated gravitational separa- tion of metal and sulphide downwards and of a silica-rich feldspathie melt upwards.

In all the enstatite aehondrites the original pyroxene was orthoenstatite. However, Pollack (1966) has shown that in brecciated meteorites this has been partly converted into disordered enstatite and elinoenstatite, evidently by the mechanical deformation accompanying brecciation. This mode of clinoenstatite formation was demonstrated experimentally by Turner et al. (1960). Clinoenstatite in meteorites can be formed in two ways: either by inversion of protoenstatite, or by mechanical deformation of orthoenstatite.

INCLUSIONS IN IRON METEORITES

A few iron meteorites contain silicate inclusions (Mason 1967@ These inclusions are usually granular aggregates of olivine and pyroxenes, with minor amounts of sodic plagioclase. In all these meteorites the olivine and pyroxenes show a narrow composition range and are magnesium-rich, but contain a moderate amount of iron. The compositions of tile coexisting ortho- and clinopyroxenes are illustrated in Fig. 1. These silicate inclusions have considerable paragenetic significance. From the study of the Wid- manst{itten structure of the metal, the cooling rates of several of these meteorites have been estimated at 2-3~ years (Short & Goldstein 1967). This extremely slow cooling rate should have provided adequate opportunity for equilibration between the silicate minerals, specifically between the two pyroxenes. In the Woodbine meteorite the orthopyroxene contains about 3% (by weight) CaMgSi206, the clinopyroxene 94%. Exsolution lamellae were looked for, but not found. Boyd & Schairer's diagram (1964) for the solvus in the system MgSiOa-CaMgSi206 indicates that these compositions represent equiIibrium at or below about 800~176 This suggests that, at low temperatures and in the absence of\rater, diffusion rates within these silicate minerals are effectively zero, even when cooling times of geological magnitude are invoh'ed.

UREILITES

The ureilites form a small and unique group of stony meteorites, classified among the achondrites but not obviously related to any other class of achondrite in either composition or structure~ Among their unique qualities is the presence of diamond of unquestioned extraterrestrial origin, whose structure and textural relations indicate that it was shock-produced from

I'YROXENF~ IN METEORITES 7

graphite (Lipschutz 1962). The silicate minerals are olivine (about Fal0-20) a n d clinobronzite.

In meteorites the customat T compositional division of the MgSiO3-FeSiO3 system, following Prior (1920), is enstatite Fs010, bronzite Fsl0_z0 , hypersthene Fs ~>zu and this nomenclature is followed in this paper.

This clinobronzite, however, is quite different in appearance from clinopyroxene of similar composition in chondrites. In chondrites the clinopyroxenes (Fig. 4) show close-spaced polysynthetic twinning, a feature characteristic also of those produced in the laboratory by quenching the proto-form. In the ureilites the clinobronzite is untwinned (Fig. 5) or occasionally coarsely twinned (Fig. 6) like calcic plagioclase.

In some ureilites the olivine shows no obvious signs of mechanical deformation, but in the Goalpara and North Haig ureilites individual olivine crystals have been converted to a mosaic of small granules. In these meteorites the clinobronzite is completely untwinned. This suggest that the presence of coarsely twinned clinobronzite may perhaps be correlated with lower shock intensity than the completely untwinned form.

Table 3. Chemical and mineralogical variation within the chondrite classes (all figures are weight per cent, except FeO/FcO+MgO, which is ira mole per cent)

Enstatite Olivine-Bronzite Olivine-hypersthene

Pyroxene 50-60 (Fs0)* 20-35 (Fsl~-tg) 25-35 (Fsz0-z~) Olivine none 25-40 (Fat~-.,0)* 35-60 (Fa,,-jz) Nickel 13-28 15-20 1 -10 Oligoclase 0-10 0-10 O-10 Troilite 7-15 ~ 5 N 5 Total Fe 20-35 25-30 19-24 FeO <1 7-12 12-21 FeO/FeO + MgO < 1 15-20 22-32

* Fs=mole per cent FeSiO~, Fa=mole per cent FezSiO~

Presumably the clinobronzite in these meteorites was originally in the ortho-form, although the only direct evidence for this is tile size and shape of the crystals. Untwinned clinopyroxenh has been formed from orthopyroxene in laboratory experiments, for example, by exerting a shearing stress on Bamle enstatite at 30 kilobars in the temperature range 500~176 (Pollack, pers. comm.).

BRONZITE AND HYPERSTHENE C H O N D R I T E S

The most common chondrites are the bronzite and hypersthene chondrites, which comprise over 80~/o of all meteorites seen to fall.

Compared to terrestrial rocks, these meteorites show a remarkably small variability in chemical and mineralogical composition (Table 3). T h e silicate

8 BRIAN MASON

material has essentially peridotitic composition, except that the plagioclase is sodic (about Ans-t5 ). Although so similar in composition, the bronzite and hypersthene chondrites are separated by small but distinct hiatuses in olivine and pyroxene composition, and in total iron content.

The bronzite and hypersthene chondrites show the same trend in structural features as noted for the enstatite chondrites, i.e. a variation from highly chondritic types to those in which chondritic structure has almost disappeared. Van Schmus & Wood (1967) have utilized this trend, together with other features, to establish four groups showing successive degrees of recrystallization. In the least-recrystallized group the pyroxene belongs to the clinoenstatite-clinohypersthene series and is usually variable in composition, as is the olivine. In the more recrystallized groups, orthopyroxene is the dominant pyroxene, and is of uniform composition, as is the olivine.

A typical representative of the least-rccrystallized group is the Chainpur meteorite, described in considerable detail by Keil et al. (1964). Microprobe analyses were made of 193 olivine crystals and 224 calcium-poor clinopyroxene crystals from this meteorite, with the results shown in Fig. 7.

28

24

zo

(J

12 24 �9 36 48

MOL- PERCENT Fo Fe + M~

(J

,=,,

52

28

24

2O

16

12

8 z 4 3'z

MOL-PERcENT F~ F e §

Fig. 7. Composition of 193 individual olivine crystals (a) and 224 individual pyroxene crystals (b) in the Chalnpur chondrite (Keil et al. 1964).

The pyroxene crystals all show the polysynthetic twinning typical of the inverted proto-form. The cutoff of pyroxene composition at Fs26 may have some significance in delimiting the extent of the proto-field. T h i s may help to answer the question posed by Boyd (1966): 'The nature of the polymorphism of the pyroxenes along the MgSiO3-t:eSiO3 join is perhaps the most important unsolved problem in pyroxene phase relations. We know that

PYROXENES IN METEORITES .9

protoenstatite has a field of stability above about 1000~ for pure MgSiO3. How far does it extend into the join ? What is the relation between protoenstatite, elinoenstatite, and pigeonite ? We do not have sufficient experimental data to answer these questions and such data will not be easy to obtain.'

Pigeonites with FeO/FeO+MgO (mole per cent) less than 30 have not been recorded, whereas the meteoritic clinoenstatite-clinohypersthene series appears to cover the 0-30 mole per cent range. This suggests that the proto- form is the phase crystallizing from a melt with up to 30 mole per cent FeO/FeO+MgO, while at higher iron contents pigeonite is the primar T phase. The situation is complicated by the CaO content of pigeonite, which averages about 5 % and does not appear to fall below 4 %. It appears that pigeonite and the clinoenstatite-clinohypersthene series, although structurally related, may be separated by a compositional gap.

The calcium content was measured in each of the 224 pyroxene crystals in the sampled sections of the Chainpur chondrite. This was usually low, 0.1--0.3 %, but higher amounts, up to 2.1 ~ were occasionally recorded. In grains with higher calcium contents, the calcium usually varied from point to point within the grain. This appears to be due to the presence of minute inclusions of diopside, either primary or formed by exsolution.

In the more-recrystallized ehondrites the association is orthopyroxene- diopside. Van Schmus & Koffman (1967) have recently made an extensive study of the compositions of coexisting orthopyroxene and diopside in these meteorites, and their results are incorporated in Fig. 1. They conclude that these results indicate that equilibration of the pyroxenes was established at about 800~ they comment: 'The temperature of 800~ must be interpreted cautiously; it is not considered to represent the maximum temperature experienced by the ordinary chondrites, nor does it represent the true crystallization temperature of the minerals implicated; it is interpreted as the lowest temperature at which Fe 2 and Mg 2 ions possess sufficient mobility to effect equilibration among phases'. This is consistent with the relationship between diopside and enstatite in the silicate inclusions in iron meteorites, and with equilibration temperatures established for the enstatite chondrites. It may be noted that diopside in chondrites usually occurs as discrete grains; it occasionally mantles orthopyroxene, but is not present as exsolution lamellae in the latter mineral. This is consistent with the formation of the orthopyroxene by the inversion of calcium-poor clinobronzite or clinohypersthene.

In this connection the Shaw meteorite, recently described by Fredriksson & Mason (1967), provides an interesting contrast. Shaw is a hypersthene chondrite, but with scarcely a trace of chondritic structure; except for the presence of a little nickel-iron and troilite, and interstitial oligoclase, its structure and composition is similar to that of a terrestrial peridotite nodule. It contains olivine(Faz3), orthopyroxene (Ca6Fel sMg~6),and diopside (C%8Fell Mgs,); the calcium distribution between the two pyroxenes indicates an equilibration temperature above 1 IO0~ much higher than for any other chondrite.

10 BRIAN MASON

It is conceivable that the parent body from which Shaw was derived reached a much higher temperature than other chondrite parent bodies, but tile preservation of the high-temperature calcium distribution seems to require unusually rapid cooling from these high temperatures.

Some of the common chondrites contain secondary clinobronzite or clinohypersthene evidently formed by shock. This is typically found in the so-called black chondrites, the black color being due, not to carbonaceous matter, as in the carbonaceous chondrites, but to finely divided metal and sulphide particles evidently distributed thoughout tile meteorite by the shock. The clinopyroxene is usually subordinate in amount to the orthopyroxene and is difficult to distinguish optically, since it often lacks the twinning characteristic of clinopyroxene produced by inversion of the proto-form.

ACtIONDRITES AND MESOSIDERITES

The achondrites (except the ureilites and enstatite achondrites) show rather close analogies to terrestrial basic and ultrabasic igneous rocks, and their pyroxenes are correspondingly similar. In the hypersthene achondrites orthopyroxene has a narrow composition range (around Fs23 ) and its calcium content is low, generally about 1% CaO; in the Johnstown meteorite the orthopyroxene has 1.39% CaO, and shows a few exsolution lamellac of diopside. Discrete grains of diopside are absent from the hypersthene achondrites.

In the merosiderites, and the eucrites and howardites, pyroxenes with a wide range of composition are present, even in a single meteorite. Many of these meteorites are polymict breccias, (i.e., mixtures of fragments from different parent bodies or different layers of a differentiated asteroid). Pigeonite is a major phase in the eucrites, and a minor constituent in many howardites and mesosiderites (Mason 1967b). It is probably significant that the composition range of pigeonite in meteorites is comparable with that in terrestrial rocks; it contains about 10% Ca in C a + M g + F e , and has a minimum F e / F e + M g percentage around 40. Augite is present as discrete grains in some of these meteorites, commonly as exsolution lamellae in the pigeonite. Binns (1967a) has recently published a diagram showing the range in composition of the pyroxenes in the Shergotty and Zagami achondrites, and his data are incorporated in Fig. 1. Hess & Henderson (1949) i)rovided a detailed description of pigeonite in the Moore County eucrite, and outlined the sequence of crystallization and exsolution of the pyroxene in this meteorite. Many of the eucrites have very similar chemical and mine/'alogical composition;: they consist almost entirely of plagioclase (about Ango ) and pigeonite (average composition about Ca,oFe6sh'Ig2~) with accessory tridymite. This probably corresponds to a eutectoid composition.

PYROXENES IN METEORITF-S 11

ACKNOWLEDGEMENTS. 1 am indebted to F. R. Boyd, R. F. Fudali, G. M. Brox.vn, and s . s . Pollack for reading and commenting on the manuscript. Part of the expenses of my inves- tigations have been defrayed by a grant (NsG-688) from the National Aeronautics and Space Administration.

U.S. National 3Iuseum

IVashington D.C. 20560.

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x r ,~ Scn.xtus, W. R. & KOFF.XI.~,~, D. M. 1967: Equilibration temperatures of iron and magnesium in chondritie meteorites. Science 155, 1009-11.

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Accepted for publication May 1967 Printed December 1967

pyroxenes in meteorites

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