THE ADELAIDE METEORITE

Transcription

1 THE ADELAIDE METEORITE R. Davy* and S.G. Whitehead Australian Mineral Development Laboratories (A mdel) Flemington Street, Frewville, South Australia 5063 G. Pitt Department ofmines Greenhill Road, Eastwood, South Australia 5063 A preliminary description and chemical analysis of the newlydiscovered Adelaide meteorite is given. This meteorite is a C2 chondrite with C3 affinities. However, the meteorite is unlike both C2 and C3 chondrites in having a CalAI ratio of The subtype is ambiguous but is closer to the Vigarno than the Ornans subtype. The meteorite is unusual in that it contains primary magnetite. In many other respects this meteorite resembles the Allende meteorite. INTRODUCTION In June 1972 an unlabelled specimen of rock which had been in the possession of the South Australian Department ofmines for several years was submitted to the Australian Mineral Development Laboratories by the Department for examination as a possible meteorite. An initial petrographic examination confirmed the meteoritic nature of this specimen and the Department of Mines then made strenuous efforts to identify the collection site; however, no record of origin could be found. It was considered likely that it had been picked up in the south-east of South Australia, because the specimen had long been considered to be basaltic, possibly derived from the Late Tertiary-Quaternary volcanoes of the Mount Gambier-Millicent area. In the absence of a recognised location of discovery for this meteorite, it is proposed that it be called the Adelaide meteorite. The specimen was microscopically examined in thin and polished-thin section and this was supplemented by the use of a Jeol electron probe micro-analyser where appropriate. Probe analyses were carried out by using an enlarged beam of 10 micrometres diameter, generated by an accelerating voltage of 15 kv and giving a specimen current of 0.08 microamps. Standards used included chemically analysed olivine and pyroxene and results were computer-corrected for mass absorption, secondary fluorescence and atomic number effects. Chemical analyses were carried out by the Analytical Chemistry Section of the Australian Mineral Development Laboratories using conventional *Currently of West Australian Geological Survey, Perth, Western Australia, Meteoritics, Vol. 13, No.1, March 31,

2 methods for the determination of silicate components. Iron was determined by reaction with mercuric chloride and subsequent titration with potassium dichromate. Total sulphur was determined by evolution of the sulphur as oxides which were collected and titrated. Carbon dioxide from carbonate was determined by acid evolution, the CO 2 evolved being collected and weighed. Carbon was determined by conversion to CO 2, collection and weighing of the CO 2 and subtraction of the CO 2 due to carbonate. Trace element determinations were made by comparative emission spectroscopy. The compositions of various silicate and sulphide phases present were determined by electron probe microanalysis. Specific gravity was determined conventionally using a water immersion method. The remainder of the meteorite is at present held by the South Australian Department of Mines. DESCRIPTION Morphology The meteorite is of irregular, subangular, polyhedral shape (Figs. I and 2). It is bounded by nine almost flat surfaces of different sizes. The longest dimension (diameter) is approximately 12 cm and the shortest approximately 9 em. The edges and corners of the meteorite have been rounded by passage through the atmosphere. The meteorite was received in two pieces, comprising about 90% of the complete stone. The larger piece weighed 1931 g; this piece had a specific gravity of 3.2. The weight of the smaller piece was not determined but was probably about 100 g: this piece alone was used for mineralogical and chemical analysis. The fractured surface of the meteorite shows a discontinuity between the core of the meteorite and a weathered skin which is 4 to 5 rnm thick. Separate analyses have been made of the weathered skin and of the core (Table 3). The broken surfaces of the meteorite are patchily covered with a thin layer of white powder. This is presumed to be a sulphate formed by weathering of sulphides. Petrography The meteorite is characterized by the presence of large well-formed chondrules up to 1.5 mm in diameter together with irregular aggregates, or "spongy chondrules," and angular fragments of single crystals in a dark brown, almost opaque, isotropic matrix (Fig. 3). The texture closely resembles that of the Vigarano sub-type of chondrite illustrated by Van Schmus (1969a). 122

3 Most chondrules, both whole and broken, are between 0.1 and 1 mm in size, averaging mm, but there are also a few larger examples. The majority are composed of olivine, either as an aggregate of small euhedral to subhedral crystals with or without glass or as "barred" olivine composed of parallel plates of olivine in optical continuity, with interstitial glass Or divitrified glass. This latter type commonly has a complete rim of olivine. Other chondrules consist of granular olivine with minor clinopyroxene, of single olivine crystals (most of these are "half-moon" broken chondrules), of low calcium clinopyroxene and of feldspar. In a few chondrules feldspar occurs with clinopyroxene and, very rarely, forms almost the whole of the chondrule. There is a complete range from spongy chondrule to aggregate material, with the form ranging from sub-ovoid to highly irregular (Fig. 3). This material is also composed mainly of olivine but commonly contains droplets of opaques (metal, sulphide or oxides), small amounts of pyroxene and patches of very fine-grained, optically unidentified material, possibly once glass. Some aggregates have been penetrated by the matrix. Chondrules and aggregates together constitute about 65% of the meteorite. Large and small chondrules and aggregates are randomly packed and most are separated from each other by films of matrix at least 0.01 to 0.05 mm thick. The matrix is so fine-grained that it cannot be fully resolved even at a magnification of looox. It contains abundant iron oxides, probably including both magnetite and goethite. The latter possibly has formed during either alteration or weathering and is more common in the "rim" region. The matrix also contains scattered minute droplets of sulphide. Goethite occurs along numerous very small fractures in the aggregates of silicate minerals and chondrules (Fig. 4) and it has also replaced many of the droplets and larger aggregates of sulphide (Fig. 5). Magnetite clearly has been recognised in association with troilite. MINERALOGY Olivine Olivine constitutes 40 to 50% of the meteorite; it occurs in chondrules mainly as crystalline aggregates and, less commonly, as single crystals. Most of the single crystals are angular fragments but some have one "round" side, indicating a chondrule origin, and a few are euhedral crystals. In some chondrules crystalline aggregates of olivine contain minor interstitial pyroxene which crystallized after the olivine. A few chondrules are composed of barred olivine and one was found with intersecting sets of olivine bars. Interstices in many crystalline aggregates of olivine and also in barred olivine 123

4 Fig. 1 View of the Adelaide meteorite showing its approximately hexagonal outline. In this photograph it is sitting on a flat base and the upper irregular surface represents a fracture surface, the missing fragment having been used for most of the work described in this paper. Scale in mm,,20mm Fig. 2 Side view of the Adelaide meteorite. Scale in mm, 124

5 Fig. 3 Thin section (transmitted light) showing chondrules and broken fragments of various shapes in a matrix which is almost completely opaque. Chondrules and fragments are composed predominantly of olivine and contain droplets of opaque material now composed of goethite, magnetite and remnants of sulphide. Fractures contain films of goethite. 125

6 Fig. 4 Polished section showing small octahedral crystals of magnetite (m) in an interstice in a barred olivine (01) chondrule, and associated with an olivine fragment. The small light grey veins are goethite. 126

7 Fig. 5 Polished section showing an elongate mass of magnetite (m) within a chondrule composed of olivine (01) and pyroxene. The magnetite partly encloses a mineral, which was probably once troilite, but which is now goethite (g). 127

8 contain glass which varies in colour from colourless to purple-grey, and also to dark brown or green. Most of the olivine crystals contain small opaque inclusions which vary in shape from globular to irregular and elongate droplets. Most of these inclusions were composed of varying proportions of sulphide and magnetite but are now composed largely of goethite with remnants of sulphide and some unaltered magnetite. There also are a few metallic globules which are much smaller and also much less abundant than the sulphide droplets. The cores of twenty grains of olivine were analysed by the electron probe with the following results (Table I): Table 1 Electron probe microanalyses of olivine Number of Ions on basis of Analysis FeO* MgO sio, Total 4 Oxygen Atoms Mol % No. % % % % Fe Mg Fe+Mg Si Fayalite L LOa La ~ L o.di LOO 47.5 *Total iron as FeO. Analyst: P.K. Schultz, Australian Mineral Development Laboratories. The composition of olivine in this meteorite is therefore very variable, indicating that the meteorite is non-equilibrated. The olivine is apparently unaltered, but in places has been fractured. The fractures are commonly sealed with goethite. Some of the glassassociated with olivine in chondrules was analysed by the electron probe. The colourless glass was found to be predominantly 128

9 aluminium and calcium silicate with very little (0.2%) iron and very little magnesium. The brown glass was found to have slightly higher concentrations of iron (up to 2% Fe) and magnesium than the colourless glass, but to be otherwise similar. Pyroxene Pyroxene is associated with olivine in a significant but subordinate proportion of chondrules and there are a few chondrules which are composed almost entirely of pyroxene. Pyroxene constitutes 10 to 15% of the meteorite overall. When pyroxene is intergrown with olivine, most of it occurs in interstices between olivine crystals, but in some chondrules there is coarser-grained pyroxene which contains small inclusions of olivine. Some pyroxene-rich chondrules contain euhedral to subhedral crystals. The pyroxene is colourless and most of it shows polysynthetic twinning. The maximum extinction angle (Z 1\ cleavage) recorded was 28. Nineteen grains of pyroxene were analysed by the electron probe and these gave compositions consistent with (clino-) enstatite as follows (Table 2). No determination of manganese was attempted; nickel was found to be absent (at a detection limit ofapproximately 0.2%). Most pyroxenes were of a low calcium, low iron type with the molecular proportion of ferrosilite less than 2.5%. However, three grains contained up to 11% ferrosilite. Calcium contents also varied; three-quarters ofthe grains analysed contained less than 1%CaO. The proportion ofthe ions suggests that aluminium is more related to the cations than present as a replacement of silicon. The composition is that of enstatite, and as the extinction angle is appreciable the mineral may be called clino-enstatite. Binns (1970) has discussed extinction angles ofmeteorite pyroxenes, relating changes of crystal system and extinction angle to the meteorite cooling history. The presence of the "clino" pyroxene supports the idea that a non-equilibrated assemblage is present. Calcium-aluminium rich chondrules One chondrule of supposed clinopyroxene was shown by electron probe microanalysis to have the following composition (values in weight percent): FeO 0.31 CaO 15.4 MgO 9.0 Ah Si Total

10 Table 2 Electron probe microanalyses of pyroxenes - IN 0 No of ions on basis of 6 oxygen atoms Analysis FeO* CaO MgO Ah0 3 Si02 Total Fe Ca MgAI Mol % No. % % % % % % Fe Ca Mg AI ~ Si Ferrosilite** ' *Total iron expressed as FeO. **Calculated on a calcium-free basis. Analyst: P.K. Schultz, Australian Mineral Development Laboratories.

11 The mineral identity of this chondrule has not been established but it is clearly not pyroxene. Gehlenite, fassaite and other calcium-magnesiumaluminosilicates have been reported from C3 type meteorites (e.g. Allende, Clark et al., 1970) but the observed composition, although of similar affinity, has too much silica for these types of minerals. Sulphide minerals Sulphide minerals constitute about 5% of the meteorite, although they were once more abundant, having been replaced by iron oxides to at least 50%. Most of the sulphide was originally troilite but very little completely fresh material remains, because this appears to have been the phase most susceptible to alteration or weathering. However, crystalline aggregates of slightly porous but still anisotropic troilite are common within the matrix. The troilite also occurs as droplets, averaging 0.02 to 0.03 mm in diameter, included within olivine chondrules and aggregates. The troilite of these droplets shows extensive replacement by goethite and the remnants of sulphide left in the droplets are very small. Some sulphide occurs as porous, irregular masses, in association with magnetite, in parts of the matrix, and a little troilite occurs as small discrete "chondrules" where it has been either partly replaced by goethite or was intergrown with a second mineral phase which has now been replaced by goethite. The sulphide-bearing chondrules are small (up to 0.1 mm) and do not contain silicates. Except for their larger size and discrete nature, they are very similar to the droplets included within much of the olivine. A few very small droplets within olivine chondrules are composed of what is believed to be pentlandite (determined by electron-probe analysis to contain nickel, iron and sulphur but too small for accurate analysis). This mineral is now present in trace amounts but it is possible that some of the goethite now associated with troilite may represent altered pentlandite. The pentlandite remaining contains a much higher percentage of nickel than the small metallic iron globules which are included in the olivine. Metallic iron Metallic iron was found only as minute globules less than 0.01 mm in diameter, included in some olivine crystals. Generally, only a few globules are present in anyone chondrule, but two to three small chondrules of olivine were found which contain 5 to 10%metal globules.in one of these the metal is distributed along SUb-parallel lines approximately mm apart in optically continuous, possibly barred, olivine. Another distinctly barred olivine contains a few metal globules in the spaces between the "bars" of olivine. 131

12 Most of the metal appears to be homogeneous, but a few ofthe slightly larger globules show a trace of a darker phase. Electron-probe microanalysis of some of the globules failed to show the presence of any elements other than iron and (up to 5 to 10%) nickel. A few have very minor sulphur. It is tentatively suggested that an iron carbide may also be present but carbon could not be determined by the electron probe. Metallic iron is not now present either in the matrix or in the sulphide-bearing chondrules, but in view of the amount of oxidation it is impossible to be certain that there has been no metal in these areas. Chromite A few octahedral crystals of chromite were found in the polished section. These are about 0.01 rom in size and occur on, or just within, the boundary of some of the olivine chondrules. One chromite crystal has been cut by a thin veinlet of goethite. Magnetite Magnetite comprises 3 to 5% of the meteorite (in less weathered zones); much of it occurs within chondrules, and some in the matrix. Most of the magnetite within chondrules is intergrown with, or occurs in interstices between, olivine crystals and varies in grain size from a few microns up to about 0.1 mm. One chondrule of barred olivine sectioned has a few octahedral magnetite crystals 5 to 10 microns in size in interstices between olivine bars (Fig. 4). Some of the coarser-grained magnetite within chondrules partly encloses smaller globules of troilite which locally has been replaced by goethite. In these locations it is clear that, under terrestrial weathering conditions, goethite has first preferentially replaced the troilite leaving magnetite unaltered (Fig. 5). In more extensively weathered zones, magnetite also has been replaced by goethite. Some globules and irregularly-shapedmasses of troilite (and of goethite after troilite) within chondrules and also in the matrix have trace to minor amounts of magnetite just within the margins of the globules. The textures suggest the possibility that, at some time in the history of this meteorite, some troilite may have been partly replaced by magnetite. This almost certainly occurred before terrestrial weathering. Most, if not all, of the visible magnetite in the matrix is intergrown with troilite or with goethite which has replaced troilite, but it is possible that some of the extremely fine-grained, unidentified, submicroscopic material in the matrix is magnetite. Feldspar A few chondrules composed of clino-enstatite (with minor olivine) also contain laths of twinned calcic plagioclase. A probe examination of feldspar in one of these chondrules showed no trace of sodium. One chondrule was found which consisted mainly of feldspar and glass. 132

13 CHEMISTRY Chemical analyses of the core and of the weathered rim are given in Table 3 together with data for the Allende and Karoonda meteorites. Semi-quantitative trace element chemistry is given in Table 4. Table 3 Major element analyses (wt %) of Adelaide, Allende and Karoonda meteorites Si Ah Fe metal FeS* Remaining Fe 2+ as FeO** Fe 3+ as Fe CaO MgO MnO Cr K 20 om 0.02 Na vo, < 0.05 < 0.05 P20S Ti Ni Co C 0~5 0~5 CO H Loss at 105 C(H 2 0 ) TOTAL Si02/MgO Adelaide In core In rim Allende Karoonda <0.1 o o ot Fe metal/fe total Fe total/si s equivalent **AllFe 2+as FeO tcarbon content given as 0.10% by Van Schmus (1969a). Data on Allende meteorite taken from Clarke et al. (1970), p. 45, Table III, column 3. Data on Karoonda meteorite taken from Mason and Wiik (1962), p.4, Table I, column A. 133

14 Table 4 Semi-quantitative spectrographic trace element analysis (ppm) of Adelaide meteorite Core Rim Co Cr V Cu Pb I 1 Zn Sn 400 Not detected Ga Ge Li I 1 The following elements were sought, but were not found in excess of their detection limits. The detection limits are quoted in brackets after the element symbol: W (50), Mo (3), Ta (100), Nb (20), Be (1), Th (100), Pt (10), Pb (10), Os (10), Ir (2), Rh (2), Ru (2), Cd (3);Bi (1), Ag (0.1), Au (3), As (50), Sb (30), Te (20), Tl (1), Rb (10), Cs (30), Ba (50), Sr (10), Y (10), La(100), Ce (300), Nd (300), Pr (~,itr (100), Sc (50) and Eu (50). The analyses distinguish between metallic iron, ferrous iron (as sulphide and "oxide") and ferric iron. Points of interest include the high water content, the unusually low value of calcium and low values for sodium and potassium. The presence of calcium in feldspar and pyroxene and also in glass associated with olivine has been established by use of the electron probe but the location of sodium and potassium is unknown. Weathering has been considered as a cause of the low calcium and sodium values but in the absence of obvious alteration of, for example, plagioclase, there is no reason to justify it. There is a significant proportion of carbon, although this is not as high as in many carbonaceous chondrites. More unusually, a significant amount of carbon is reported as CO 2, which may be present either as carbonate or as hydrocarbons, since more CO 2 is reported from the core than from the rim. The rim of the meteorite undoubtedly has been affected by weathering. The presence of goethite replacing sulphide and also filling small fractures indicates that, at least in places, the core is also partly weathered. Associated changes apparently involve loss of silicon, nickel and sulphur and an increase in the oxidation state of the iron. According to Urey and Craig (1953) the presence of ferric iron usually denotes a weathered meteorite. The presence 134

15 of apparent primary magnetite in the core of the meteorite almost certainly indicates that ferric iron was present in the meteorite's pre-terrestrial environment. Because there is a distinct possibility that the magnetite has replaced some unidentified sulphide mineral, a two-stage origin for the meteorite is suspected. Variations in composition of olivines and pyroxenes have been discussed by Wood (1967) and Dodd (1974). The former has shown that both C2 and C3 type carbonaceous chondrites have a scatter of compositions for both olivine and pyroxene and that the range of the scatter is greater for the olivines than for the pyroxenes. These observations are consistent with the nature of the olivine and pyroxene in the Adelaide meteorite. Only twenty crystals of olivine and nineteen of clino-enstatite have been analysed in the present study. It is impossible to relate the compositions to the histograms of composition given by these authors; however, the fayalite content of the Adelaide meteorite ranges from 0.5 to 47.5%, whereas the ferrosilite content of the pyroxene varies only from 1.0 to 10.8%. No attempt has been made to trace the metamorphic history of the chondrules, CLASSIFICATION OF THE METEORITE Classification of the meteorite has been attempted following the concepts of Van Schrnus and Wood (1967), Van Schrnus (1969a, b), Mason (1971) and Van Schmus and Hayes (1974). The Adelaide meteorite is an unequilibrated carbonaceous chondrite with properties lying between Omans and Vigarano sub-types, although possibly closer to the Vigarano sub-type. The classification is probably C2, although there are many similarities with C3 chondrites. The CalAl ratio is inconsistent with either. Classification into the fundamental C2-C3 type has been effected mainly by reference to Van Schrnus and Wood (1967, table 4, p. 757). This classification is justified by: a) the wide variation ofolivine composition (in particular with some variation of pyroxene composition) b) the presence of clear glassin chondrules c) the presence of a chondritic texture with well-defined chondrules having distinct edges d) the presence of clinoenstatite as the only pyroxene identified e) an opaque matrix f) a carbon content which falls within the range 0.2 to 1%. The average nickel content of the sulphides present is probably less than 0.5%, although both troilite (dominant) and pentlandite (traces) occur. The presence of a nickel-bearing sulphide phase, however, even in trace amounts, suggests a C2 classification. Similarly the apparent high proportion 135

16 of olivine with fayalite content below 4% also suggests C2 classification (Wood, 1967). The water content (H 2 C1) of 4.30% is like that of C2 chondrites. However, this high value may not be an entirely primary feature of the meteorite, because the presence of minor goethite suggests that part of the high water content Il)ay be due to weathering. The high proportion of matrix (35%) lies between figures conventionally accepted for C2 and C3 chondrites. The specific gravity (3.2) is more in keeping with C3 than with C2 chondrites (Van Schmus and Hayes, 1974). The metallic globules, although small, are larger than those of normal C2 chondrites (Grossman and Olsen, 1974). Van Schmus (1969a) notes that the pyroxene composition of C3 chondrites is more magnesian than the olivine compositions in the same meteorite. Although there is a wide scatter of olivine composition in the Adelaide meteorite, it seems likely, on the basis of a mean ferrosilite-tofayalite ratio, that this meteorite also has an overall more magnesian pyroxene. Chemical ratios used by Van Schmus and Wood (1967) are listed in Table 3. Of these, the ratio of iron as metal to total iron is probably inappropriate for discussion, since oxidation may have affected some of the metallic iron present. The differing compositions of olivine and pyroxene allow no easy calculation of a suggested FeOj(FeO + MgO) ratio. The SiO,fMgO ratio of 1.45 for the core of the chondrite is almost exactly that suggested by Van Schmus and Wood (1967, p. 750) for C group chondrites. The atomic ratio for CajAI is This is much lower than values for both C2 and C3 chondrites quoted by Van Schmus and Hayes (1974), which are in the range If this is a fundamental distinction of major importance, some new class may need to be erected for this type of meteorite. Assuming that this is truly either a type C2 or a C3 chondrite, further classification ought to be possible. C2 and C3 chondrites have been divided by Van Schmus (1969a, b) and Van Schmus and Hayes (1974) into Vigarano and Omans sub-types on a combination of physical, textural and chemical grounds. The Adelaide meteorite does not fit readily into one of these sub-types, although overall it is closer to the Vigarano than the Omans sub-type. The physical characters for the table attached are derived mainly from Van Schmus and Hayes (1974): 136

17 .. W -:l Character Omans Vigarano Adelaide SubType SubType 1. chondrule size 0.5mm mm mm, most common mm 2. proportion of matrix minor significant, but below 50% 35% 3. dominant silicate, olivine olivine olivine range of composition 5-50 mole percent Fa < 4-30 mole percent Fa mole percent Fa 4. sulphide phase troilite troilite troilite «1% Ni) «0.5% Ni) (Ni;;;" 1%) pentlandite 5. opaque inclusions in chondrules sparse profusely distributed common 6. Ca, AI, (Ti) rich silicate common common present inclusions in chondrules

18 Chemical factors cited by Van Schmus (1969a), Mason (1971) and Van Schmus and Hayes (1974) are as follows: Character Omans Vigarano Adelaide A tomic Ratios Fe x 10/Si >7.9 < Ca x 100/Si <7.0 > Al x 10O/Si <9.5 > Ti x 103/Si <3.1 > Cu x 104/Si >3.4 < Ca/Al Weight Percentages Fe (total) >25% <25% 21.6% H 2O 0.53% 2.64% 4.3% C 0.47% 0.96% 0.85% Sources Atomic ratios: Van Schmus and Hayes (1974). Fe data: Mason (1971). H 2 0 and C data: Derived f~om Van Schmus (1969a). Examination of these tables shows the difficulties of exact classification. Of the physical characters one favours Vigarano and one favours Omans, with four uncertain. Of the chemical characters three favour Vigarano, two Omans, and three are indeterminate but nearer Vigarano. The nature of the properties listed strongly suggests that, although there may be two dominant sub-types of carbonaceous chondrites, as further discoveries are made a continuously variable series willbe established between these types. The properties cited also suggest that the boundary between C2 and C3 chondrites is possibly artificial and that additional data will bridge present gaps. COMPARISON WITH OTHER METEORITES A review of the literature has confirmed that the Adelaide meteorite is a C2-C3 chondrite of intermediate sub-type, and has established points of similarity and points of dissimilarity with other meteorites. The meteorite 138

19 recorded in the literature which seems to have the greatest affinity with the Adelaide meteorite is the Allende meteorite (Clark et al., 1970). This meteorite (or group of meteorites) is C3, Vigarano sub-type with a similar combination of chondrules, aggregates and matrix to the Adelaide meteorite, although the proportion of matrix is much larger (57% against 35%). The Allende meteorite contains dominant magnesium-rich chondrules but calcium-aluminium-rich chondrules also are present. Most of the Adelaide meteorite chondrules and aggregates identified have been magnesium-rich but two calcium-aluminium rich chondrules have been identified, and more may yet be discovered, particularly in the fine-grained material. The Allende meteorite contains small areas darker than surrounding areas. The composition of these areas is similar to that of the whole, but the chondrules within these dark patches are smaller than in the main mass. One similar dark area has been noted in the Adelaide meteorite. The chemistry of the Allende and Adelaide meteorites is compared in Table 3. The Adelaide meteorite has conspicuously more water in its composition, and more carbon and iron sulphide. It contains proportionately less silicon, aluminium, calcium, magnesium, nickel and sodium, although the differences are reduced if the analyses are compared on a water-free basis. Trace element values for the Allende meteorite are given in Clark et al. (1970, p. 47). Methods of analysis for the trace elements are not directly comparable because of the' semiquantitative nature of the data in Table 4; but, taking the results at face value, figures obtained for the two meteorites, where comparison is possible, are closely similar except for enhanced zinc, and possibly, tin in the Adelaide meteorite. The only other carbonaceous chondrite reported from South Australia is the Karoonda meteorite (Grant and Dodwell, 1931; Mawson, 1934; and Mason and Wiik, 1962). This meteorite is classed as C4 by Van Schmus (1969a). It has rather similar chemistry (except for a very low carbon content) but it has been so thoroughly recrystallised that its original texture cannot be categorized definitely. However, an unusual point of similarity with the Adelaide meteorite is the presence of apparently primary magnetite in both meteorites. ACKNOWLEDGMENTS This paper is based on work carried out on behalf of the South Australian Department of Mines. The permission of the Director of the Department of Mines, and also the Director of Amdel, to publish this work is gratefully acknowledged. The authors are indebted to RA. Binns, KJ. Henley and J.D. Lewis for helpful criticism. A.L. Graham reviewed the first draft of this paper. 139

20 REFERENCES Binns, R.A., Pyroxenes from non-carbonaceous chondri tic meteorites. Min. Mag. 37, Clarke, R.8. Jr., E. Jarosewich, B. Mason, J. Nelan, M. Gomez and J.R. Hyde, The Allende, Mexico, meteorite shower. Smithsonian Contributions to the Earth Sciences No.5. Smithsonian Institute Press, City of Washington. Dodd, R.T. Jr., The petrology of chondrites in the Hallingeberg meteorite. Contrib.MineraL Petrol. 47, Dodd, R.T. Jr., W.R. Van Schmus and DM. Koffman, A survey of the unequilibriated ordinary chondrites. Geochim. Cosmochim. Acta 31, Grant, K. and G.F. Dodwell, The Karoonda (S.A.) meteorite of November 25, Nature 127, Grossman, L. and E. Olsen, Origin of the high-temperature fraction of C2 chondrites. Geochim. Cosmochim. Acta 38, Mason, B., The carbonaceous chondrites - a selective review. Meteoritics 6, Mason, B. and H.B. Wiik, Descriptions of two meteorites: Karoonda and Erakot. American Museum Novitates No Published by the American Museum of Natural History. Mawson, D., The Karoonda meteorite. Trans. Proc. Roy. Soc. South Australia 58, 2-5. Urey, H.C. and H. Craig,'1953. The composition of the stone meteorites and the origin of the meteorites. Geochim. Cosmochim. Acta 4, Van Schmus, W.R., 1969a. M'reralogy, petrology and classification of types 3 and 4 carbonaceous chondrites. In "Meteorite Research," Millman PM. (ed). D. Reidel Publishing Company, Dordrecht, Holland, Van Schmus, W.R., 1969b. The mineralogy and petrology of chondritic meteorites. Earth Sci. Rev. 5, Van Schmus, W.R. and J.M. Hayes, Chemical and petrographic correlations among carbonaceous chondrites. Geochim. Cosmochim. Acta 58, Van Schmus, W.R. and la. Wood, A chemical-petrological classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, Wood, J.A., Olivine and pyroxene compositions in Type II carbonaceous chondrites. Geochim. Cosmochim. Acta 31, Manuscript received 12/23/77 140