Cana.dian Mineralogist Vol. 15, pp. 97-fi1 (1977)

RINGWOODITEAND MAJORITEIN THECATHERWOOD METEORITE

LESLIE C. COLEMAN Department of Geological Sciences, University ol Saskatchewan,Saskatoon, Saskatchewan s7N 0w0

AssrRAc"r 1964 or 1965 when it became entangled in fanning equipment of the Lorne E. Horton A network of dark v€ins in the Catherwood oli- farm near Catherwood, about 55 km west of vine-hypersthene chondrite contains ringwoodite and Saskatchewan. It was fust recog- majorite as discrete transparent grains, and as inti- Saskatoon, ,meteorite through the mato mixtur€s with minor maskelynite, which form nized as a in l97L and, the dark brown matrix of tle veins. All of the ring- offices of its Meteorite Observation and Re- woodito and majorite is polycrystalline and extreme- covery Project, was acquired by the National ly fine-grained. Minor amounts of olivine, pyrox- Research Council of Canada. It is now part of ene, and plagioclase are associated with some ths national collection that is housed at the tramparent grains. Geological Survey of Canada in Ottawa. The occurrences and properties of ringwoodite Catherwood is ovoid with a maximum dimen- and majorite iu Catlerwood are similar to those in sion of about L7 cm and a weight of 3,963 gm. the Tenham and Coorara meteorites of Australia. some of the fusion crust is still present, Tho ringwoodite is Fa2s in Although composition, as is the weathered and Catherwood olivine, and has a=8.122!O.Ol2A. it has been fairly extensively prominent Tho majorite is Wo1En76Fs21in composition, as is much of its interior is rusty brown. A the Catberwood pyroxene, and has a-11.543t feature of this meteorite that is evident on any 0.018A. Ringwoodite and majorite in Catherwood sawn surface is a netrvork of dark veins (Fig. 1). are considered to be high-pressure products of oli- Work on Catherwood in the Mineralogy Section vine and pyroxene which were localized along frac- of the Geological Survey of Canada during the tures in tbe meteorite and which were transformed summer of. I97l establishedthe presenceof ring- by shock resulting from extralerrestial collision. woodite and majorite in and associated with these veins (Coleman & Fillo 1973). sourvrerrg Un r6seau de filons de couleur sombre recoupent GBNSRAI-DsscnrPrtoN la chondrite i olivine-hyporsthdne de Catherwood. Ils contiennent de la ringrroodite et de la majorite In most respects,Catherwood appears to be en grairs isol6s transparents et, avec un peu de mas- a typical olivine-hypersthene chondrite of type kelynite, en agr6gats qui fornrent la gangue brun L6 according to the classification of Van Schmus fonc6 des filons; Tous les sp€cimens de ringwoodite & Wood (1967), Chondrules, from about 1 to polycrystallins grain et do majorite sont et de ex- 6 mm iu diameter, are readily discernible and tr6mement fin. De petites quantitds dolivine, de comprise about L57o of the meteorite by vol- pyrox0ne et de plagioclase sont associ6es i quel- boundaries are diffuse with respect ques grains transparents. Le mode de gisement et ume; their les caractEres de la ringwoodite et de la majorite to the enclosing matrix. They are composed sont les m6mes dans la chondrite de Catherwood principally of olivine and pyroxene of similar que dans les m6t6orites de Tenham et Coorara, eD compositions and in similar proportions to those Australie: la ringwoodite (a=8.722L0.0124) y a in the matrix. Most chondrules have a barred la composition- de I'olivine Fazs et la majorite (a- appearance;some appear to be granular with in- 11.543:t0.018A), celle du pyroxdne Wo1En7sFe21; dividual grains about 0.1 mm in diameter; others l'olivine et le pyroxdne, originellement localis6s dans harre a fan-like lamellar structure, and some aP- les fractures de la m6t6orite, s trandormCrent sous pear to be an aggregate of several part-chon- haute pression en ringwoodite et majorito, trans- formation peut-Ctre d6clench6e par le choc d'une drules. collision extraterrestre. The matrix surrounding the chondrules con- sists of anhedral to subhedral grains generally (fraduif par la R6daction) ranging from 0.015 to O.5 mm and most com- monly about 0.2 mm in diameter. The matrix seem to have been subjected fNtnouucttoN and the chondrules to extensive brecciation which is considered to The Catherwood meteorite wa$ discovered in have been responsible for the small size of ma- 97 98 TTIE CANADIAN MINERALOGIST

Frc. 1. Surface of sawn slab of Catherwood displaying network of dark veins. The broken specimen was glued togetler along fracture F-F.

Frc. 2. Part of a dark vein in Catherwood with transparent segregations of ringwoodite (R) and majorite (M) within fine-grained dark brown matrix containing spheroidal opaque inclusions (black). trix grains at the lower end of the size range. mm and averaging about 0.01 mm, and sub- Catherwood is heterogeneous in terms of the angular to subrounded aggregates of several proportions of the minerals present from one transparent minerals (Figs. 2, 3). part to another. Ilowever, an average ,modal Microscopic examination and X-ray diffrac- composition is approximately 55 7o olivine, 25 Vo tion and microprobe investigations indicate that lorr-Ca pyroxene, I37o opaqueminerals (kama- these transparent aggregates are composed prin- cite-taenite, troilite, chromite), 7Vo plagioclase cipally of ringwoodite, the spinel of olivine (or maskelynite), and accessorydiopsidiCpyrox- composition, and .majorite, the garnet of bron- ene atrd apatite. The minerals seem to be homo- zite composition, and that some of the aggre- geneou{t: olivine is about Fax and the low-Ca gates also contain the minerals which occur in pyroxene is about WorEnzeFsa. The plagioclase, the matrix of the meteorite. The aggregatesgen- which in most places appearsto have been con- erally range from 0.2 to 1.0 mm (Fig. 3) but verted to maskelynite and has a composition of some have maximum dimensions in excess of about AnroOraAbezr occurs as irregular grains 2.0 mm. Some aggregatesup to 1 mm in diame- rarely more than 0.05 mm in diameter and in- ter are monomineralic and consist of either ring- terstitial to olivine, pyroxene and opaque min- woodite or majorite ,(Fig. 2), and others consist erals. of adjoining segregationsof these two minerals. Evidence of strain is the wavy extinction Small grains of olivine, orthopyroxene, and pla- com,mon in the larger olivine and pyroxene gioclaseare included by some of the aggregates. grains, and abnormally low birefringence in oli- Laue photographs of individual, apparently ho- vlne. mogeneous,transparent grains display no spots but only circles concentric about their centers; grains OccunrNcE, oF RrNGwooD AND MAJoRTTE this indicates the are polycrystalline and extremely fine-grained. A network of dark veins is a prominent fea- Both ringwoodite and majorite are isotropic ture on sawn surfaces of Catherwood; these and, therefore, can be difficult to distinguish veins range from about 0.1 to about 1O.Omm from dach other in thin section. However, much in apparent width (Fig. 1), but the averagetrue of the ringwoodite is pale purple to smoky grey width is estimated to be 0.5 mm. A visual esti- whereas much of the majorite is pale yellowish mato is that the veins make up about IVo of. the brown. Both are colorless in some occurrences rneteorite by volume. Microscopic ex4mination and, in such cases, can only be differentiated of the veins reveals that they consist of a very readily with a microprobe. fine-grained translucent dark bro'wn matrix with Microprobe and X-ray diffraction examina- spheroidal opaque inclusions (Fig. 2) ranging in tion of the translucent matrix of the veins in- diameter from more than 0.04 to less than 0.001 dicates that it consists principally of majorite RINGWOODITE AND MAJORITE 99

Frc. 3. Photomicrograph displayrng the full width of one of the dark veins in Catherwood.

Frc. 4. Photomicrograph of a part of a relatively narrow dark vein in Catherwood. Greyish purple ring' woodite (R) occuri as segregations within the vein matrix (black) and between the vein and the oli- vine (OL) of the body of ihe meteorite. Segregalions of majorite (M) are also present within the vein' and somewhat more abundant ringwoodite, and yzed with a microprobe by A. G. Plant at the minor amounts of maskelynite. The opaque Geological Survey of Canada during the initial inclusions appear to consist of an intimate in- examination of the meteorite in 1971. These tergrowth of troilite and meteoritic Ni-Fe alloy. minerals and the olivine 'and pyroxene in the All rnajorite detected in this study is con- main body of the meteorite have been analyzed tained within the dark veins, but some ring- at the University of Saskatchewan and at the woodite occurs otrtside of them. Most of this University of Manchester as part of the present ringwoodite is along the vein margins where it study (Tables 1, 2). Results of the analysesper- is continuous. with, and apparently replaced, formed at the Geological Survey of Canada are grains of olivine (Fig. ). A few small patches also reported in these tables as are those of of ringwoodite were observedon the rnargins of linns el al. (1969) of ringwoodite and olivine olivine grains near the dark veins. No dng- in Tenham and of Smith & Mason (1970) of ma- woodite was observedmore than 0.5 rnm from the jorite in Coorara. veins and, in general, it is within 0.2 mm of Analyses at the University of Saskatchewan them. were carried out with an Acton M$64 rnicro- The above aspectsof the occurrencesof ring- probe using wavelength dispersive methods and woodite and majorite in Catherwood appear to those at the University of Manchester with a be identical to those described by Mason el a/. Cambridge Geoscan microprobe using energy (1968), Binns e/ al. '(1969), Smith & Mason dispersive methodq. A rnuch larger number of (1970),and Binns (L97O) for the sametwo min- analysesof each of the phaseswas performed in erals in the Coorara and Tenham meteodtes of the latter case and their results are considered Australia. Thin sections of a fragment of Ten- to be the more reliable. ham have been examined by the author and Ringwoodite in Catherwood has a composi- appear very similar but not identical to those tion that is practically identical to that of the from Cathenvood; the most notable differences olivine in the main body of the meteorite (fable are that the purple color of the ringwoodite in 1). A similar relationship was reported for ring- Tenham is more vivid than in Catherwood, and woodite and olivine in Tenham @inns er al. the size of the transparent aggregates in the 1969) and in Coorara (Smith & Mason 1970). dark veins, and width of the veins, are some- Binns (197,0) found that purple ringwoodite what greater in Catherwood than in Tenham. within the veins in Tenham is not as Fe-rich as that which is grey and outside the veins. IIe shock pressures CorraposmoNsoF RINcwooDrrE AND Meronng suggestedthat a diminution of away from the veins at the time that olivine was Clear grains of ringwoodite and majorite in being transfor.med to ringwoodite could have the dark veins of Catherwood were first anal- been responsible for this compositional differ- 100 THE CANADIAN MINERALOGIST

ence. Minor differences werd noted between ritic alloy. Of the others, ten yielded results that analyses of different ringwoodite grains during could be accounted for entirely by mixtures of the present study, but no systematic difference variable amounts of ringwoodite, majorite, and of the type noted by Binns v{4s observed. maskelynite as indicated in Table 3. The analyses of Catherwood majorite (Iable 2), except for those performed"at the University X-Rev DrrrnectroN Sruprps of Manchester, contain significant amounts of Ti, Al, Cr, and Na. Of the 15 clear maiorite Laue photographs of ringwoodite grains, ma- grains analyzed at the University of MaricfrLster, jorite grains, and fragments of the ringwoodite- minor amounts of Ti were detealed in 5 and Cr majorite-maskelynite matrix display smooth rings in 3; Al and Na were detected in n

TAELE3. MINERALPROPORIIONS IN II.IE VEIN MATRIXOF CA'IHERS@[) produce ringwoodite and majorite are most CAICULATEDFROM MICROPROBE AMLYSES readily attained if a meteorite's olivine and py- Range of wt.g Average wt.U roxene have these compositions. RingHoodite 39 - 89 60 tibjorite. ll - 56 34 lhskelynlte 0-12 6 AcKNowLEDcEMENTs The author is grateful to the Geological Sur- vey of Canada and the National Research Coun- by Smith & Mason (1970) for Coorara majorite. cil for making a portion of the Catherwood The fragments of matrix produced composite meteorite available for study. Members of tle patterns in which the majorite pattern is gen- Mineralogy Section of the Geological Survey of erally more intense than that of ringwoodite. Canada, and particularly Dr. A. G. Plant, have been extremely helpful with suggestions con- cerning the work and in supplying analytical DrscussroN data. Some of the analytical work and the revi- sion of the manuscript was carried out in the The sirnilarities of their compositions support Department of Geology, University of Man- the idea that ringwoodite in Catherwood has chester; the generosity of that department in formed from olivine and the majorite from making available its facilities is gratefully ack- pyroxene. Furthermore, the relative abundances nowledged. The work was in part supported by of the ringwoodite and majorite as clear grains National Research Council Grant 41495. and in the matrix of the dark veins (Iable 3) are simils to those of olivine and pyroxene in the main body of the meteorite. The fineness of the RrrEr.eNcps grarn size of the ringwoodite and majorite, the character of the powder patterns of the olivine BtNxs,R. A. (1970): (MgFe)rSiOa spinelin a me- and pyroxene, and the texfure of the meteorite teorite.Phyt. Earth Planet.Interiors 3, 156-160. indicate tlat these two transformations mav -, Devls, R. J. & Rnnp, S. J. B. (1969): have been brought about by severe shock sucL Ringroodite, natural (MgFe)zSiOa spinel in as would accompany extraterrestriat impact. All the Tenham meteorite. Nature 221, 943-9M. of this is in agreement with the ideas of Mason CorEMeN, L. C. & Fu.r.o, W. J. (L973): Ring- et al. (1,968)"linns et al. (L969), Binns (1970), woodite and majorite in tle Catherwood chon- and Smith & Mason (1970) for the origin of dritg Saskatchewan,Canada. Geol. Soc. Amer. ringwoodite and majorite in Coorara and in Program Abstr, 6, 580-581. Tenham. HrweNN, D. (1966): On the origin of hnrr- Mason et al. {1968) md ginns (1970) discuss stheno chondrites:agesand shock effects of black the physical conditions necessaryfor the olivine- chondrites. Icarus 6, L89-22L. ringwoodite and orthopyroxene-majorite inver- Mesou, B., Nnr-w, J. & Wruu, J. S. (1968): Oli- sions and mention temperatures in the order of vine-garnettransformation in a meteorite. Science 100O'C and shock pressures of as much as L6A,66-67. 450 kbar. Conditions s,imils to tlose attained SMrrH,J. Y. & MlsoN, B. (1970): Pyroxene-garnet rn Coorara and Tenham would have been neces- transformation in Coorara meteorite. Science168, sary for these transformations to take place in 832-833. Catherwood because of the great simihfly e1 VeN Scruug W. R. & Wooo, J. A. (1967): A the mineral compositions. These compositional chemical-petrologic classification for chondritic similarities may reflect a common parentagefor meteorites.Geoohim. Cosmochim. Acta !1, 747- Catherwood, Coorara, and Tenham. Alternative- 765. ly, the compositional similarities could indicate Manuscriptreceived August 1976, emendedNovem- only tlat the physical conditions necessary to 1976.