Geo log i cal Quar terly, 2001, 45 (3): 281–288
Two spinel-bearing com pound chon drules from the Baszkówka me te or ite
Jerzy BORUCKI and Mar ian STĘPNIEWSKI
Borucki J. and Stępniewski M. (2001) — Two spinel-bearing com pound chon drules from the Baszkówka me te or ite. Geol. Quart., 45 (3):
281–288. Warszawa.
The pres ent pa per de scribes two com pound chon drules la belled panda and chev ron, dis cov ered in the Baszkówka me te or ite. Op ti cal mi - cro scope, elec tron mi cro scope and elec tron microprobe stud ies re vealed an un usual min eral com po si tion and fab ric. Both chon drules in - clude “pri mary” and “sec ond ary” com po nents. The panda and chev ron chon drules have ap par ent di am e ters of ~0.86 and ~0.54 mm re spec tively with spinel dom i nant in pri mary com po nents and ol iv ine-plagioclase in sec ond ary com po nents over lain by thin microcrystalline rims. Euhedral or subhedral Mg, Fe2+/Al, Cr-spinel crys tals from the panda’s pri mary com po nent (zoned, cav ern ous or ho mo ge neous) were not found in Mg-Fe chon drules in Baszkówka, where Cr oc curs ex clu sively as chro mite. Ol iv ine crys tals from panda’s pri mary (Fa27.1) and sec ond ary (Fa26.5) com po nent have sim i lar com po si tions to those from pan da’s rim (Fa26.4) and com pa ra ble to ol iv ine from other Baszkówka Mg-Fe chon drules (av er age ~Fa25.7). The Ca-plagioclase laths from the groundmass of panda’s pri mary component has an ex tremely vari able com po si tion (~An49 to ~An60), ev i dently more calcic than those in other chon drules of the me te or ite (mean ~An13). The pri mary com po nents of the two spinel-bearing chon drules may have been formed in a fire ball gen er ated by pow er ful im pact dur ing early ac cre tion of a protoplanet. Af ter crys tal li sa tion the spinel crys tals were abraded and pol ished, prob a bly dur ing rapid flight through a dense cloud of in ter plan e tary dust. The mod er ately re duc ing con di tions of the hy po thet i cal fire ball were re placed by a more re duc ing en vi ron ment dur ing the con den sa tion of the rims of both chon drules.
Jerzy Borucki, Peszteńska 3 m 24, PL-03-925 Warszawa, Poland; Mar ian Stępniewski, Pol ish Geo log i cal In sti tute, Rakowiecka 4, PL-00-975 Warszawa, Po land (re ceived: No vem ber 21, 2000; ac cepted: Jan u ary 19, 2001).
Key words: Baszkówka, me te or ites, or di nary chondrites, spinel.
INTRODUCTION
In spite of the ther mal al ter ation of Baszkówka, some of its pri mary fab ric and com po si tion re main, prob a bly be cause of the neg li gi ble com pac tion and the ab sence of shock ef fects (Stępniewski et al., 1996, 1998a, b; Wlotzka et al., 1997). One of the pe cu liar ity of Baszkówka’s com po si tion is the re cent find of two com pound, spinel-bearing chon drules termed panda and chev ron. The O-isotopic anal y sis of pan da’s min er - als dis played a com plex pat tern of an 16O-rich group com pris - ing pri mary ol iv ine and spinel crys tals ver sus an 16O-poor group com posed of sec ond ary ol iv ine and groundmass plagioclase (Maruyama et al., 1999).
The most fre quent spinel (Ru bin, 1997) in me te or ites is a Mg-Al-spinel co ex ist ing with hibonite, melilite, fassaite, Ti-pyroxene, an or thite and mag ne tite in Ca-Al-inclusions (CAI’s) of dif fer ent groups and types of car bo na ceous chondrite (e.g. Wark and Lovering, 1982; Bis choff et al., 1993;
Da vis and MacPharson, 1996; McKeegan et al., 1998). Other, fre quently rel ict spinel grains, are of ten pres ent within chon -
drules (Rambaldi, 1981; Kracher et al., 1984; Misawa and Fujita, 1994; Con nolly and Hewins, 1996; Con nolly and Bur - nett, 1998) or in chon drule rims (El Goresy et al., 1984) of the car bo na ceous chondrites.
Chro mite, an other mem ber of the spinel group, is a fre quent ac ces sory com po nent of the ma trix and chon drules in or di nary chondrites (OCs) (e.g. Ramdohr, 1963; Nagahara, 1982; Mc - Coy et al., 1997). Mag ne tite, an other spe cies of spinel, has been ob served in some meta mor phosed chondrites (Noguchi, 1993). Spo radic Al-rich, Fe-Mg spi nels have also been no ticed in some OCs (e.g. Fudali and Noonan, 1975; Mc Coy et al., 1991; Krot et al., 1993; Semenenko and Girich, 1998).
In Baszkówka, as in the other mem bers of the L-group, ac - ces sory chro mite is pres ent in the ma trix of chondrite, in chon - drule rims and in side chon drules (Stępniewski et al., 1998b).
Some mi cro scopic ob ser va tions, partly con firmed by EMA (Elec tron Microprobe) anal y sis, sug gest also that mag ne tite might be pres ent on weath ered (ox i dised) sur faces of Fe/Ni-metal and Fe-sulphide grains (Stępniewski et al., 1998a). Here, we de scribe the min er al ogy and pe trol ogy of the
chon drules chev ron and panda and dis cuss the or i gin of these ob jects.
SAMPLES AND METHODS
Sam ples for chem i cal and petrographic anal y sis were cut off one side of the me te or ite us ing a di a mond disc. This yielded a heel up to 15 mm thick, and then a slice about 10 mm thick, which was saw up into pieces for prep a ra tion of pol ished, un - cov ered thin sec tions about 50 µm thick (Borucki and Stępniewski, 2001). Af ter the optical microscopy they were cov ered with a thin layer of sil ver for ob ser va tion with a scan -
ning elec tron mi cro scope (SEM) and for elec tron microprobe anal y sis (EMA).
Petrographic ob ser va tions were made us ing an Ernst Leitz po lar iz ing mi cro scope (Wetzlar, Ger many) and a MIN-8 (USSR) equipped with a uni ver sal 5-axes ta ble. A set of bi - refrin gence com para tors was used. Mag ni fi ca tions were from 17.5 x to 1500 x (im mer sion). Panda was an a lysed by elec tron mi cro scope; the semiquantitative de ter mi na tions were made with an Ox ford elec tron microprobe and SEMQant an a lyt i cal pro gram. Dif fer ent sets of ar ti fi cial and/or nat u ral stan dards were se lected for each ob ject an a lysed. Alu mi num, Si, Fe, Mg pres ent in quan ti ties over 10 wt.% were de ter mined with good pre ci sion, rel a tive stan dard de vi a tion: RSD = ~1 to 5 rel a tive
%. The de ter mi na tions of Na and Ca in the range of 1 to 10 wt.% were de ter mined with RSD = ~5 to ~15 rel a tive %. The er rors in de ter min ing el e ments at and be low 1 wt.% means the val ues lack quan ti ta tive sig nif i cance.
RESULTS
The two spinel chon drules (panda and chev ron) found in the chondrite Baszkówka are both com pound chon drules con - sist ing of the spinel dom i nated pri mary com po nents “...which were rigid enough to re tain their orig i nal shape...” in the sense of Wasson et al. (1995) and an ol iv ine-plagioclase ad her ing sec ond ary com po nent “... which had vis cos i ties low enough to al low them to con form to the shape to the pri mary in the su ture zone...” (op. cit.). Both chon drules are cov ered with microcrystalline con den sa tion rims.
282 Jerzy Borucki and Marian Stępniewski
T a b l e 1 SEMQuant anal y ses of spinel (mean ± stan dard er ror of mean)
(in wt.%)
El e ment El e ment (11 cases) [wt.%]
Atoms per 4 oxygens
Mg Al Ti Cr Mn Fe Zn O To tal
6.8±0.4 22.4±0.7 0.20±0.04
14.9±1.0 0.6±0.1 17.9±0.5 0.29±0.1 36.8±1.6 99.89
0.48 1.45 0.01 0.50 0.02 0.56 0.01 4.00 7.03
T a b l e 2 SEMQuant anal y sis of ol iv ine (mean ± stan dard er ror of mean), ox y gen iso topes,
and Fo/Fa con tents (in wt.%)
El e ment Pri mary
phenocryst
Sec ond ary com po nent
Panda’s rim Baszkówka*
Mg-Fe chondr.
NC Na Mg Si Ca Ti Cr Mn Fe O To tal δ17O**
δ18O**
For ster ite (Fo) Faya lite (Fa)
1 nd 22.5 18.6
<dl
<dl
<dl
<dl 18.9 41.2 101.2
–3.3 –3.5 72.9 27.1
1 nd 21.5 18.9
<dl
<dl
<dl
<dl 17.6 41.4 99.4 2.8 –1.1 73.5 26.5
1 0.3 21.1 18.8 1.0
<dl
<dl 0.4 17.4 41.0 100.0
– – 73.6 26.4
5
<dl 22.5±0.17 18.0±0.11 0.07±0.01 0.01±0.009
0.03±0.02 0.35±0.05 17.5±0.2 40.9±0.3 99.36
– – 74.3 25.7
* — four PO and one BO chon drules; ** — in re la tion to SMOW, re cal cu lated from Maruyama et al. (1999); NC — num ber of cases; nd — no data; <dl — be low the limit of de tec tion
Two spinel-bearing compound chondrules from the Baszkówka meteorite283
.
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I E T A L P
284Jerzy Borucki and Marian Stępniewski
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I I E T A L P
Panda is a glob u lar chon drule with an ap par ent di am e ter of about 0.86 mm and a por phy ritic pri mary with spinel and ol iv - ine pheno crysts in a groundmass com posed of Ca-plagioclase laths (Pl. I, Fig. 1). The up per quar ter of the pri mary’s sur face is smooth, ex cept for a spinel phenocryst and small hol lows in the groundmass. This part of the pri mary is cov ered with an ad her - ing sec ond ary con fined to a sin gle elon gated, anhedral ol iv ine crys tal. The sec ond ary ol iv ine ad heres di rectly to the sur face of the groundmass but is sep a rated from the spinel sur face by a
~10 µm thick oligoclase layer. Such tex ture in the sec ond ary could re sult from the ex ter nal cool ing with crystallisation in - ward from the outer sur face of the chon drule. The re main ing three quar ters of the pri mary ex te rior are ex tremely rough, cov - ered with mul ti ple sharp points, ir reg u lar voids and deep, nar - row hol lows. This part of the pri mary ex te rior is filled and cov ered with a microcrystalline ag gre gate of ol iv ine, pyroxene, plagioclases and op aques mak ing the rim of the chon drule.
The SEMQuant anal y ses of spinel pheno crysts from the pri mary dis played high con tents of alu mi num, chro mium, iron and mag ne sium. Atomic re la tions, set to 4 at oms of ox y gen on the ba sis of the mean from 11 anal y ses of three pheno crysts (Ta ble 1), ap prox i mate the com po si tion of spinel. Cal cu lated con tents of at oms (set to 4 oxygens) show an ex cess of bi va lent ions (Mg + Fe(to tal) + Mn + Zn = 1.07) to gether with a smaller de fi ciency of tri va lent ions (Al + Cr + Ti = 1.96) sug gest ing that less than 10% of Fe(to tal) is in the tri va lent state.
One spinel grain (Pl. I, Fig. 2) shows etch pits and euhedral ac cre tions, and a ghost crys tal is pres ent in side an other one (Pl. I, Fig. 3) — both fea tures of a rapid, in ter rupted and unequilibrated crystallisation. The rounded and pol ished sur - face of the left-up per most spinel grain (Pl. I, Fig. 1; Pl. II, Fig. 1) has an other or i gin; it is prob a bly caused by abra sion and pol ish ing of the grain, per haps by a jet of solid par ti cles.
Abraded, pro trud ing grains of ol iv ine were found at the edges of the Cold Bokkeveld (CM2) chon drules (Green wood et al., 1994). Such abraded sur faces should face an in com ing, hy po - thet i cal jet of par ti cles, or the flight di rec tion of the spinel grain through a cloud of in ter plan e tary dust.
Ol iv ine. The main fea tures of ol iv ine crys tals from the panda pri mary and sec ond ary and com par i son with other ol iv - ine grains from Baszkówka are shown in Ta bles 2 and 3.
The sim i lar i ties be tween both ol iv ine crys tals within panda, in its rim and in other chon drules of Baszkówka are ev i dent in the Fo/Fa con tents, sizes, frac tur ing and char ac ter of ex tinc tion.
Other fea tures, how ever, re veal im por tant dif fer ences be tween the pri mary and sec ond ary ol iv ine grains.
The in ter nal con tour of the sec ond ary ol iv ine crys tal matches the curved and smooth sur face of pri mary spinel (Pl.
II, Fig. 1) along their mu tual con tact. How ever, the min er als do not touch each other, since their in ter face is lined by a ~10 µm thick oligoclase layer (Ab78, An17, Or5). Thist is prob a bly the re sult of ex ter nal cool ing, where crystallisation started from out side to wards the pri mary core. No oligoclase layer is pres ent at the con tact of sec ond ary with pri mary, where the ol iv ine crys tal ad heres to the groundmass (Pl. II, Fig. 2, bot tom right).
Here the con tact is hazy and rough, with a few frac tures in the ol iv ine filled with plagioclase sim i lar to that of the groundmass.
Prob a bly the plagioclase melt of the sec ond ary was resorbed by the partly so lid i fied groundmass of the pri mary. The ex ter -
nal con tact of the sec ond ary ol iv ine with the rim is smooth with large and shal low si nuses (Pl. II, Fig. 2). At the bor der of the ol - iv ine crys tal, a layer of fine-grained plagioclase ma trix up to
~20 µm thick , con tain ing some dis persed small (~2 to ~6 µm) chro mite grains, cov ers the con tact of the ol iv ine with the rim and fills notches in the ol iv ine. Al though four small ag gre gate of ol iv ine lo cated at the lower left bor der of panda have an am - big u ous struc tural sta tus, they seems to be sim i lar to clus ters of gran u lar olivines found in the POI’s (plagioclase- ol iv ine-in clu - sions) from some car bo na ceous chondrites (Sheng et al., 1991).
The groundmass of panda, al most opaque in trans mit ted light with rare trans par ent and polar is ing patches, in re - flected-oblique il lu mi na tion (Pl. I, Fig. 1), dis plays a fab ric com posed of gray-ol ive laths mostly run ning side by side but lo cally hav ing a quasi-ra dial ar range ment. The laths have vari - able thick ness from ~1 to ~10 µm with a mean of ~7 µm and ac - cord ing to the SEMQuant anal y ses (Ta ble 4) are mostly com posed of plagioclase.
The mean com po si tion of plagioclase (Ta ble 5), ex tremely vari able on a micrometric scale, is roughly in the lab ra dor ite range, with a low orthoclase mol e cule con tent. Some un iden ti - fied sil i cate and ox ide phases com pose the re main ing frac tion of the groundmass. Opaque plagioclase mesostasis from Mg-Fe chon drules of Baszkówka dis play also rare trans lu cent and weakly polar is ing points but, in con trast to the panda groundmass, it does not have any lath struc ture. Sig nif i cant dif - fer ences be tween the panda groundmass and the mesostasis of Mg-Fe chon drules of Baszkówka are given in Ta ble 5.
Two spinel-bearing compound chondrules from the Baszkówka meteorite 285
T a b l e 3 Struc tural fea tures of ol iv ine
Features Pri mary phenocryst Sec ond ary com po nent Size ∼0.47 x 0.16 mm ∼0.48 x 0.10 mm
Frac tures few ir reg u lar and/or
pla nar –
Extinction slightly un du lat ing
(∼4–8°) –
Struc tural sta tus en closed in the groundmass
main com po nent of the sec ond ary
Crys tal faces at the con tacts
ol iv ine/groundmass:
euhedral ol iv ine/spinel:
anhedral
anhedral, with ∼10 µm thick oligoclase interlayer at the ol iv - ine/spinel con tact
Ghost crystals
few euhedral, up to
∼4 x 15 µm, pyramids and pina - coids with par al lel ori en ta tion to host
absent
Ovoid
inclusions rare abun dant, arranged in
chains or clusters
The chon drule rim mat ter, rest ing on a rough sur face of the groundmass, has filled many deep and nar row hol lows dur - ing con den sa tion of gas. Ol iv ine ag gre gates that, be side sub or - di nate plagioclases, py rox enes and op aques, are the most abun dant com po nent of the rim, con sist of a few small (1–10 µm) anhedral and/or subhedral ol iv ine par ti cles with smooth, rounded cor ners and undulose bor ders. Ol iv ine par ti - cles in side the ag gre gate have, for the most part, an al most con - cor dant op ti cal ori en ta tion with small de vi a tions gen er at ing a faint mo saic ex tinc tion un der crossed polars. From place to place the su per po si tion of par ti cles gives the ef fect of ab sent ex tinc tion.
Chev ron is a glob u lar chon drule that has an ap par ent di am - e ter of ~0.54 mm (Pl. II, Fig. 3) and a com pound struc ture, with the pri mary com posed mostly of a sin gle spinel grain, and the sec ond ary formed of ol iv ine grains with dark gray inter growths of plagioclase glass. The chon drule is cov ered with a thin microcrystalline con den sa tion rim.
The pri mary of the chev ron chon drule is com posed of the wine-red spinel grain (~0.52), which re sem bles the spinel pheno crysts from panda. The pro nounced chev ron-like struc - ture of the spinel re sulted from rapid crystallisation along the main axes and the growth of mul ti ple laths par al lel to the octahedron faces. Two cav i ties in the mid dle of the grain are filled with a plagioclase-ol iv ine ag gre gate. The larger cav ity vis i ble in the up per part (Pl. II, Fig. 3) of the crys tal is filled with plagioclase with tiny inter growths of spinel. In con trast to the euhedral faces of spinel, its anhedral sur face vis i ble at the left side of the crys tal is round and smooth with a few mi nor eroded crat ers (Pl. II, Fig. 4). In chev ron the ef fects of strong abra sion and pol ish ing is even more dis tinct than in the case of panda. Prob a bly, in the same way as in panda, the rounded and
pol ished sur face was aimed in the di rec tion of the hy po thet i cal flight across a dense cloud of in ter plan e tary dust.
The sec ond ary of the chev ron is com posed mostly of four submillimetric het er o ge neous, anhedral ol iv ine crys tals (Pl. II, Fig. 3, bot tom-right) and cover al most all of the euhedral faces of the spinel pri mary. Ol iv ine crys tals dis play an in ter nal mo - saic struc ture com posed of a few smaller do mains with fuzzy bor ders de lim ited by clus ters and chains of in clu sions (mostly oval-shaped, from 1–8 µm in di am e ter; rarely euhedral up to 40 µm long) with lines of small frac tures. An gles be tween the op ti cal vec tors in ad join ing do mains vary from zero up to 5°
giv ing the ef fect of mo saic ex tinc tion. The spaces be tween the ol iv ine crys tals and the frac tures within them are filled with the dark gray, trans lu cent, amor phous, and iso tro pic glass (black on Pl. II, Fig. 3). All along the euhedral faces of pri mary spinel,
286 Jerzy Borucki and Marian Stępniewski
T a b l e 4 SEMQuant plagioclases of panda (mean ± stan dard er ror of mean) (in wt.%)
El e ment Pri mary groundmass
Sec ond ary pellicle
Rim Baszkówka chon -
drules mesostasis NC
Na Mg Al Si K Ca Ti Cr Fe O To tal Albite Anortite Orthoclase
11 3.3±0.4
nd 14.1±0.6 25.7±0.4 0.2±0.1 7.4±0.9 0.7±0.2 0.6±0.4 1.0±0.3 47.0±0.2
100.02 44.2±5.1 54.1±5.5 1.7±0.6
1 3.9 6.4*
7.9 25.6
nd 1.5 nd nd 7.7*
45.1 99.9 81.4 12.4 6.2
2 6.0
<dl 10.9 30.3 0.9 1.24 nd nd nd 47.3 97.6 82.9 9.9 7.2
6 6.2±0.1 0.6±0.2 10.9±0.2 29.2±0.4 0.8±0.1 1.9±0.3 nd nd 0.5±0.1 46.2±0.4
96.3 80.8±0.9 13.2±1.5 6.0±0.9
* — inter growth with ol iv ine; NC — num ber of cases; nd — no data; <dl — be low the limit of de tec tion
T a b l e 5 Com par i son of plagioclase from panda groundmass
and Mg-Fe chon drules mesostasis
Char ac ter is tic Groundmass of panda
Baszkówka chon - drules mesostasis Tex ture felsitic (intersertal) apha ni tic
Plagioclase lab ra dor
∼An49–∼An60
oligoclase
∼An12–∼An15
Orthoclase in plagioclase
low
∼Or1–<Or2
high
∼Or5–∼Or7
In clu sions spinel, il men ite(?) ol iv ine, pyroxene
but only un der the cover of sec ond ary ol iv ine, there has crys tal - lised a thin (~10 µm) plagioclase pellicle, ab sent from the re - main ing part of this con tact.
The rim, com posed of granoblastic ag gre gates of subhedral grains, 6–17 µm in di am e ter (mean ~12 µm), has a vari able thick ness from less than 25 µm to over 75 µm. Di rectly on the abraded face of the spinel there has grown an ir reg u lar
~40–50 µm thick layer of ho mo ge neous ol iv ine ag gre gates which out wards be comes fine-grained and con tains sub or di - nate orthopyroxene and plagioclase (Pl. II, Fig. 4) as well as op - aques, kamacite, rare taenite, troilite, and chro mite. The thin rim (Pl. II, Fig. 3, bot tom-right) on the sec ond ary ol iv ine thick - ens on the pol ished sur face of the sec ond ary spinel (Pl. II, Fig. 3, up per-left), sug gest ing an ori ented con den sa tion- sed i - men ta tion of the rim ma te rial.
DISCUSSION AND CONCLUSIONS
1. Both spinel-bearing com pound chon drules have sub-millimetre, subhedral spi nels in their pri ma ries, mostly ol - iv ine-bearing ad her ing sec ond ar ies with a cog nate com po si tion of spinel (data for chev ron, Maruyama, 2001). The ab sence of Fe-Ni metal in pri ma ries and sec ond ar ies and the com po si tion of spinel sug gests a sub stan tial prev a lence of Fe2+ over Fe3+
and, in con se quence, a mod er ately re duc ing mi lieu of their or i - gin. Both chon drules are over laid by microcrystalline con den - sa tion rims com posed an ag gre gate of ol iv ine, pyroxene, plagioclase and metal grains in di cat ing an en vi ron men tal shift to wards more re duc ing con di tions.
2. Some fea tures of the panda pri mary are com pa ra ble to these typ i cal for POIs of car bo na ceous chondrites, youn ger than CAIs but older than Mg-Fe chon drules (Sheng et al., 1991), i.e.: (a) the main com po nents are plagioclase, ol iv ine, py rox enes and spi nel; (b) the tex tures are sub-ophitic to intersertal and por phy ritic; (c) the clus ters of equigranular ol iv - ine crys tals lo cated near the pe riph ery of the POIs roughly re - call that ob served in panda; (d) the ex te rior accretionary rims are sim i lar to the rims around the Mg-Fe chon drules and also to that of the POIs. The iso to pic Mg het er o ge ne ity in the POI spi - nels and the ig ne ous tex tures of the POIs sug gest, ac cord ing Sheng et al. (1991), that spinel crys tals may be the rel ict grains re main ing af ter in com plete melt ing of un known pre cur sors.
How ever, the iso to pic ox y gen com po si tion of spinel from chev ron does not sup port such a sup po si tion (Maruyama et al., 2000). In con se quence, spinel crys tal ex ist ing in panda pri mary could be in her ited rel ict grains, whereas the one in chev ron is not.
3. A sig nif i cant en rich ment of 16O was found in the large chro mium spinel from Murchi son (Si mon et al., 1994), while the 16O-excess found in small spinel grains, in her ited prob a bly from re frac tory in clu sion, is much higher (δ18O = –50‰). The
O-isotope com po si tion of the panda spinel (Maruyama et al., 1999) shows only a mod est 16O-enrichment, and chev ron spinel is even poorer in 16O (Maruyama et al., 2001). Con se - quently, the panda spinel pheno crysts are poorer in 16O than spinel grains “...di rectly re lated to CAI pre cur sors or CAIs it - self...” in the Allende me te or ite (Maruyama et al., 1998), but richer in 16O than the bulk of panda (Hałas et al., 1999) and they are, most prob a bly, the old est com po nents of the pri mary.
4. Among the five spi nels pres ent in the panda pri mary, three are ho mo ge neous, one con tains an in ner ghost crys tal and vaguely resemble to the pri mary-rim type (Si mon et al., 1994) coarse spi nels sep a rated from the Murchi son me te or ite. The chev ron (pri mary euhedral spinel) show ing an un usual cav ern - ous struc ture, and the hints of such struc ture vis i ble in one of the panda spi nels, sug gests a sim i lar ity to small spinel grains found at the Cre ta ceous/Ter tiary bound ary as prod ucts of an im pact fire-ball (Preisinger et al., 1996).
5. So lid ified and crys tal lised pri ma ries of both chon drules were strongly and uni lat er ally abraded. The in ten sity of the abra sion was so high that even hard spinel crys tals (hard ness 7.5–8.0° of Mohs scale) were rounded and smoothed. Hence, the enig matic, abrad ing grains have to be: (a) hard (≥ 8.0°), (b) small (micrometric?) not pro vok ing the frag men ta tion of pri - ma ries, (c) en er getic (rapid), and (d) abun dant. Such nu mer ous par ti cles may be met dur ing the high speed flight across a rel a - tively dense cloud of neb u lar dust dur ing clumpy disc ac cre tion (Boss, 1996). The sec ond ary of panda ad hered to the abraded sur face of the pri mary wit ness ing a suc ces sion of events, whereas that of chev ron is stuck on the euhedral (non-abraded) spinel faces. In both cases, how ever, the ex ter nal cool ing re - sulted in the crys tal li sa tion from the out side of the ol iv ine to an in ter nal oligoclase layer at the sur face of the pri mary. These cir cum stances are con sis tent with the for ma tion of the com - pound chon drules in a jet flow (Liffman et al., 1996).
6. Both chon drules are cov ered with a fine-grained ol iv ine ag gre gate with sub or di nate plagioclase, kamacite, troilite and chro mite. In ter nal struc tures of the ol iv ine grains, con sist ing of a few small anhedral and/or subhedral par ti cles with smooth, rounded cor ners and un du lated, blurred bor ders, sug gest a con - den sa tion or i gin rather than melt so lid i fi ca tion and crys tal li sa - tion. The ap pear ance of op aques in the rims of panda and chev ron, par tic u larly of kamacite, ab sent from the pri ma ries and sec ond ar ies of both chon drules, in di cate a sig nif i cant mod - i fi ca tion of the mi lieu which be came more re duc ing. Sec ond ary vari a tions of tex tures and com po si tion in both rims may be the re sult of grad ual changes in en vi ron men tal con di tions and or of frac tional crys tal li sa tion.
Ac knowl edge ments. We thank E. Starnawska and L. Giro for microprobe anal y ses and BSE-images and A. Kozłowska and M. Kuberska for kind as sis tance with the prep a ra tion of pho tos.
Two spinel-bearing compound chondrules from the Baszkówka meteorite 287
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288 Jerzy Borucki and Marian Stępniewski
