Com par a tive Mössbauer stud ies of the Baszkówka or di nary chondrite and some other me te or ites
Jolanta GAŁĄZKA-FRIEDMAN, Erika R. BAUMINGER, Is rael NOWIK, Nonna BAKUN-CZUBAROW, Mar ian STĘPNIEWSKI and Jacek SIEMIĄTKOWSKI
Gałązka-Friedman J., Bauminger E. B., Nowik I., Bakun-Czubarow N., Stępniewski M. and Siemiątkowski J. (2001) — Com par a tive Mössbauer stud ies of the Baszkówka or di nary chondrite and some other me te or ites. Geol. Quart., 45 (3): 319–326. Warszawa.
It was sug gested that per haps Mössbauer pa ram e ters of troilite could serve as a cri te rion for an in ter pre ta tion of the or i gin of var i ous me - te or ites. It was found that in three me te or ites com ing from the same par ent body (Baszkówka, Mt. Tazerzait and Songyuan) the an gle be - tween the prin ci pal axis of the elec tric field gra di ent (EFG) and the di rec tion of the mag netic field in troilite is around 60°. Further in ves ti ga tions showed that in some other me te or ites, not be long ing to this group, this an gle is close to 90°.
Jolanta Gałązka-Friedman, In sti tute of Phys ics, War saw Uni ver sity of Tech nol ogy, Koszykowa 75, PL-00-662 Warszawa, Po land;
Erika R. Bauminger and Is rael Nowik, Racah In sti tute of Phys ics, The He brew Uni ver sity, Je ru sa lem 91904, Is rael; Nonna Bakun-Czubarow, In sti tute of Geo log i cal Sci ences, Pol ish Acad emy of Sci ences, Twarda 51/55, PL-00-818 Warszawa, Po land; Mar ian Stępniewski, Pol ish Geo log i cal In sti tute, Rakowiecka 4, PL-00-975 Warszawa, Po land; Jacek Siemiątkowski, Lower Silesian Branch, Pol ish Geo log i cal In sti tute, Jaworowa 19, PL-51-122 Wrocław, Po land (re ceived: Sep tem ber 11, 2000; ac cepted: De cem ber 13, 2000).
Key words: or di nary chondrites, par ent body, Mössbauer spec tros copy, Θ an gle, troilite, kamacite, ol iv ine, pyroxene, weath er ing, ox i da - tion.
INTRODUCTION
As tro nom i cal anal y sis as well as petrographic data in di cate that some or di nary chondrites could come from the same par ent body. For ex am ple, the ma jor ity of strongly shocked L chondrites are be lieved to orig i nate from one par ent body, whereas al most unshocked and highly po rous L chondrites like Baszkówka, Mt. Tazerzait, and Tjerebon are sup posed to be frag ments of an other par ent body (Pilski and Walton, 1998).
An at tempt was made to see whether Mössbauer spec tros copy can find a com mon fea ture in spec tra of me te or ites orig i nat ing from the same par ent body, which can dis tin guish them from spec tra ob tained for me te or ites from other par ent bod ies.
Mössbauer stud ies of four sam ples taken from dif fer ent parts of the Baszkówka me te or ite showed a vari abil ity in min eral com - po si tion. This ob ser va tion proves that the dis tri bu tion of iron
among dif fer ent min eral phases can not be a good cri te rion for the or i gin of var i ous me te or ites. It was sug gested that the Mössbauer pa ram e ters of me te or itic troilite could serve as such a cri te rion (Gałązka-Friedman et al., 1999).
MÖSSBAUER SPECTROSCOPY
Mössbauer spec tros copy is ap pli ca ble to dif fer ent iso topes, but is most sen si tive for 57Fe. The Mössbauer spec tra re flect the prop er ties of the en vi ron ment of the iron at oms. Each iron-bearing com pound gives a char ac ter is tic Mössbauer spec - trum. The spec tra show the count ing rate af ter the ab sorp tion of the gamma rays (14.4 keV for 57Fe emit ted by a ra dio ac tive
57Co source) in the in ves ti gated sam ple (the ab sorber), as func - tion of the source ve loc ity. Mössbauer spec tra are char ac ter ised by the fol low ing Mössbauer pa ram e ters: the iso mer shift (IS),
the quadrupole split ting or quadrupole in ter ac tion (QS) and the value of the in ter nal mag netic hyperfine field (Heff). The iso mer shift, quadrupole split ting and line widths are given in ve loc ity units (mm/s). The re la tion be tween these units and en ergy units can be eas ily cal cu lated from the equa tion:
∆E = Eγ(v/c)
where: Eγ — the en ergy of the emit ted gamma rays (14.4 keV for 57Fe); v — the ve loc ity of the ra dio ac tive source, c — the ve loc ity of light; for 57Fe, a change in ve loc ity of 1 mm/s is equiv a lent to a change in en ergy of 4.8 ⋅ 10–8 eV; the val - ues of the mag netic hyperfine fields are usu ally given in kOe or Tesla.
In min er als there are usu ally two types of Mössbauer spec - tra for 57Fe: dou blets and sex tets (con sist ing of two or six lines).
Sex tets are typ i cal for iron com pounds with mag netic or der ing;
in nonmagnetic com pounds only dou blets are ob served. In dou blets the dis tance be tween the two lines yields the quadrupole split ting and the cen tre be tween the two lines — the iso mer shift. In mag netic spec tra the po si tions of the lines de pend on the value of the iso mer shift (IS), the quadrupole in - ter ac tion (QS), the hyperfine field (Heff) and the an gle be tween the prin ci pal axis of the elec tric field gra di ent (EFG) and the di - rec tion of the in ter nal mag netic field.
Mössbauer stud ies of me te or ites started shortly af ter the dis cov ery of the Mössbauer ef fect in 1958. The first pa per deal - ing with this sub ject was pub lished in 1964 (Sprenkel-Segel and Hanna, 1964). Mössbauer mea sure ments were used to iden tify dif fer ent min eral phases and to de ter mine their rel a tive amounts.
A typ i cal Mössbauer spec trum of non-weathered or di nary chondrite is a su per po si tion of two sex tets and two dou blets.
One sex tet is con nected with me tal lic iron within an iron-nickel al loy (kamacite), the other with Fe2+ in iron sul phide (troilite) of com po si tion FeS. Dou blets cor re spond to fer rous iron pres - ent in sil i cate phases such as ol iv ine (Mg, Fe)2[SiO4] and orthopyroxene (Mg, Fe)[SiO3] .
Some times subspectra cor re spond ing to fer ric iron ap pear in me te or itic Mössbauer spec tra. Many pa pers were de voted to the prob lem of the or i gin of this fer ric iron. Some fer ric com - pounds in me te or ites could be the prod uct of ter res trial weath -
er ing (Berry et al., 1994; Shinonaga et al., 1994) and oth ers may be due to pre-terrestrial ox i da tion (Knudsen, 1989; Burns, 1994). Many pa pers were de voted also to kamacite (Scorzelli, 1991).
The first Mössbauer stud ies of the Baszkówka me te or ite were pre sented in 1996 (Gałązka-Friedman et al., 1996). The Baszkówka chondrite con sists of chon drules and their frag - ments, rock frag ments, sil i cate min eral crys tals as well as of ir - reg u lar grains of kamacite and troilite (Gałązka-Friedman et al., 1998; Stępniewski et al., 1998). The main sil i cate min er als of both the chon drules and the ma trix are ol iv ine and orthopyroxene. The com po si tion of ol iv ine de ter mined by X-rays is about Fa20 (Siemiątkowski, 2001), while that de ter - mined by an elec tron microprobe ranges from Fa23 to Fa26, giv - ing an av er age value of Fa25.3 (Stępniewski and Borucki, 2001).
The microprobe anal y ses of orthopyroxenes give re sults rang - ing from Fs18 to Fs23 with an av er age value of Fs20.4
(Stępniewski and Borucki, op. cit.).
Mössbauer stud ies per formed on four dif fer ent sam ples of the Baszkówka me te or ite (Gałązka-Friedman et al., 1996, 1998) showed the pres ence of the four above-mentioned iron- bear ing min eral phases: ol iv ine, pyroxene, kamacite and troilite and a small amount of tri va lent iron. The Mössbauer spec trum of this tri va lent iron was a dou blet with IS = 0.45 mm/s and QS
= 0.75 mm/s. These pa ram e ters are some what dif fer ent from those of the prod ucts of ter res trial weath er ing (Burns, 1994).
As the Baszkówka me te or ite is a com par a tively young fall, we sug gest that this tri va lent iron was cre ated dur ing the cos mic life of the me te or ite. This as sump tion is con firmed by Mössbauer mea sure ments of iso lated frac tions of the Baszkówka me te or ite (see below).
T a b l e 1 Iron dis tri bu tion (atomic %) among min eral phases in four sam ples
of the Baszkówka me te or ite
Subspectrum Sam ple 1 Sam ple 2 Sam ple 3 Sam ple 4
Kamacite 23 14 13 29
Troilite 25 28 33 26
Ol iv ine 33 35 28 26
Pyroxene 16 21 24 18
Fe3+ 3 2 2 1
Fig. 1. Struc tures of FeS from left to right: NiAs-type, MnP-type and troilite struc tures; at the top, oc ta he dral iron sites viewed from above are shown
The per cent ages of iron found in dif fer ent min eral phases in four sam ples of the Baszkówka me te or ite are listed in Ta ble 1.
The vari abil ity of the min eral com po si tion ob served in the four sam ples of the Baszkówka me te or ite in di cates that the dis - tri bu tion of iron among dif fer ent min eral phases can not be a cri te rion for de ter min ing the par ent body of var i ous me te or ites.
MÖSSBAUER STUDIES OF TROILITE PRESENT IN DIFFERENT METEORITES
THE CRYSTAL STRUCTURE OF TROILITE
Iron monosulphide phases are char ac ter ised by a mul ti plied unit cell with re spect to the NiAs-type struc ture, and of ten pos - sess a lower lat tice sym me try (Hafner and Kalvius, 1966;
Kruse and Ericsson, 1988; Kruse 1993; cf. Fig. 1). At room
tem per a ture FeS oc curs as hex ag o nal troilite with P62c space group. In this struc ture, all Fe at oms form tri an gu lar three-atom clus ters in planes per pen dic u lar to the c axis. Each Fe atom is sur rounded by a dis torted sul phur octahedron. The Fe clus ter breakup may be caused ei ther by in creased tem per a ture, in - creased pres sure, or by va can cies at Fe sites. On heat ing, the tran si tion of troilite into the orthorhombic MnP-type struc ture (Pnma) can be com pleted at 140°C, but this trans for ma tion in me te or itic ma te rial is not fully re vers ible, as af ter slow cool ing to 7°C, 20% of the Fe at oms may re main in the MnP struc ture.
On heat ing, MnP-struc tured iron monosulphide trans forms into the NiAs struc ture with space group P63/mmc at about 225°C.
HYPERFINE PARAMETERS OF TROILITE
Ta ble 2 shows Mössbauer pa ram e ters of troilite ob tained at room tem per a ture from 19 dif fer ent me te or ites in 9 dif fer ent
T a b l e 2 Mössbauer pa ram e ters of troilite from dif fer ent me te or ites
Me te or ite Ref er ence IS
[mm/s]
QS [mm/s]
H [T]
Numakai Shima et al. (1975) 0.76 –0.06 31.6
Toluca Ousef et al. (1979) 0.76 0.26 31.5
Rittersgrun Just et al. (1984) 0.67 – 31.3
Putinga de Oliveira et al. (1988) 0.76 0.30 31.1
Torino Ortalli and Pedrazzi
(1990) 0.75 0.26 30.6
Alfinello Bonazzi et al. (1992) 0.73 0.17 31.1
Alenak Bonazzi et al. (1992) 0.79 0.18 31.1
Siena Bonazzi et al. (1992) 0.75 0.17 31.1
Aigle Bonazzi et al. (1992) 0.74 0.16 31.1
Pieve di Casa Bonazzi et al. (1992) 0.77 0.19 31.6
Xinyang Zhang et al. (1994) 0.61 0.31 30.2
Jilin Zhang et al. (1994) 0.67 0.16 31.2
Al Kidirate Gismelseed et al. (1994) 0.78 –0.15 31.2
RC 063 Berry et al. (1994) 0.76 – 31.5
RC 099 Berry et al. (1994) 0.68 – 30.6
RC 035 Berry et al. (1994) 0.43 – 28.7
Y74155 Shinonaga et al. (1994) 0.81 0.15 31.4 Y791428 Shinonaga et al. (1994) 0.82 0.15 31.4 ALH77299 Shinonaga et al. (1994) 0.81 0.15 31.4
lab o ra to ries. The data were pub lished in 10 dif fer ent pa pers be - tween 1975 and 1994. In all these anal y ses of the troilite subspectrum it was as sumed that the prin ci pal axis of the EFG is colinear with the di rec tion of the mag netic hyperfine field.
In Ta ble 2 iso mer shifts are given rel a tive to iron metal.
Typ i cal val ues given for the er rors of iso mer shifts (IS) are
±0.02 mm/s, of quadrupole split ting (QS) ±0.04 mm/s and of mag netic fields (Heff)±0.5 T. A large spread in the val ues ob - tained for these pa ram e ters can be eas ily no ticed. For the iso - mer shift the val ues range be tween 0.67 and 0.82 mm/s and for QS be tween –0.15 and +0.31 mm/s. The rea sons for such a spread could be dif fer ent amounts of va can cies or im pu ri ties in dif fer ent troilites (Ok et al., 1982; Baek et al., 1984; Kruse, 1990) or the ne glect ing of the an gle be tween EFG and Heff in these fits. As in all above mea sure ments troilite was only a small frac tion of iron-con tain ing com pounds in the sam ples, we de cided to iso late troilite from Baszkówka and Mt.
Tazerzait me te or ites in or der to ob tain more ac cu rate Mössbauer pa ram e ters.
NEW MÖSSBAUER STUDIES OF DIFFERENT IRON-CONTAINING COMPOUNDS ISOLATED FROM BASZKÓWKA AND MT. TAZERZAIT
METEORITES
MATERIALS
In or der to avoid iron ox i da tion, the sam ples of bulk me te - or ites were crushed, then ground into fine pow der in an ag ate mor tar un der ethyl al co hol. Af ter grind ing, the sam ples were dried at room tem per a ture and loaded, as soon as possible, into Mössbauer plas tic hold ers. Troilite was iso lated from the grad - u ally crushed chondrites us ing com bined mag netic and
Fig. 2. Mössbauer spec trum of troilite iso lated from the Baszkówka chondrite
The solid lines are the the o ret i cal lines ob tained from com puter fits to the ex per i men tal data points and show the subspectra: sex tet a for troilite, dou blet b for ol iv ine and dou blet c for pyroxene; A — fit with Θ = 0; B — fit with Θ = 61.4°; ob serve the bad fit to lines 1 and 2 in A
hand-picking with a nee dle tech niques. First, kamacite was re - moved from the crushed ma te rial us ing a per ma nent mag net, then troilite grains were iso lated from the re main ing min er als by hand-picking un der a ste reo scopic mi cro scope. The last method was also ap plied for ob tain ing the con cen trates of ol iv - ine. The fine-grained sep a rates were loaded into the plas tic hold ers. The sep a ra tion into the dif fer ent phases was not com - plete.
METHOD
Mössbauer spec tra of 57Fe were mea sured at room tem per a - ture with a con ven tional Mössbauer spec trom e ter. A 25 mCi
57Co-in-Rhodium source, giv ing a nar row unsplit emis sion line, was used. The 14.4 keV gamma rays were de tected us ing a pro por tional coun ter. The ve loc ity scales were cal i brated us ing an iron foil ab sorber at room tem per a ture. All ex per i men tal Mössbauer spec tra were fit ted with Lorentzian lines, the line widths, quadrupole splittings (QS), iso mer shifts (IS), the mag - netic hyperfine fields (Heff) and the rel a tive in ten si ties of var i - ous subspectra be ing free pa ram e ters in these fits. At first all spec tra were ana lysed as sum ing that the an gle be tween the prin ci pal axis of the EFG and Heff is zero, but later it was real - ised that no good fits to the troilite subspectrum could be ob - tained with this as sump tion and that an ad di tional pa ram e ter Θ
= 0, rep re sent ing this an gle, has to be in tro duced.
RESULTS
In Fig ure 2 the Mössbauer spec trum of troilite iso lated from Baszkówka is shown. Fig ure 2A shows the fit as sum ing Θ = 0, Fig ure 2B the fit with Θ as a free pa ram e ter, yield ing Θ = 61.4°.
The points with er ror bars are the ex per i men tal points and the lines rep re sent the best the o ret i cal fits to the ex per i men tal spec - tra. In the fig ure subspectrum a rep re sents the sex tet at trib uted
to troilite, subspectrum b — the dou blet at trib uted to ol iv ine and subspectrum c — the dou blet at trib uted to pyroxene. All subspectra are shown. Num bers 1 and 2 point to sec ond and the fifth line of the troilite sex tet. It is easy to no tice, that in Fig ure 2A the the o ret i cal lines do not fol low the ex per i men tal points of these two peaks. In Fig ure 2B, when the an gle Θ was in tro - duced as a free pa ram e ter, the fit of the the o ret i cal lines to the ex per i men tal points is per fect.
The spec trum ob tained from troilite iso lated from the Mt.
Tazerzait me te or ite is shown in Fig ure 3. This spec trum was also fit ted with Θ as free pa ram e ter.
Mössbauer pa ram e ters ob tained from the best fits to the subspectra of troilite iso lated from Baszkówka and Mt.
Tazerzait me te or ites are given in Ta ble 3. Within the lim its of er rors, these pa ram e ters are the same.
T a b l e 3 Mössbauer pa ram e ters of troilite pres ent
in seven or di nary chondrites
Me te or ite IS [mm/s]
QS [mm/s]
H [T]
Θ [deg]
Baszkówka
(isol) 0.76±0.01 1.03±0.02 30.9±0.1 61.4±0.1 Mt. Tazerzait
(isol) 0.78±0.02 1.05±0.05 31.3±0.1 61.3±1.0 Songyuan 0.76±0.01 0.89±0.05 30.9±0.1 61±5.0 El Hammami 0.75±0.01 0.90±0.02 31.1±0.1 63±2.0
Grzempy 0.63±0.01 0.68±0.03 30.1±0.1 90±10
Mbale 0.68±0.01 0.68±0.02 30.7±0.1 90±10
Tsarev 0.66 ±0.01 0.88±0.06 30.3±0.1 90±15
Fig. 3. Mössbauer spec trum of troilite sep a rated from the Mt. Tazerzait chondrite
Subspectra a, b and c as in Fig. 2; dou blet d cor re sponds to Fe3+; in the the o ret i cal spec trum Θ = 61.3°
Mössbauer spec tra of the ol iv ine and sil i cate (ol iv ine and pyroxene) phases iso lated from the Baszkówka me te or ite were also ob tained. In Fig ure 4 the Mössbauer spec trum ob tained for ol iv ine (b) is shown and a weak subspectrum cor re spond ing to pyroxene (c) as well as a weak dou blet with pa ram e ters char ac - ter is tic for tri va lent iron (d) are also ob served.
Fig ure 5 shows the Mössbauer spec trum ob tained for two sil i cate phases: ol iv ine (b) and pyroxene (c). Again weak subspectra cor re spond ing to other phases are also ob served: (d)
— the dou blet cor re spond ing to tri va lent iron and (a) — the subspectrum due to troilite is seen here. The fact that the tri va - lent iron was sep a rated with both sil i cate phases in di cates that it is within the struc ture of these phases and is not a nano-phase prod uct of ter res trial weath er ing.
PRELIMINARY MÖSSBAUER STUDIES OF SIX OTHER ORDINARY CHONDRITES
Mössbauer stud ies of six other or di nary chondrites:
Songyuan (L6), Tsarev (L5), Mbale (L6), Grzempy (H5), El Hammami (H5) and Mar low (L5) were per formed. The sam - ples were pre pared from bulk pieces of the me te or ites, with out iso la tion of the min eral phases. In five me te or ites: Songyuan, Tsarev, Mbale, Grzempy and El Hammami the same min er als (kamacite, troilite, ol iv ine and pyroxene) as in the Baszkówka me te or ite were iden ti fied. In the sixth one (Mar low) there was no kamacite and only a very small amount of troilite. We were able to iden tify subspectra of ox ides and hy drox ides. It seems that the Mar low me te or ite is strongly weath ered and that the me tal lic and sul phide phases were oxi dised in it. In the five non-weathered me te or ites we were able to de ter mine the an gle Θ of troilite.
The value of Θ ob tained for the Songyuan me te or ite (61±5°) is very close to the value ob tained for Baszkówka and Mt. Tazerzait. This re sult cor re lates well with other ob ser va - tions, which sug gest that Songyuan be longs also to “the Baszkówka fam ily” (Mor gan, 1999, pers. comm.). The val ues of Θ ob tained for other me te or ites, not be long ing to this group, are close to 90°. All ob tained pa ram e ters are sum ma rised in Ta - ble 3. There are dif fer ences also in the other Mössbauer pa ram - e ters be tween the var i ous me te or ites.
Thus the iso mer shift for the Grzempy, Mbale and Tsarev troilite is be tween 0.63 and 0.68 mm/s, whereas for oth ers it is 0.77±0.02 mm/s. The QS of troilite from the 3 above men - tioned me te or ites is be tween 0.68 (Grzempy and Mbale) and 0.88 mm/s (Tsarev), whereas it is 1.02 for Baszkówka and Mt.
Tazerzait troilite. For the Songyuan troilite it is 0.89±0.05 mm/s and for El Hammami 0.90±0.02 mm/s.
The Mössbauer pa ram e ters of the sil i cates are the same, within lim its of er rors, in all 7 me te or ites and are: IS = 1.17±0.01 and QS = 3.00±0.03 mm/s for ol iv ine and IS = 1.17±0.01 and QS = 2.13±0.03 mm/s for pyroxene.
As can be seen from Ta ble 3, Θ was de ter mined more ac cu - rately for the Baszkówka and Mt. Tazerzait me te or ites than for the other me te or ites. For the Baszkówka and Mt. Tazerzait me - te or ites the pa ram e ters were eval u ated from iso lated troilite, whereas for the other me te or ites, bulk sam ples were used.
CONCLUSIONS
The Mössbauer stud ies of four bulk sam ples, taken from four dif fer ent parts of the Baszkówka me te or ite, showed the pres ence of four iron-bearing com pounds: kamacite, troilite, ol iv ine and pyroxene. This is the typ i cal min eral com po si tion
Fig. 4. Mössbauer spec trum of ol iv ine (b) iso lated from the Baszkówka chondrite Small amounts of pyroxene (dou blet c) and of tri va lent iron (dou blet d) are also pres ent
of or di nary non-weathered chondrites. The dif fer ent dis tri bu - tion of the four min eral phases in var i ous parts of the same me - te or ite shows that this dis tri bu tion can not serve as a good cri te rion for dis tin guish ing be tween the par ent bod ies of var i - ous me te or ites.
Mössbauer stud ies of troilite iso lated from a large part of the Baszkówka me te or ite point to the ne ces sity of in clud ing the an gle Θ (the an gle be tween the main axis of the elec tric field gra di ent and the di rec tion of the mag netic field) in the fit ting of Mössbauer spec tra of troilite. As as tro nom i cal anal y sis and petrographic fea tures in di cate that Baszkówka and Mt.
Tazerzait me te or ites could come from the same par ent body,
the equal ity of the an gles Θ and the other Mössbauer pa ram e - ters ob tained for troilite iso lated from Baszkówka and Mt.
Tazerzait me te or ites, brought about the hy poth e sis that these pa ram e ters could yield a good cri te rion for iden ti fy ing the par - ent body of dif fer ent me te or ites. Pre lim i nary Mössbauer stud - ies of six other or di nary chondrites seem to sup port this hy poth e sis, how ever, in or der to prove it, more spec tra of troilite iso lated from dif fer ent me te or ites have to be mea sured.
The pres ence of Fe3+ in the sam ples of iso lated ol iv ine and sil i cate phases (ol iv ine and pyroxene) is in agree ment with the sug ges tion that fer ric iron found in the Baszkówka me te or ite is a prod uct of pre-terrestrial ox i da tion (Ostertag et al.,1984).
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