39. T Etrataenite in Carbonaceous Chondrites

No. 10] Proc. Japan Acad., 66, Ser. B (1990) 183

39. T etrataenite in Carbonaceous Chondrites

By Takesi NAGATA, M. J. A., and Barbara J. CARLETON*)

(Communicated Dec. 12, 1990)

Abstract. Presence of tetrataenite grains has been recently confirmed in Yamato 791717 carbonaceous (C03) chondrite. The observed coexistence of tetrataenite fine grains with more abundant magnetite and kamacite in this C03 chondrite suggests that the tetrataenite grains were directly formed by a coalescence process of Fe and Ni fine particle smokes in the solar nebula.

1. Introduction. Since the discovery of presence of tetrataenite (FeNi-1" phase of a CuAu type ordered crystal structure) in iron meteorites'),21 and in an ordinary chondrite,3~ tetrataenite has been found in a fairly large number of chondrites as well as in iron and stony-iron meteorites.4)-7) Among 72 meteorites containing tetrataenite previously reported, only the Kainsaz (C03) is a carbonaceous chondrite.4 Experimental techniques employed for identifying tetrataenite are (i) Moss- bauer spectrum, (ii) X-ray diffraction pattern, (iii) anisotropic optical reflection of smoothed mineral surface, and (iv) anisotropic magnetic properties. Electron microprobe analyses (v) of metals to determine their chemical composition is carried out whenever possible, and an electron diffraction pattern analysis (vi) is also undertaken for determining crystal structures of some samples. More than one of these techniques are employed jointly in the identification process. All of the above experimental techniques with the exception of (iv) magnetic analysis require the test sample to be at least 10 pm x 10 pm for the analysis purpose. In nature, however, chondrites often contain fine tetrataenite grains smaller than 1 pm in mean diameter. In contrast, magnetic analysis method has proven to be successful in identifying fine tetrataenite grains distributed in a rock mass that consists mostly of non-ferromagnetic silicate minerals. 7).10) In the present study, 4 Antarctic carbonaceous chondrites, Y-693 (C4), Y- 74662 (CM2), Y-791717 (C03) and Y-81020 (C03), are magnetically analyzed in an effort to identify the presence of tetrataenite grains. The magnetic analysis was performed in conjunction with petrographic and chemical analyses. Among the four carbonaceous chondrites analyzed, the presence of tetrataenite is defi- nitely detected only in Y-791717 (C03) chondrite. 2. Magnetic analysis. The basic magnetic properties of a single crystal of tetrataenite at 293°K are as follow.u (i) Saturation magnetization (Js) : Js =1.31.3>< 10 emu cm3, (ii) Uniaxial magnetocrystalline energy components (K1 and K2): K1= 3.2 x 106 ergs/cm3, K0 = 2.3 x 106 ergs cm3, (iii) Critical upper limit temperature (Tc) of ordered tetrataenite : Tc = 593°K, (iv) Curie point temperature (Be): (Be) 1000°K,

*' National Institute of Polar Research , Kaga, Itabashi, Tokyo 173, Japan. 184 T. NAGATA and B. J. CARLETON [Vo1. 66(B), where Bc is a roughly approximated value because ec>Tc. In the present study, the dependence of characteristic parameters Js and K1 of tetrataenite on temperature is evaluated by use of natural tetrataenite grains. The specific intensity (Is) of natural tetrataenite grains separated from chondrites is approximately constant with deviations smaller than 3 % throughout a temperature range from 4°K to 500°K. Hence J5(T)=1.3 X 103 emu/cm3 may be acceptable even if given a possible error of 3 % for 4°K < T < 500°K. The magnetic coercive force (HC) of a magnetically independent ensemble of relatively large grains (mean diameter > 7 nm) of natural tetrataenite is nearly constant, gradually changing from HC(295K) = 2550 Oe at 295°K to HC(4K) _ 2765 Oe at 4°K. The average of HC for the entire temperature range is given by HC = (2643 ± 74) Oe. HC of a magnetically independent ensemble of uniaxially anisotropic magnetic grains oriented at random is given by HC = 0.958K1/Js. (1) Putting K1 and Js values of a single crystal of tetrataenite obtained at 293°K into eq (1), its theoretical value of IIC is given by HC (293K) = 2360 Oe. The observed and theoretically derived magnitudes of HC at T ' 295°K are in approximate agreement with each other with a possible deviation of about 10% or less. It may thus be concluded that the K1 value also remains nearly constant, as far as its order of magnitude is concerned, throughout a temperature range between 4°K and about 300°K. Namely, K1(T) _ (3.2 ' 3.5) X 10'6 erg/cm3 in the low temperature range. In the present magnetic analysis scheme for chondrites, the principal procedures are the determination of (a) magnetic hysteresis cycle curves between 15 kOe and -15 kOe in magnetic field at temperatures between 4°K and 300°K and (b) repeated thermomagnetic curves in 10 kOe in a temperature range from 290°K to 1100°K, or from 4°K to 1100°K when necessary. Natural remanent magnetization (NRM) characteristics and paramagnetic susceptibility (Zp) at various temperatures are also measured for subsidiary data. With respect to (a), the magnetic hysteresis measurements provide values for specific intensity (Is) of saturation magnetization, specific intensity (IR) of saturated remanent magnetization, magnetic coercive force (HC), and specific paramagnetic susceptibility (Zp). Remanence coercive force (HRC) also is de- termined in each hysteresis measurement in order to clarify an approximate binary component system structure of the magnetic constituent of chondrites.1°) The determinations Is, IR, HC, HRC and xp are undertaken both for the original state of a chondrite sample before any heating and its thermally treated state after heating up to 1100°K so as to cause the ordered structure of tetrataenite to break down. The experimentally evaluated relaxation time for the transformation of more than 95% of tetrataenite to disordered taenite is about one day at 770°K and about 0.2 hours at 1100°K. In a chondritec sample whose magnetic constituents consist of tetrataenite and other low coercivity minerals such as kamacite, disordered taenite and magnetite, the transformation from the tetrataenite phase to a disordered taenite phase by heat treatment is associated with a drastic decrease of HRC fol- lowing the transformation.10) With respect to (b), the repeated thermomagnetic (TM) curve measurements lead to an approximately quantitative identification of ferromagnetic or fern- magnetic constituents together with their magnetic transition temperatures, No. 10] Tetrataenite in Carbonaceous Chondrites 185 including Curie point, a -->r and r -~ a transitions of the kamacite / taenite phase, as well as the break-down of tetrataenite structure at elevated temperatures beyond Tc. 3. Tetrataenite in the Yamato 791717 (C03) chondrite. The Yamato 791717 carbonaceous chondrite (Y-791717) is classified into (CO3) chondrite group,12) and its bulk chemical composition is given in Table I.13) As indicated by 1.98 wt% of Fe2O3 in the table, this (C03) chondrite contains magnetite (Fe304) of about 2.9 wt% in bulk content. Electron microprobe analyses of opaque mineral grains of larger than about 20 pm in diameter in this chondrite show their chemical compositions as sum- marized in Table II. Magnetic minerals detected in these analyses are magnetite, a and a2 phases of kamacite and tetrataenite. While all iron sulfide grains are antiferromagnetic troilite (FeS), no ferrimagnetic pyrrhotite is detected.

Table I. Bulk chemical composition of X-791717 (C03)

Fig. 1. First and second thermomagnetic curves of Y-791717 (CO3) chondrite.

Table II. Chemical composition of opaque grains in Y-791717 (CO3) (Atomic %) 186 T. NAGAT'A and B. J. CARLETON [Val. 66(B),

Table III. Magnetic hysteresis parameters Y-791717 (C03)

Table III summarizes the magnetic hysteresis parameters of a Y-791717 specimen at 295°K in the original state and after the second-run and third-run heatings to 1100°K to break down the tetrataenite structure. Fig. 1 illustrates the first-run and the second-run thermomagnetic (TM) curves. The third-run TM curves are practically identical to those of the second- run, and the magnetic hysteresis parameters measured after the third thermal treatment are almost exactly the same as those after the second heating as shown in Table III. These results indicate that the break-down of tetrataenite to disordered taenite is almost completed by the second heat treatment. In Fig. 1, a horizontally flat initial heating TM curve of Is, which charac- teristically decreases slowly with increasing temperature up to about 400°C, decreases sharply as it approaches the Curie point of disordered 50Fe50Ni taenite around 560°C. These two observed characteristics together represent the existence of a tetrataenite component. After a break-down of the tetrataenite phase at elevated temperatures, this characteristic feature of the heating TM curve never returns in repeated TM curves. The second-run heating TM curve can be decomposed into a super- imposed component of magnetite and disordered taenite which can be present at temperatures below 615°C, and a kamacite component which has its a --* Y transition at 780°C. The cooling TM curves are composed of magnetite, disordered taenite and kamacite at temperatures below about 600°C which cor- responds to the r -~ a transition point of a kamacite of about 5 wt% in Ni content. Because the ferromagnetic constituent of this chondrite consists of three main components, i.e. tetrataenite, magnetite and kamacite, a unique solution for determining the exact magnetic mineral composition in this chondrite cannot be directly derived using the magnetic binary system model analysis.10) How- ever, a roughly approximate binary system model consisting of an extraordi- narily high coercive tetrataenite component and the other low coercivity group can be used to derive approximate magnitudes of HC, HRC and bulk content of the tetrataenite component, where HRC/HC -1.5 is assumed. From the observed data of magnetic hysteresis parameters of Specimen (A) and (B) for temperatures between 22°C and -200°C, an approximate value of HC of tetra- No. 101 Tetrataenite in Carbonaceous Chondrites 187 taenite in the Y-791717 (C03) chondrite is estimated to be He 1250 Oe, which is roughly constant regardless of temperature, and the bulk content of the tetrataenite component is about 0.6 and 0.4 wt% in Specimen (A) and (B), respectively. The He value of the tetrataenite component of the Y-791717 chondrite thus estimated is considerably smaller than HC ' (2400 '' 2600) Oe of a synthesized single crystal of tetrataenite," or the almost pure grains of tetrataenite found in the Olivenza (LL6) chondrite. It is reported on the other hand that tetrataenite grains of 10 ' 40 nm in diameter synthesized by a coalescence process of ultra-fine particles of Fe and Ni are mostly composed of a tetrataenite nucleus and a kamacite outer layer enveloping the nucleus or a kamacite nucleus and a tetrataenite outer layer enveloping the nucleus.14) The apparent total coercive force of such a particle of double structure must be appreciably smaller than He of pure tetrataenite, depending on the mutual geometrical configuration of the two components. As an example, the measured value of He of a dilute homogeneous mixture of tetrataenite grains produced by the coalescence process of Fe and Ni fine smoke particles with silica (Si02) grains is only He = 740 Oe. Assuming then such a double component structure as mentioned above for the tetrataenite-rich grains having Hc~ 1250 Oe, the smallest grain size of the tetrataenite-rich grains in the Y-791717 (C03) chondrite is estimated with the aid of a magnetic granulometry analysis9),1~~ on the basis of observed data of Specimen (B) given in Table III. Results show that the smallest grains less than 6.5 nm in diameter occupy about 0.1 wt %, and all other grains which occupy about 0.4 wt% have a grain size larger than 10 nm. All these results lead to a conclusion that the Y-791717 (C03) chondrite contains tetrataenite grains of about 0.5 wt % together with magnetite and kamacite. The observed extreme stability of the NRM of this chondrite, despite thermal and alternating magnetic field demagnetization procedures, a characteristic uniquely common in all tetrataenite-containing chondrites, may also support this conclusion. 4. Tetrataenite in some other carbonaceous chondrites. The other three Antarctic carbonaceous chondrites, Y-693 (C4), Y-74662 (CM2) and Y-81020 (C03) examined in the present study contain no detectable amount of tetrataenite. According to results of studies on anisotropic optical reflection characteristics of smoothed surfaces of metallic mineral grains,4a however, it seems most likely that the Kainsaz (C03) chondrite contains a small amount of tetrataenite phase, which is optically named "the clear taenite phase." The average chemical composition of 3 specimens of clear taenite phase is given by 53.2 Ni, 0.36 Co, 0.02 Cu in wt%, Fe comprising the remaining wt%. On the other hand, TM curves of various selected carbonaceous chondrites obtained in a study for the main purpose of evaluating their magnetite content have been reported.1h' The TM curves of all these chondrites are re-examined in the present work. The initial heating TM curve of the Kainsaz (CO3) chondrite well exhibits the standard characteristic behaviour of tetrataenite, though its cooling TM curve is considerably deviated from the standard one. Other carbonaceous chondrites, TM curves of which look likely to represent the existence of a tetrataenite phase, are the Felix (CO3), Isna (C03) and Lance (CO3) chondrites. As 188 T. NAGATAand B. J. CARLETON [Vol. 66(B),

no other measured data for identifying a tetrataenite phase in these (C03) chondrites have been obtained, further discussions on these chondrites will be postponed. 5. Concluding remarks. In the present study, it is confirmed that some (C03) chondrites contain tetrataenite grains together with more abundant magnetite grains. It is quite unlikely that all these independent tetrataenite fine grains were formed as a result of a slow quasi-equilibrium cooling of 50Fe50Ni metal through a temperature range below 593°K in the solar nebula environment that would still allow the presence of Fe304 grains. It seems likely, in comparison, that these fine tetrataenite grains in (C03) chondrites were formed, at least in part, by the coalescence process of Fe and Ni smoke ultra-fine particles, either in the process of accretion of the (C03) chondrites at temperatures of 200 400°C,14),17) or in the process of their reaccretion after Fe 'Ni metals were vapourized at high temperatures beyond 1700°C into Fe and Ni smokes, perhaps as a result of a certain collision event in the early solar nebula. The authors' thanks are due to H. Kojima and M. Funaki of National In- stitute of Polar Research for their assistance in electron microprobe analyses and magnetic analyses, respectively. This research is supported in part by a research grant from the Japan Academy.

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