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39. T Etrataenite in Carbonaceous Chondrites

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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 tetra- taenite 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 identi- fying 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 % through- out 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 near- ly 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 mag- netic 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 ap- proximate 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 pro- cedures 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 tae- nite 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 tempera- tures 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 magne- tite, 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 thermo- magnetic 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 tae- nite 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, dis- ordered 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.
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