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33. Tetrataenite in Chondritic Meteorites Tetrataenite Is an Ordered Phase of Feni with a Superlattice Crystal Struc- Ture Like

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33. Tetrataenite in Chondritic Meteorites Tetrataenite Is an Ordered Phase of Feni with a Superlattice Crystal Struc- Ture Like No. 6] Proc. Japan Acad., 65, Ser. B (1989) 121 33. Tetrataenite in Chondritic Meteorites By Takesi NAGATA, M. J. A., and Barbara J. CARLETON*) (Communicated June 13, 1989) Tetrataenite is an ordered phase of FeNi with a superlattice crystal struc- ture like that of AuCu having lattice parameters, a=2.533A and c=3.582A.1> With the crystal anisotropy energy (E) of the tetragonal unit crystal lattice expressed by E=k1 sin2 O+k2 sin4 0, (where the angle 0 is measured from the c-axis, k1=3.2 X100 ergs/cm3 and k2=2.3 X105 ergs/cm3) and the saturation mag- netization (Js) of Js=1300 emu /cm3,2> the magnetic coercive force (Hc) of a single domain crystal of tetrataenite can attain a maximum value of about 8000 Oe. A single crystal of ordered FeNi (tetrataenite) was first produced by neutron irradiation of a single crystal of disordered FeNi in the presence of a magnetic field of 2500 Oe along the (100) axis at 295°C.2),3) The artificial for- mation of tetrataenite has also been experimentally demonstrated by irradiating a disordered FeNi specimen by an electron beam.4) It was thus established that the tetrataenite phase is stable at temperatures below the order-disorder trans- formation temperature of 3200C.2) 3) In 1979, natural crystals of the tetrataenite structures were first discovered in iron meteorites, the Toluca and the Cape York, with the aid of Mossbauer spectral analysis and X-ray diffraction.J) Since then, the presence of the tetra- taenite phase of FeNi alloys has been found in metallic grains in some chondritic meteorites as well as in iron meteorites and mesosiderites. The large crystal anisotropy of tetrataenite is associated with both an optical anisotropy of its polished surface and a magnetic anisotropy. On the basis of the optical anisotropy and the unique chemical composition designated as 50Fe50Ni in atomic ratio, Clarke and Scott identified the ordered FeNi phase in metallic grains in a number of H-, L- and LL- chondrites, as well as in iron meteorites and mesosiderites.l) They gave the mineral name "Tetrataenite" to the ordered FeNi metallic phase in the meteorites. The magnetic properties of tetrataenite are characterized by: (a) anomalously large magnetic coercivity (larger than 103 Oe in magnetic coercive force, Hc); (b) an order-disorder transformation at 320°C; and (c) a high Curie temperature (about 750°C as compared with a Curie temperature of 560°C for the disordered taenite of the same chemical composition). Taking into account the representative characteristics (a), (b) and (c), to- gether with chemical composition data of metallic grains measurable by the EMPA method, a procedure for magnetic identification of the tetrataenite phase in meteorites has been developed with parallel checks by means of Mossbauer spectral analyses of the same samples.~~-10) A similar procedure for magnetically identifying tetrataenite in meteorites has been promoted by Wasilewski, mostly based on representative characteristics (b) and (c).11) In the authors' analysis system, a binary structure model consisting of a high magnetic coercivity com- ponent (A) having a large magnetic uniaxial anisotropy, and a low coercivity component (B) of ferromagnetic grains of multidomain structure, has been spe- *' National Institute of Polar Research , Kaga, Itabashi, Tokyo 173, Japan. 122 T. NAGATA and B. J. CARLETON [Vol. 65(B), cifically introduced in order to analyze the multi-component structure of the magnetic properties of meteorites.0),10) Table I. Composition of metallic grains in LL chondrites The data in Table I show experimental findings based upon magnetic analysis compared with those based upon Mossbauer spectral analysis with respect to kamacite (a), disordered taenite (r) and tetrataenite (r") in 4 LL chondrites. The apparent disparity in the results of the alternative analyses may be explained in part by recognizing that the examined specimens in these two types of analyses are not exactly the same and in part by the fact that both types of analysis involve probable errors of ±10%. Taking these factors into consideration, it may Fig. 1. Bulk content (in weight) of ferromagnetic tetrataenite phase in chondrites in a low tempera- ture range above the liquid helium temperature. (a) St. Severin (LL6) chondrite metal-rare matrix. (b) Y-791717 (C3) chondrite. No. 6] Tetrataenite in Chondrites 123 be concluded that the results are in fact in rough agreement with each other . The magnetic identification method for quantitatively detecting the presence of tetrataenite has been applied in analyzing a large number of H-, L- , LL- and C-chondrites collected from Antarctica.6)-10) To the present, the tetrataenite- containing Antarctic chondrites detected in this manner include 2 H-, 5 L-, 1 LL- and 1 C-chondrites. An extremely slow cooling rate (i.e. 1''102 degrees/106 years) of Ni-rich Fe-Ni at temperatures below 320°C has been proposed as the most probable process in forming the tetrataenite phase in Fe-Ni metallic grains occurring in meteorites.1),12),13) Such an interpretation seems reasonable in many instances in that tetrataenite phases are often found in Ni-rich iron meteorites and metallic parts of mesosiderites, while a stratification structure involving a thin tetrataenite layer placed between a kamacite domain and a disordered taenite domain has typically been observed in tetrataenite-rich Fe-Ni metals.1),13) In fact, the experimental production of tetrataenite induced by irradiating a dis- ordered FeNi specimen with neutrons or electrons2)-4) is equivalent to an artificial acceleration of the diffusion process of Ni atoms through the FeNi crystal lattice. It has been observed, however, that the tetrataenite phase is also present in some very fine Fe-Ni metallic grains in chondrites. Fig. 1(a) illustrates a sharp increase in the ferromagnetic tetrataenite abundance at temperatures below -200°C in metal-rare parts of St . Severin chondrite matrix. The amount of ferromagnetic tetrataenite in the sample is about 0.03 wt% at temperatures between 25°C and -200°C and sharply increases at temperatures below -200°C to 0.26 wt% at -269°C. This experimental result suggests that 88 wt% of tetrataenite is in the superparamagnetic state at temperature above -200°C, while these same particles behave f erromagnetically only at temperatures below -200°C . According to Neel's theory of superparamagnetism of uniaxially anisotropic fine particles,14) the relaxation time (zo) of fine ferromagnetic particles of volume v is given at temperature T by 1/zo=fo exp (-Ku v/kT), where f o is a pseudo-material constant equivalent to 109 sec-1. Taking z6 N 102 sec as a typical experimental procedure time, ku=3X 106 ergs/cm3 and T=70°K, v is evaluated as v N 1.1 X 10-19 cm3. If z~ N 103 sec, v is estimated as v N 1.2 X 10-19cm3. It is concluded therefore that 88% of the tetrataenite present at temperatures 0 below -200°C is in the form of particles with mean diameters smaller than 50 A. As the observed He value of the fine tetrataenite particles is 2250 Oe, reduced to 290 Oe after their order-disorder transformation at temperatures above 320°C, it is certain that these particles have a composition and a structure close to those of tetrataenite. C. Kaito has recently experimentally demonstrated the formation of tetra- taenite fine particles by combining a Ni smoke stream with a Fe smoke stream at a temperature around 200°C)- ~) The formation of tetrataenite nuclei by coalescence of ultra-fine particles of Fe and Ni at temperatures below 320°C with a potential for growth of the nuclei may be hypothesized as one possible process by which fine particles of tetrataenite are formed in chondritic mete- orites. The widely believed hypothesis that the disordered Ni-rich taenite phase which is stable at higher temperatures becomes separated into three stable phases, that is, a, r and r", during a course of extremely slow cooling,l),12 '13) is hardly plausible in explaining a formation process for the isolated fine 124 T. NAGATAand B. J. CARLETON [Vol. 65(B), particles of tetrataenite, though the hypothesis is plausible for understanding a formation process of a tetrataenite phase coexisting with c and r phases in Ni-rich iron meteorites and mesosiderites. Another newly observed fact which appears to support the coalescence mechanism hypothesis for forming fine tetrataenite particles is the discovery of tetrataenite in a carbonaceous chondrite, Y-791717. Results of magnetic and chemical analyses show that about 0.4 wt% of tetrataenite particles (Hc=1400 Oe) are present in Y-791717 carbonaceous chondrite, as shown in Fig. 1(b). Additional magnetic components in this C-chondrite include 2.9 wt % of magnetite and 0.45 wt % of kamacite. It is highly improbable that the generally proposed hypothesis of extremely slow cooling of Ni-rich disordered taenite is adequate to explain the formation of the tetrataenite phase in the case of the most primordial meteorite such as the carbonaceous chondrite Y-791717. The authors' thanks are due to M. Funaki for his cooperation in magnetic analysis and to J. A. Danon for his cooperation in Mossbauer analysis. This re- search work is supported in part by the research grant from the Japan Academy. References 1) R. S. Clarke Jr., and E. R. D. Scott (1980) : Amer. Miner., 65, 624-630. 2) L. Neel et al. (1964) : J. Appl. Phys., 35, 873-876. 3) J. Pauleve et al. (1962) : C. R. Acad. Sci., Paris, 254, 365-368. 4) A. Chamberedp et al. (1979) : J. Magn. Magn. Matr., 10, 139-144. 5) J. F. Albertsen et al. (1978) : Nature, 273, 453-454.
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