Study of Bursa L6 ordinary chondrite by X‐ray diffraction, magnetization measurements, and Mössbauer spectroscopy

Abstract

We report the results of the complex study of the bulk interior of Bursa L6 ordinary chondrite using optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, electron microprobe analysis (EMPA), X-ray diffraction (XRD), magnetization measurements, and M€ ossbauer spectroscopy. The main and minor ironbearing phases and their chemical compositions were determined by these techniques. The detected iron-bearing phases in the bulk interior of Bursa L6 are the following: olivine; orthopyroxene; Ca-rich clinopyroxene; troilite; chromite; hercynite; ilmenite; the a 2-Fe(Ni, Co), a-Fe(Ni, Co), and c-Fe(Ni, Co) phases; and ferrihydrite resulting from meteorite terrestrial weathering. Using the EMPA, the values of fayalite and ferrosilite were obtained as~25.2% and~21.4%, respectively. The unit cell parameters for silicate crystals were determined from XRD, then the Fe 2+ and Mg 2+ occupations of the M1 and M2 sites in these crystals were estimated. Further calculations of the ratios of the Fe 2+ occupancies in the M1 and M2 sites in olivine and orthopyroxene based on XRD and M€ ossbauer spectroscopy appeared to be in a good agreement. The temperatures of equilibrium cation distributions for olivine and orthopyroxene obtained from these techniques are consistent: 623 K (XRD) and 625 K (M€ ossbauer spectroscopy) for olivine and 1138 K (XRD) and 1122 K (M€ ossbauer spectroscopy) for orthopyroxene.

Applications of Mössbauer Spectroscopy in Meteoritical and Planetary Science, Part I: Undifferentiated Meteorites

“…The authors decomposed their spectrum in two magnetic sextets and five quadrupole doublets assigned to the following phases: (i) Fe-Ni alloy (δ = 0.09 mm/s, H eff = 331 kOe, A = 2.1%); (ii) troilite (δ = 0.75 mm/s, H eff = 311 kOe, A = 23.0%); (iii) olivine (M1: δ = 1.14 mm/s, ∆E Q = 3.02 mm/s, A = 20.3% and M2: δ = 1.13 mm/s, ∆E Q = 2.80 mm/s, A = 21.8%); (iv) pyroxene (M1: δ = 1.10 mm/s, ∆E Q = 2.29 mm/s, A = 5.0% and M2: δ = 1.11 mm/s, ∆E Q = 2.06 mm/s, A = 11.3%); (v) ferric compound (δ = 0.44 mm/s, ∆E Q = 0.57 mm/s, A = 16.4%). Further, ordinary chondrites Tsarev L5, Ozerki L6, Kemer L4, and Bursa L6 were studied by Mössbauer spectroscopy with a high velocity resolution [98][99][100][101] and these spectra were decomposed using the fitting model with simulation of the full static Hamiltonian for troilite component and accounting for additional minor components (see Figure 29). Additionally, new components associated with the M1 and M2 sites in clinopyroxene (if clinopyroxene content was not less than 4 wt%), with chromite, hercynite, ilmenite, non-stoichiometric troilite Fe 1−x S and components for Fe-Ni-Co phases with Ni content variations were revealed in these spectra.…”

“…Further, modal analysis for the Mössbauer spectra of equilibrated H, L, and LL ordinary chondrites measured with a high velocity resolution in [84,[98][99][100][101]106,107,111] and fitted using a simulation of the full static Hamiltonian and minor spectral components is shown in Figure 39. The total relative areas for: (i) the α 2 -Fe(Ni, Co), α-Fe(Ni, Co) and γ-Fe(Ni, Co) phases; (ii) ferric compounds and the M1 and M2 sites in (iii) olivine; (iv) orthopyroxene; and (v) clinopyroxene; (vi) troilite and Fe 1−x S; (vii) chromite; (viii) hercynite; and (ix) ilmenite were used.…”

“…These equations were used for T eq calculations for olivine and orthopyroxene in ordinary chondrites considered in [100,101,107,112] and references therein. The T eq values deduced from Mössbauer spectroscopy were compared with T eq values obtained using XRD data and listed in Tables 2 and 3 for olivine and orthopyroxene, respectively.…”

“…Table 2. The values of the distribution coefficient and the temperature of equilibrium cation distribution in olivine crystals in ordinary chondrites determined by means of X-ray diffraction and Mössbauer spectroscopy (data were taken from References [100,101,107,112]). Then, the use of Mössbauer parameters (relative areas of spectral components) for ordinary chondrites classification was continued for the results obtained by Mössbauer spectroscopy with a high velocity resolution considered first in [77] and continued in [83,99,106,107,111].…”

“…This is related to the higher total relative area of Fe-Ni-Co alloy and ferric components (in the case of low weathering grade) in the Mössbauer spectra than that observed for corresponding ordinary chondrite groups. The total relative areas A total of Fe-Ni-Co alloy and ferric components in the case of weathering for these meteorites are the following: A total =~53% for Annama H5 [84], A total =~29.9% for Kemer L4 [100], A total =~31.6% for Bursa L6 [101] and A total =~10.2% for Chelyabinsk LL5, fragment No 2 [106]. In the case of equal f -factor for all phases, A total in the arbitrary units in fact is the metallic iron fraction Fe 0 /Fe total considered in [36].…”