Druse clinopyroxene in D'Orbigny angritic meteorite studied by single-crystal X-ray diffraction, electron microprobe analysis and Mössbauer spectroscopy

A. YASSIR; R. B. SCORZELLI; M. E. VARELA; G. KURAT; I. SOUZA-AZEVEDO; S. J. STEWART; F. C. HAWTHORNE

METEORITICS & PLANETARY SCIENCE

METEORITICAL SOC

Año: 2009 vol. 44 p. 581 - 587

1086-9379

The crystal structure of druse clinopyroxene from the D’Orbigny angrite, (Ca0.944 Fe2+0.042 Mg0.010Mn0.004) (Mg0.469Fe2+ 0.317Fe3+ 0.035Al0.125Cr0.010Ti0.044) (Si1.742Al0.258) O6, a = 9.7684(2), b =8.9124(2), c = 5.2859(1) Å, β = 105.903(1)˚, V = 442.58 Å3, space group C2/c, Z = 2, has been refined to an R1 index of 1.92 % using single-crystal X-ray diffraction data. The unit formula,calculated from electron microprobe analysis, and the refined site-scattering values were used to assign site-populations. The distribution of Fe2+ and Mg over the M1 and M2 sites suggests a closure temperature of 1000 ºC. Mössbauer spectroscopy measurements were done at room temperature on a single-crystal and a powdered sample. The spectra are adequately fit by a Voigt-based quadrupole-splitting distribution model having two generalized sites, one for Fe2+ with two Gaussian components and one for Fe3+ with one Gaussian component. The two ferrous components are assigned to Fe2+ at the M1 site, and arise from two different next-nearest-neighbour configurations of Ca and Fe cations at the M2 site: (3Ca,0Fe) and (2Ca,1Fe). The Fe3+/Fetot ratio determined by Mössbauer spectroscopy is in agreement with that calculated from the electron-microprobe analysis.  The results are discussed in connection with the redox and thermal history of D’Orbigny.