Crystal structure and compositional variation of Angra DOS Reis fassaite

Details Crystal structure and compositional variation of Angra DOS Reis fassaite Crystal structure and compositional variation of Angra DOS Reis fassaite

Jun 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977e%26psl..35..357h&link_type=abstract

Earth and Planetary Science Letters, vol. 35, no. 2, June 1977, p. 357-362.

Computer Science

7

Chemical Bonds, Crystal Structure, Meteoritic Composition, Pyroxenes, Electron Probes, Microanalysis, Molecular Structure, Octahedrons, Tetrahedrons, X Ray Analysis

Scientific paper

The crystal structure of fassaite from the Angra dos Reis meteorite has been determined by least-squares refinement of three-dimensional X-ray data to an R value of 3.3%. The pyroxene is monoclinic, space group C2/c, with unit-cell dimensions a = 9.738(1), b = 8.874(2), c = 5.2827(5) Å, β = 105.89(1)°, and V = 439.1(1) Å3. Average bond lengths are (Si,Al)-O = 1.651, M1-O = 2.061, and M2-O = 2.489 Å. The distribution of iron and magnesium between M1 and M2 suggests a temperature of equilibration greater than 1000°C. Electron microprobe analysis of several fassaite grains reveals small but statistically significant variations of (Mg + Si) versus (Al - Ti). The range of fassaite composition may be represented by En3Hd22TiCpx6(Di53+/-2CaTs16-/+2) where En = Mg2Si2O6, Hd = CaFeSi2O6, TiCpx = CaTiAl2O6, Di = CaMgSi2O6, CaTs = CaAl2SiO6. Most fassaite analyses calculated on the basis of four cations yielded greater than six anions, suggesting that part of the titanium or chromium might be reduced to Ti3+ or Cr2+.

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