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Crystal structure and compositional variation of Angra dos Reis fassaite

1977; Elsevier BV; Volume: 35; Issue: 2 Linguagem: Inglês

10.1016/0012-821x(77)90138-8

1385-013X

Robert M. Hazen, L. W. Finger,

Geological and Geochemical Analysis

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 anR value of 3.3%. The pyroxene is monoclinic, space groupC2/c, with unit-cell dimensionsa = 9.738(1),b = 8.874(2),c = 5.2827(5)Å, β = 105.89(1)°, andV = 439.1(1)Å3. Average bond lengths are (Si,Al)-O = 1.651, M1-O = 2.061, and M2-O = 2.489Å. 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 byEn3Hd22TiCpx6(Di53±2CaTs16∓2) whereEn=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+.