Crystal Structures of Pyroxene-Type ZnSiO_3 and ZnMgSi_2O_6(Physics)

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The crystal structures of two pyroxene polymorphs of ZnSiO_3, and the orthopyroxene of ZnMgSi_2O_6 have been studied. The monoclinic ZnSiO_3 crystallizes in the space group C2/c with a=9.787, b=9.161, c=5.296A, β=111.42°, Z=8 ; orthorhombic ZnSiO_3, Pbca, a=18.204, b=9.087, c=5.278A, Z=16 and orthorhombic ZnMgSi_2O_6, a=18.201, b=8.916, c=5.209A, Z=16. In the structure of the monoclinic ZnSiO_3, Zn atoms are coordinated octahedrally (at M1 sites) and tetrahedrally (at M2 sites). The Zn atoms at the M2 sites are not coordinated by the bridging oxygen [O(3)] of the SiO_3 chains. In the orthorhombic ZnSiO_3, however, Zn atoms at the M2 sites have an irregular octahedral coordination including O(3) atoms. Because of the difference in coordination of O(3) to Zn atoms, the shape of the SiO_3 chains in the two polymorphs of ZnSiO_3 is different. The structure of the orthorhombic ZnMgSi_2O_6 is intermediate between that of enstatite (MgSiO_3) and the orthorhombic ZnSiO_3. Zn atoms are partially ordered in M1 and M2 sites with site occupancies of 36 and 64% respectively.