2.輝石の結晶構造に関連する諸問題
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Recent studies on the crystal structures of pyroxenes are summerized. The structural differences of clinopyroxenes are explained based on the metal atoms at the M1 and M2 sites. Space group P2 for omphacite is strongly doubted. Close structural relations between clinopyroxenes and pyroxenoids are explained based on the close packing of oxygen and arrangements of metal atoms. Structural relations between orthopyroxenes and clinopyroxenes are explained by the "unit-cell twinning" as proposed by Ito. Strong tendency for tetrahedral arrangements around Zn atoms in the pyroxene-type ZnSiO_3 results in an exception of polymorphic relation between clinopyroxenes and orthopyroxenes. The structural change in the pyroxene solid solutions of the common quadrilateral composition is explained based on the site occupancy at the M1 and M2 sites and the tetrahedral sites of the silicate chains. The changes of the polyhedra around the M2 sites and of the M1-O and M2-O distances in the solid solutions are shown based on the recent data. The domain structure ofpigeonite indicates the high-low transition of pigeonite from C2/c to P2_1/c. The structural change of pigeonite in this transition is explained by the elongation of the SiO_3 Chains with displacements of O3. The structural changes of clinopyroxenes due to temperature increase are generally explained by the difference in the rate of increase of the Si-O and M-O distances. The lattice distortions of pyroxenes are commonly observed in fine textures of exsolution of pyroxenes. Pigeonite crystals with exsolution of two kinds of augite from the Moore County meteorite are presented as an example.
- 1974-09-30