抄録
The crystal of synthetic rhodonite, Mn0.685Mg0.315SiO3, has cell dimensions, a=7.545(2) Å, b=11.782(3) Å, c=6.663(2) Å, α=92.69(2)°, β=94.32(3)°, γ=105.71(2)°, and space group P-1; Z=10, Dx=3.55g·cm−3. An isotropic refinement of the structure to R=4.0% has revealed the ordering scheme of Mg in the rhodonite type structure; with the use of Takéuchi’s site notation for cations (Takéuchi, 1977), the site occupancies of Mg are expressed as follows: 0.304(6) and 0.254(6) respectively at M10 and M11, and 0.132, 0.519(6) and 0.366(6) respectively at M21, M22 and M23. The geometrical characteristics of the octahedron about M22 can account for the preferential concentration of Mg in the site. The mode of structural variation of pyroxenoids according to the change in chemistry has been discussed in terms of the mean M–O distances of M1i cations (i=0, 1 for rhodonite) and the lengths of lj and l0, the former being the distance between apical oxygen atoms, associated M1i of a jth pair of adjacent tetrahedra in the same silicate chain, and the latter the distance between similar oxygen atoms of a specific pair of tetrahedra which are linked to the offset tetrahedron between them; l0 which is in fact an edge of M10 octahedron, significantly varies with the change in chemical composition. An analysis on the mode of structural variation has led to a view that pyroxenoids having silicate chains longer than those of ferrosilite III would not exist.
