Optical spectroscopic study of tuhualite and a re-examination of the beryl, cordierite, and osumilite spectra

摘要: Polarized optical absorption spectra of a rare Fe^(2+), Fe^(3+)-bearing silicate mineral, tuhualite, (Na,K)_2 Fe^(2+)_2 Fe^(3+)_2 Si_(12)O_(30), were measured at room temperature in the range 350–4000 nm (ca. 28500–2500 cm^(−1)). The display number strongly pleochroic bands visible and NIR range, which are attributed to ferric ferrous ions distributed over octahedral tetrahedral sites structure. No caused by H_2O or OH stretching vibrations have been observed 3000 4000 cm^(−1) range. A narrow, weak line ~422 (γ ≈ β) is ^6A_(1g) → ^4A_(1g),^4E_g spin-forbidden transition six-coordinated Fe^(3+) that occupy Fe2 position tuhualite broad, intense band ~573 > β >> α) assigned Fe^(2+)/Fe^(3+) intervalence charge transfer (IVCT) between Fe^(2+) Fe^(3+). Together with high-energy edge, 573 causes violet color spectacular pleochroism tuhualite. An ~1040 much weaker ~ 2150 (α) spin-allowed dd transitions distorted Fe1 sites. On basis data obtained for structurally related minerals beryl, cordierite, osumilite discussed re-assigned. re-interpreted on distribution these structural positions.