The origin of pleochroism in erythrite

Abstract

Optical absorption measurements show that the pleochroism of erythrite is due to variations in the main absorption envelope in the 400 to 600-mn region as crystallographically oriented sections are rotated in linearly polarized light. The absorption envelope arises from the 4T1 (F) --> 4 T, (P) transition, with contributions from transitions derived from 2G and 'H levels, in octahedrally coordinated Coil- ions, of which there are two kinds (Coi and Con) in erythrite. The pleochroism is related to the interaction of the electric vector of light with electrons located in specific t2g orbital lobes of ions in the Coi and Coil sites. The validity of the present interpretation rests on the assumptions that erythrite is isostructural with vivianite and symplesite and that the principal optical directions are essentially coincident with the "molecular axes" of the Co2+-bearing octahedra.

Electrodes modified with synthetic clay minerals: Evidence of direct electron transfer from structural iron sites in the clay lattice

“…The pink color is characteristic of octahedrally coordinated Co 2+. Absorption bands between 19,000 to 20,000 cm -1 attributed to 4T1(F)---->4TI(P) transition of high-spin Co 2 § in octahedral coordination are found for many Co-containing minerals (Faye and Nickel 1968;Rossman 1988). Bruce et al (1986) attributed the blue color of their NaCo-smectites to tetrahedral Co 2+ substitutions.…”

Mixed-Valence Minerals of Iron and Titanium: Correlations of Structural, Mössbauer and Electronic Spectral Data

“…The major feature in the visible-region spectra of blue kyanites is an intense polarization-dependent band at 16,500 cm-l with a prominent shoulder in the region 11,500 -12,500 cm-l (78,79). The 16,500 cm-l band was assigned to Fe2+ + Fe 3 + CT (75,78) between Fe 2 + and Fe 3 + ions located in chains of edgeshared [Al061 octahedra in the kyanite structure in which AI-AI distances are 276-288 pm (77).…”

“…The M-Tl distance across the isolated M octahedron-Tl tetrahedron clusters is only 274pm. The absorption spectra of cordierite (2,3,7,41) show a broad intense band at about 17,000 cm-l polarized in the plane of the edge-shared M and Tl polyhedra, the intensity of which increases at low temperatures (7). The Mossbauer spectra of such cordierites, how~ver, show no evidence of Fe3+ at 293K, (42), even in specimens reported to contain as much as 22% of the iron in the ferric state ( Figure 12).…”

“…8H20, is the best known mineral in which a Fe Z + ~ Fe3~ intervalence transition occurs across isolated pairs of edge-shared octahedra (2,3,(8)(9)(10). The crystal structure of vivianite illustrated in Figure 1 contains two distinct cation sites (11).…”

Optical absorption and EPR studies on enargite

“…These are defined as be strong and all other spin forbidden transitions are very weak [7,8]. Thus, the 5 T 2g → 5 E g transition gives an intense, but broad absorption band.…”