The crystal chemistry of (Mn^{3 +}, Fe³⁺)-substituted andalusites (viridines and kanonaite), (Al_1−x−yMn³_x⁺Fe³_y⁺)₂ (OǀSiO₄): crystal structure refinements, Mössbauer, and polarized optical absorption spectra

Abstract

The crystal chemistry of viridines and kanonaite, (Al_1-x-yMn³_x⁺ Fe³_y⁺ )₂ (OǀSiO₄), has been evaluated with special reference to the behavior of Mn³⁺ in this andalusite type structure. Five natural samples (from Ultevis: x = 0.012, y = 0.028; from Yakutia: x = 0.076, y = 0.046; from Tanzania: x = 0.091, y = 0.031; from Darmstadt: x = 0.171, y = 0.048; from Kanona: x = 0.340, y = 0.009) and two synthetic viridines (P135: x = 0.173, y = 0.005; P150: x = 0.22, y = 0) have been studied by means of X-ray powder diffraction, single crystal-structure refinements, ⁵⁷Fe y-resonance spectroscopy, and optical absorption microspectroscopy.

The structure refinements reveal that the transition metal ions substitute for Al almost exclusively in the distorted octahedral Al(l) site of the andalusite structure type within the entire mixed crystal series. This is independently proven by the Mössbauer results for ⁵⁷Fe, which show that only 10 to 15 % of total iron is present in the Al(2) trigonal-bipyramidal site. With increasing substitution, the octahedral (c/a)_oct ratio increases. This result is corroborated by the increasing energy of the Mn³⁺⁵B_1g → ⁵A_1g transition as determined from the optical spectra. The increasing octahedral elongation leads to a tilting of both the Al(2)O₅ trigonal bipyramids and SiO₄ tetrahedra, and to slight changes of several of the Al(2)– O bond distances. These changes, although similar to those observed at high temperatures (Winter and Ghose, 1979), are considerably stronger than those caused by high temperature (e.g.,Δ(c/a)_oct/(c/a)_oct to be extrapolated for x = 0.5 is approximately 0.1 while at 1000°C this relative change is only 0.035]. In the optical spectra, spin-allowed and spin-forbidden transitions of Mn^{3 +} are identified near 15000 cm-¹ [⁵B_1g→ ⁵A_1g(D)], 18000 cm^-1 {⁵B_1g→[³T_1g(H)]}, 19700cm^-1 {⁵B_1g→[³T_1g(H)]}, 21800cm^-1 [⁵B_1g→⁵B_2g(D)], 23300cm^-1 [⁵B_1g→ ⁵E_g(D)] and spin forbidden Fe³⁺ transitions near 19700cm^-1 {⁶A_1g→[⁴T_2g(G)]}, 20800 cm^-1 {⁶A_1g →[⁴T_2g(G)]}, 22300 cm^-1 {⁶A_1g→ [⁴A_1g, ⁴E_g](G)}, 23300cm^-1 {⁶A_1g→[⁴A_1g, ₄E_g](G)}. The crystal-field parameter 10 Dq for Mn³⁺ decreases in the whole series by approximately 10%. However, this effect is compensated by increasing groundstate splitting such that the crystal field stabilazation energy of Mn^{3 +} is nearly constant, 198 ± 2 kJ/g-atom Mn^{3 +}, in the whole range of solid solutions, 0.0≤ x ≤ 0.4, which were studied.