Structure and polarized absorption spectra of Mn3+-substituted andalusites (viridines)

Reverse dichromatism in gem andalusite related to total pleochroism of the Fe2+-Ti4+ IVCT

We document a dichromatism effect in gem andalusite that shifts from light brownish pink at low thickness to medium green at high thickness. This is roughly reverse to that usually observed in other dichromatic materials. We show that this is due to a very strong pleochroism in andalusite, when dichromatism in other materials is due to an alexandrite effect. Here, colors varies from a strongly absorbing orangey-brown in the c direction to much less absorbing medium green in a and b directions. The brown absorption is so strong that it becomes opaque (total absorption) for thickness above about 1.5 mm (this value may vary according to concentration of the chromophore and intensity of the light source), and hence does not contribute to coloration anymore. For such high thicknesses, only the a and b directions remain transparent and hence only the green component contributes to the color: the sample appears green. For lower thicknesses, the green color combines with the orangey-brown color that becomes transparent along the c direction, resulting in a light brownish pink. From trace elements analysis in zoned samples, we also confirm that the dark orangey-brown color is due to Fe²⁺-Ti⁴⁺, and bring evidences that the light green color is mostly due to Fe, either in the form of isolated Fe³⁺ or Fe²⁺-Fe³⁺ IVCT, with possibly some contributions from isolated Mn³⁺.

The crystal chemistry of (Mn3 +, Fe3+)-substituted andalusites (viridines and kanonaite), (Al1−x−yMn3x+Fe3y+)2(OǀSiO4): crystal structure refinements, Mössbauer, and polarized optical absorption spectra

The crystal chemistry of viridines and kanonaite, (Al1-x-yMn3x+Fe3y+)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,57Fey-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 for57Fe, 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)octratio increases. This result is corroborated by the increasing energy of the Mn3+5B1g→5A1gtransition as determined from the optical spectra. The increasing octahedral elongation leads to a tilting of both the Al(2)O5trigonal bipyramids and SiO4tetrahedra, 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)octto be extrapolated forx= 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 Mn3 +are identified near 15000 cm-1[5B1g→5A1g(D)], 18000 cm-1{5B1g→[3T1g(H)]}, 19700cm-1{5B1g→[3T1g(H)]}, 21800cm-1[5B1g→5B2g(D)], 23300cm-1[5B1g→5Eg(D)] and spin forbidden Fe3+transitions near 19700cm-1{6A1g→[4T2g(G)]}, 20800 cm-1{6A1g→[4T2g(G)]}, 22300 cm-1{6A1g→ [4A1g,4Eg](G)}, 23300cm-1{6A1g→[4A1g,4Eg](G)}. The crystal-field parameter 10Dqfor 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 Mn3 +is nearly constant, 198 ± 2 kJ/g-atom Mn3 +, in the whole range of solid solutions, 0.0≤ x ≤ 0.4, which were studied.