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

Abstract

AbstractThe 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.