ELECTRONIC ABSORPTION AND LUMINESCENCE SPECTROSCOPIC STUDIES OF KYANITE SINGLE-CRYSTALS - DIFFERENTIATION BETWEEN EXCITATION OF FETI CHARGE-TRANSFER AND CR3+ DD TRANSITIONS

A selected set of five different kyanite samples was analysed by elect ron microprobe and found to contain chromium x(Cr3+) between <0.001 an d 0.055 per formula unit (pfu). Polarized electronic absorption spectr oscopy on oriented single crystals, R-1, R-2-sharp line luminescence a nd spectra of excitation of lambda(3)- and lambda(4)-components of R-1 -line of Cr3+-emission had the following results: (1) The Fe2+-Ti4+ ch arge transfer in c-parallel chains of edge connected M(1) and M(2) oct ahedra shows up in the electronic absorption spectra as an almost excl usively c(parallel to Z')-polarized, very strong and broad band at 160 00 cm(-1) if x(Cr3+)<0.001pfu, in this case the only band in the spect rum, and at an invariably lower energy of 15400 cm(-1) in crystals wit h x(Cr3+)greater than or equal to 0.003 pfu. The energy difference is explained by an expansion of the O-f-O-k, and O-b-O-m edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr 3+ substitutes for Al compared to the chromium-free case. (2) The Cr3 is proven in two greatly differing crystal fields a and b, giving ris e to two sets of bands, derived from the well known dd transitions of Cr(3+ 4)A(2g)-->T-4(2g)(F)(I), -->T-4(1g)(F)(II), and -->T-4(1g)(P)(II I). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a,A), 17200(a,E), 16000(b,A ), 16200(b,E); II: 24800(a,A), 24400(a,E); 22300(b,A), 22200(b,E); III : 28800(b,A) cm(-1). Evaluation of crystal field parameters from the b ands in the electronic spectra yield Dq(a)=1730 cm(-1), Dq(b)=1600 cm( -1), B(a)=790 cm(-1), B(b)=620 cm(-1) (errors ca. +/-10 cm(-1)), again in agreement with values extracted from the lambda(3),lambda(4) excit ation spectra. The CF-values of set a are close to those typical of Cr 3+ substituting for Al in octahedra of other silicate minerals without constitutional OH-as for sapphirine, mantle garnets or beryl, and are , therefore, interpreted as caused by Cr3+ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which a re crystallographically different but close in their mean Al-O distanc es, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees o f distortion. Hence, band set a originates from substitutive Cr3+ in t he kyanite structural matrix. The CF-data of Cr3+ type b, expecially B , resemble those of Cr3+ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a pre cursor of a corundum type exsolution.