Alexej N. Platonov

Optical absorption investigation of Cr3+ ion-bearing minerals in the temperature range 77–797 K

AbstractThe effect of raising temperature on spin-allowed dd-transitions of octahedral Cr3+ was studied for various point symmetries of the Cr3+-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr3+-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30 000 to 11 100 cm-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr3+-transitions derived from 4A2g→4T2g(F) and →4T1g(F) as well as of Dq- and B-values derived, had the following results:In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr3+, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr3+-centered octahedra, αloc, were derived on the basis of the RM-0-5-proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging rm-o for all the respective octahedral positions.On increasing temperature, the half band widths increase. The evaluation for spinel, pyrope and uvarovite showed n

Fe2+-Ti4+ charge-transfer in dumortierite

Delta tilde v_{{1 mathord{left/ {vphantom {1 2}} right. kern-nulldelimiterspace} 2}}

The crystal chemistry of the humite minerals: Fe2+-Ti4+ charge transfer and structural allocation of Ti4+ in chondrodite and clinohumite

n to increase by about 60%, for both types of Cr3+ transitions, in the temperature range studied.The temperature dependence of band intensities is complex, because it depends on the site symmetry and related symmetry selection rules: symmetry forbidden transitions increase strongly on heating. This was observed for Cr3+ in centosymmetric sites n

Electronic absorption spectra of chromium-bearing sapphirine

bar 3

Local relaxation around [6]Cr3+ in synthetic pyrope–knorringite garnets, [8]Mg3[6](Al1−X CrX3+)2[4]Si3O12, from electronic absorption spectra

nm of spinel and n