Luminescence and other spectroscopic properties of purple and green Cr-clinochlore

Abstract

For the first time ever, the luminescence spectra of Cr 3? centers in two chlorite crystals are presented. Chromium ions occupy the strong crystal-field site M4 in the brucite sheet and the intermediate crystal-field site in the inner octahedral sheet for purple and green chlorite, respectively. We discuss the influence of an effective positive charge on the Cr 3? ion and an effective negative charge of ligands on the differences in the values of the Dq and B parameters. It is concluded that the presence of Fe 2? ions and other point defects, as well as concentration quenching, causes the very short luminescence lifetimes of chromium ions.

First Identification of Sudoite in Caribbean Ceramic-Age Lapidary Craftsmanship

“…It is also the samples richest in vanadium that show the strongest absorption bands at 440 and 850 nm and exhibit this luminescence, but V 3+ is not a common emitter and there are many other possibilities, such as a color center or another trace element ion. Although there are several emissions known for chlorites, the only cause of emission explicitly identified is Cr 3+ (Gaft et al, 2005;Czaja et al, 2014).…”

Color and genesis of californite from Pakistan: insights from μ-XRF mapping, optical spectra and X-ray photoelectron spectroscopy

“…Furthermore, there are two additional bands at 203 and 549 cm −1 for the green site FB-4G1, and the band at 551 cm −1 also appears on the yellowish-green and green sites of FO-1, and these bands fit the standard data for clinochlore (RRUFF R060725) as well as the Raman spectra for clinochlore measured by Gopal et al 30 . The band at 549-551 cm −1 could presumably be attributed to the Si-O b -Si stretching mode, and the vibrations at 203 cm −1 have been assigned to the brucite Me-OH libration 31 . Remarkably, for FB-4G1 the Raman band at 637 cm −1 has a much lower intensity (I 637 ) than that at 679 cm −1 (I 679 ), while the intensity of the band at 637-639 cm −1 is higher than that at 687-691 cm −1 in other sites.…”

Shedding light onto the spectra of lime—Part 2: Raman spectra of Ca and Mg carbonates and the role of d‐block element luminescence

“…The equilibrium bond distances in the ground and first excited state are almost the same, according to the Franck-Condon principle favouring absorptions and emissions between the ground vibrational states of both electronic levels. This situation is inherent to complexes with small distances between the central Cr 3+ [9] and (Mg,Fe,Al) 6 (AlSi 3 O 10 )(OH) 8 :Cr 3+ (clinochlore), [10] leading to well-defined transitions and sharp bands usually in the red spectral range, therefore termed R lines. [9] Weaker interactions between central ion and ligands, for example, in e g 1 , same multiplicity ('high spin') and significantly different bond distance resulting in the excitation into an (almost) continuum of levels and a spectrally broad emission, [9] that is, a fluorescence more familiar to Raman spectroscopists, because known as unwanted 'background' and baseline raise.…”

“…The equilibrium bond distances in the ground and first excited state are almost the same, according to the Franck–Condon principle favouring absorptions and emissions between the ground vibrational states of both electronic levels. This situation is inherent to complexes with small distances between the central Cr 3+ and surrounding O 2− , further than MgO:Cr 3+ including Al 2 O 3 :Cr 3+ (ruby or sapphire), Mg 3 Al 2 (SiO 4 ) 3 :Cr 3+ (pyrope), Y 3 Al 5 O 12 :Cr 3+ (yttrium aluminium garnet, YAG), MgAl 2 O 4 :Cr 3+ (spinel [ 9 ] and (Mg,Fe,Al) 6 (AlSi 3 O 10 )(OH) 8 :Cr 3+ (clinochlore), [ 10 ] leading to well‐defined transitions and sharp bands usually in the red spectral range, therefore termed R lines. [ 9 ] Weaker interactions between central ion and ligands, for example, in Mg 2 SiO 4 :Cr 3+ (forsterite) and CaMgSi 2 O 6 :Cr 3+ (diopside), favour the transition of an electron into another orbital over spin reversal.…”