Fluorite luminescence

Fig. 4.4. a-d Laser-induced time-resolved luminescence spectra of apatite with uranyl (a-c) and luminescence (d) after oxidizing heating (possibly in fluorite inclusions)... 

The fluorite in our study consisted of 40 samples from different environments. Concentrations of luminescence impurities in several samples are given in Table 4.6. By using laser-induced time-resolved spectroscopy we were able to detect and ascribe the following emission centers Eu +, Ce ", Gd +, Sm +, Dy3+, Eu +, Pr +, Er +, Tm +, Ho +, Nd +, Mn + and the M-center (Figs. 4.10-4.12). 

The structure of pyrochlore is considered to be an anion deficient derivative of the fluorite structure type. Ca atoms are in eight-fold coordination, while Nb atoms are in six-fold coordination. Steady-state luminescence spectra of pyrochlore revealed emission of REE, such as trivalent Dy and Nd (Gorobets and Rogojine 2001). The natural pyrochlore in our study consisted of four... 

The steady-state luminescence of Pr + in minerals was found only in scheel-ite, where the hne near 480 nm has been ascribed to this center (Gorobets and Kudrina 1976) and possibly in fluorite (Krasilschikova et al. 1986). The luminescence of Pr in minerals is difficult to detect because its radiative transitions are hidden by the stronger lines of Sm in the orange range of 600-650 nm, Dy " " in the blue range of 470-490 nm and Nd in the near IR (870-900 nm). In order to extract the hidden Pr lines time-resolved luminescence was applied. The fact was used that Pr " usually has a relatively short decay time compared to its competitors Dy ", Sm " and Nd, especially from the Po level. In order to correct identification of Pr " lines in minerals several of them were synthesized and artificially activated by Pr (Fig. 5.5). Besides, comparison has been made with CL spectra of synthetic minerals artificially activated by Pr (Blank et al. 2000). 

IR luminescence lines with relatively short decay times connected with Nd are very strong in the fluorite emission spectrum (Fig. 4.11c,d). Besides that, UV and violet hnes with a short decay time appear, which are ascribed to Nd (Fig. 4.11a,b). 

The lines of Sm + connected with several types of centers are well studied in fluorite by steady-state luminescence spectroscopy (Tarashchan 1978 Krasilschikova et al. 1986). In time-resolved spectra it is mostly prominent after long delay times and is mainly characterized by the Hnes at 562,595 and 651 nm (Fig. 4.10d). 

Together with Sm another group of lines is often detected with the main line at 685 nm, which also has a very long decay time of several ms (Fig. 4. lOd). It is very close to the known resonance line of Sm. Under low power UV lamp excitation, the luminescence of Sm in fluorite is known only at low temperatures, starting from approximately 77 K, and is composed of narrow /-/ transition lines and a broad band of 4f-5d transitions (Tarashchan 1978 Krasilschikova et al. 1986). Evidently, under strong laser excitation, luminescence of Sm + may be seen even at room temperature, where 4f-5d luminescence is usually quenched because of radiationless transition. 

The luminescence center of divalent europium in fluorite is well known (Haber-land et al. 1934 Tarashchan 1978 Krasilschikova et al. 1986 Barbin et al. 1996). It is clearly seen in laser-induced time-resolved luminescence spectra with a decay time of 600-800 ns (Fig. 4.10a). In several samples the band with a spectrum similar to those of Eu + has a very long decay time and remains even after a delay of several ms. Principally it may be connected with energy migration from a UV emitting center with a long decay time, for example, Gd ". ... 

In all minerals the gadolinium luminescence spectra are completely located in the UV part of the spectrum and consist of several lines at 310-315 nm, corresponding to transition P7/2- S7/2- The main line is characterized by a long decay time and is especially prominent in the spectra with a long delay. Gd " is known as a good sensitizer of the other rare-earth ions luminescence. It is detected in spectra of fluorite (Fig. 4.12a), zircon (Fig. 4.38h), anhydrite (Fig. 4.17a), and hardystonite (Fig. 4.20b). 

A possible candidate may be Tm ". For example, the doublets at 803 and 817 nm and at 796 and 813 nm are the strongest ones in cathodoluminescence spectra of fluorite and scheelite activated by Tm " (Blank et al. 2000). It is possible to suppose that the strong fines at 805 and 820 nm with a relatively short decay time of 60 ps in the titanite luminescence spectrum belong to Tm " ". They appear under 532 nm excitation and are evidently connected with the electron transition. Similar emission of Tm " was also detected in... 

Characteristic bands of Mn + well studied by steady-state luminescent spectroscopy (Tarashchan 1978 Gorobets and Rogojine 2001) have been found in time-resolved luminescence spectra of calcite (Fig. 4.14a), fluorite (Fig. 4.10d), datolite (Fig. 4.16d), wollastonite (two bands at 555 and 603 nm Fig. 4.42a,c), and spodumen (Fig. 4.61a). 

Spectral parameters of the structured green luminescence (Fig. 4.4d) are absolutely similar to those of luminescence in fluorite after thermal treatment (Tarashchan 1978). Principally, during the calcination of the sedimentary phosphates new mineralogical phases, including fluorite, may be formed. Taking these data into accoimt, it is possible to conclude that after thermal treatment uranium is concentrated in the fluorite lattice in the form of... 

At the present time luminescent sorters are mainly used for the processing of diamonds, but also for the scheehte, fluorite and others types of ores (Mokrousov and lileev 1979 Salter and Wyatt 1991 Gorobets et al. 1997a). The sources of luminescence excitation in these sorters are X-ray tubes and UV lamps, where the first is more powerful and the second is more selective. The UV impulse lasers employment as excitation sources enables us to combine the... 

Fluorite is used as a flux in steel making and in the smelting of ores. Luminescence sorting of fluorite is a well-known technique, which is based on the strong blue luminescence of Eu. ... 

The triboluminescence of minerals has been studied visually (see the footnotes to Table I) but only a few minerals have been examined spectroscopically. There are a few clear examples of noncentric crystals, such as quartz, whose emission is lightning, sometimes with black body radiation. Most of the triboluminescent minerals appear to have activity and color which is dependent on impurities, as is the case for kunzite, fluorite, sphalerite and probably the alkali halides. Table I attempts to distinguish between fracto-luminescence and deformation luminescence, but the distinctions are not clear cut. A detailed analysis of the structural features of triboluminescent and nontriboluminescent minerals may make it possible to draw conclusions about the nature and concentration of trace impurities that are not obvious from the color or geological site of the crystals. Triboluminescence could be used as an additional method for characterizing minerals in the field, using only the standard rock hammer, with the sensitive human eye as a detector. 

Sm luminescence has been reported from natural anhydrite samples (Gaft et al. 1985, 2001a Taraschan 1978) as a broad strong band at 630 nm. Other sharp bands are reported at 688, 700 and 734 nm. Sm emission has not been reported for apatite, but the ion size and valence are amenable to the Ca sites, so ultimately it may be observed via pulsed laser techniques. Sm is present in aqueous solution only under quite reducing conditions, so this may limit concentrations. Sm does occur in fluorite, and is responsible for the strong green coloration and sensitized Eu luminescence in that mineral (Robbins 1994). However, in fluorite the Sm may be created by radiation effects which reduce bound Sm ions. Yb emission has not apparently been reported in any minerals, but has been studied in borates and oxides (Blasse and Grabmaier 1994). Its... 

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