Benitoite

Literature that presents information on minerals often shows the composition in terms of the constituent oxides. For example, benitoite is described as 36.3% BaO, 20.2% Ti02, and 43.5% Si02. Therefore, many silicates can formally be regarded as combinations of oxides as shown in Table 14.1. 

Benitoite is a rare, strongly dichroic, blue mineral used as a gemstone. In spite of much effort in its study, the origin of color in benitoite has not been definitively estabhshed. Because traces of Fe are found, ideas proposed include the Fe -Ti or the Fe -Fe inter-valance charge transfer. While most benitoite is colorless when viewed down the c-axis, there are a very small number of exceedingly rare stones, which are pink in this direction (Rossman 1988). 

Benitoite is characterized by very intensive blue luminescence (White 1990). Laser-induced time-resolved technique enables us to detect three broad bands and one narrow line, connectet with TiOe, Ti " " and Cr or Mn, luminescence centers (Fig. 4.35). 

Combination of broad emission band and narrow line is typical for elements with d electronic configuration, such as Cr +, Mn and Manganese participation is supported by chemical analyses of benitoite, where chromium was never mentioned as micro-impurity, while Mn is known with concentrations changing from 0.03 to 0.11% (Laurs et al. 1997). Such concentrations are quite enough for luminescence generation. Substitution in Mn +form substituting... 

Comparison of this luminescence intensity in different samples reveals that any correlation is absent any impurity concentration. Thus it was supposed that the mostly probable luminescence center is Ti, which presence is quite natural in Ti bearing benitoite. The wide occurrence of Ti " minor impurities in minerals was detected by EPR. Like the other d ions (V, Mo ), Ti ions occur often in minerals as electron center (Marfunin 1979). It may be realized in benitoite, which does have some natural exposure to gamma rays in its natural setting. There could be radiation centers, such as, for example, Ti + gamma ray + electron donor Ti + electron hole. Benitoite color does not change with gamma irradiation to quite high doses (Rossman 1997) but luminescence is much more sensitive compared to optical absorption and can occur from centers at such low concentration that they do not impact the color of a benitoite. 

Eigure 5.41 summarizes the temperature behavior of decay time and quantum efficiency of red benitoite luminescence at 660 nm in the forms ln(r) and ln(q) as a functions of 1/T. In such case the luminescence may be explained using simple scheme of two levels, namely excited and ground ones. The relative quantum yield (q) and decay time (r) of the red emission may be described by simple Arhenius equations ... 

Fig. 5.41. Temperature dependence of quantum efficiency (Q) and decay time (t) of red emission band of benitoite at 660 nm...

Fig. 5.62.a. Calculated energy levels cheme for blue benitoite luminescence... 

Fig. 5. 62.b. Energy levels scheme for TiOe limiines-cence center in benitoite... 

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