Hardystonite

Luminescence spectra of hardystonite under 266 nm laser excitation reveal an extremely strong, rather narrow UV band at 355 nm, with a very short decay time of 25 ns (Fig. 4.20b). Usually such bands in minerals are attributed to Ce luminescence. However as another band was already confidently ascribed to this center (Fig. 4.20a) assignment appears problematic. In principle it is possible that several different Ce centers occur in a structure, which are formed, for example, as a result of substitutions on Ca and Zn positions or because of different types of charge compensations. The first possibihty may be excluded based on the large differences in ionic radii of Ce (115 ppm) and Zn in tetrahedral coordination (74 ppm), while the second possibihty may be taken into consideration. 

In order to check this, the excitation spectra of both UV bands have been determined (Gaft et al. 2002a). It was found that the UV band ascribed to Ce according to its liuninescence and decay, has an excitation spectrum which is typi- 

The luminescence of in synthetic Ca2ZnSi207 has been reported (Butler 

The emission and excitation peaks occur at 251 and 347 nm, respectively with a Stokes shift of 10,000 cm It is very close to luminescence and excitation bands detected in natural samples. In order to prove the possible relation of the UV luminescence band at 355 nm to Pb in natural hardystonite, its decay time as a function of temperature has been studied. These decay curves are very specific for mercury-like ions, where the emission at low temperatures is ascribed to the forbidden transition and has a long decay 

ICP analyses of the hardystonite sample in our study confirms a very high Pb content of 5,000 ppm, while the Ce concentration of 55 ppm is two orders of magnitude lower. This elevated Pb concentration may be the reason for a extremely short decay time at room temperature because of energy migration, with a corresponding decrease in decay time. The most Ukely position for Pb in the hardystonite structure is at the Ca site. The four-coordinated Zn site is less probable. Moreover, the difference between the ionic radii of Zn + and Pb is quite large. 

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Average of reported values for zircon, andalusite, silli-manite, staurolite, topaz, titanite, thortveitite, muscovite, apophyllite, hardystonite, analcite, carnegieite, sodalite, danburite, scapolite, and cristobalite mean deviation 0.02 A. 6 Average for BP04 (1.54 A.) and KHjPO, (1.56 A.). For KjSOi and other sulfates. i For Mg(C104)-6H,0. 

Most orthosilicates reacted completely with poly(acrylic acid) solution an exception was andradite, CagFOg [SiOJg. Even so, the cements of gehlenite and hardystonite were very weak and affected by water. Only gadolinite and willemite formed cements of some strength which were unaffected by water, probably because one contained beryllium and iron and the other zinc. 

Table 4.10. Concentrations of rare-earth elements in hardystonite sample (ppm)...

Fig. 4.20. a-d Laser-induced time-resolved luminescence spectra of hardystonite demonstrating Ce ", Pb +, Tm ", Dy " and Mn " centers... 

Caldte, Feldspars, Hardystonite, Pyromorphite, Scheelite, Zircon, Baddeleyite... 

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). 

Anhydrite, Feidspars, Fiuorite, Hardystonite, Zircon, Zoisite... 

Broad bands at 525 and 575 nm in the time-resolved luminescence spectra of hardystonite under 355 nm excitation (Fig. 4.20d) with very long decay time of several ms may be ascribed to strongly forbidden d-d transitions in the Mn " " luminescence center. Two bands may be connected with isomorphous substitutions on Ca in Zn structural positions. The spectriun of a famous yellow-green esperite luminescence (Fig. 4.21a) consists of a narrow band peaking at 545 nm with a very long decay time of 9 ms. Such parameters together with the typical excitation spectrum (Fig. 4.2 lb) enable confident identification of the luminescence center as Mn +. The orange emission near 600 nm of apophyllite is also evidently connected with the Mn center (Fig. 4.19c,d). 

CFsli) anhydrite, calcite, feldspars, danburite, datolite, esperite, apophyllite, charoite, fluorite, hardystonite... 

G3j2- H9j2 fluorite, anhydrite, titanite, zircon, hardystonite, pyromorphite... 

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