Hardystonite structure

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. 

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). 

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. 

A Figure 22.52 Geometrical structure of the Si207 ion, which is formed by the sharing of an oxygen atom by two silicon atoms. This ion occurs in several minerals, such as hardystonite [Ca2Zn(Si207)]. 

Local structural relaxation around Co along the hardystonite-Co-akermanite melilite solid solution. Phys, Chem. Miner., 39, 713-723. 

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