Defect dependent properties in grossularite

Two natural silicate crystal of grossularite I and II, Ca ₃Al₂ (SiO₄)₃, differing by impurities were investigated. Samples of grossularite I and II, heat treated at 600°C/1h and irradiated with different doses present peaks TL at 145, 200, 250, 335 and 440°C. The glow curves differ, however, in shape; nevertheless, they have similar sublinear dependence with radiation dose and quite different behavior with the anneal temperature. The presence of iron explains the absorption bands at 370, 431 and 505 nm and the fast increase of the absorption to the blue. The band at 409 nm is typical of Mn²⁺. In this material frequently ferrous ions substitutes Ca, giving rise to the band near 1000 nm. Some partial substitutions of (OH)₄^-4 the site of (SiO₄)^-4 explains the bands around 2750nm. The EPR spectra are characterized by a single signal around g=2.004, being intense in I than in II. Mn²⁺signals can be observed superimposed to the large signal. The spectra show too signals in g=4.3 in I, and g=4.2 e 6.0 in II, attributed to Fe³⁺. The gamma-irradiation is to increase quickly the TL intensity, but this effect is not present in the OA and EPR properties.