METAMICT AND CHEMICALLY ALTERED VESUVIANITE

Partly metamict vesuvianite samples from two localities were examined and compared. The unit cell of one is enlarged owing to volume expansi on caused by the buildup of radiation damage. The other sample sustain ed enough damage to exclude accurate determination of unit-cell parame ters. BSE imaging shows that both samples have undergone chemical alte ration, and electron-microprobe data indicate that the alteration has resulted in a heterogeneous distribution of radionuclides on the micro meter scale. HRTEM and SAED analyses reveal a wide variation in the ex tent of alpha-recoil damage, which corresponds to the heterogeneous di stribution of the radionuclides. The progressive stages of metamictiza tion also are observed in detail with TEM. Partly metamict vesuvianite recrystallizes over the range 600-850-degrees-C, which is broader tha n the range found in other metamict silicates. Combined thermogravimet ric analysis and isothermal annealing show that, upon heating (in N2 o r Ar), metamict vesuvianite begins to recrystallize at 600-degrees-C, and at 900-degrees-C decomposes into the multiphase assemblage grossul ar + gehlenite + wollastonite. Unpolarized IR spectra of both vesuvian ite samples are similar and also resemble those of radiation-damaged z ircon and titanite, suggesting that the major structural features of t he aperiodic state are similar for complex ceramics of different compo sition.