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 expansion caused by the buildup of radiation damage. The other sample sustained enough damage to exclude accurate determination of unit-cell parameters. BSE imaging shows that both samples have undergone chemical alteration, and electron-microprobe data indicate that the alteration has resulted in a heterogeneous distribution of radionuclides on the micrometer scale. HRTEM and SAED analyses reveal a wide variation in the extent of alpha-recoil damage, which corresponds to the heterogeneous distribution of the radionuclides. The progressive stages of metamictization also are observed in detail with TEM. Partly metamict vesuvianite recrystallizes over the range 600-850-degrees-C, which is broader than the range found in other metamict silicates. Combined thermogravimetric analysis and isothermal annealing show that, upon heating (in N2 or Ar), metamict vesuvianite begins to recrystallize at 600-degrees-C, and at 900-degrees-C decomposes into the multiphase assemblage grossular + gehlenite + wollastonite. Unpolarized IR spectra of both vesuvianite samples are similar and also resemble those of radiation-damaged zircon and titanite, suggesting that the major structural features of the aperiodic state are similar for complex ceramics of different composition.