Metamict and chemically altered vesuvianite

Partly metamict vesuvianite samples from two localities have been examined in detail. The unit cell of one is enlarged due to volume expansion caused by a buildup of radiation damage; the other specimen sustained enough damage to exclude accurate determination of cell parameters. Back-scattered electron imaging shows that both are chemically altered, and EPMA data indicate that alteration has resulted in a heterogeneous distribution of radionuclides on the mu m scale. Partly metamict vesuvianite recrystallizes over the range 600-850 degrees C; combined TGA and isothermal annealing show that, on heating (in N (sub 2) 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 samples of vesuvianite 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.