“…Minerals of the pyrochlore group conform to the simplified general formula where A = Ca, Na, actinides, rare earth elements (REE), Ba, Sr, Bi, Pb; 1?= Nb, Ta, Ti, Zr, Sb, W, Fe; X = O, OH, and Y = O, OH, F. On the basis of the B-site cations, three subgroups are commonly distinguished within the pyrochlore group [3]: (1) pyrochlore sensu stricto (Nb-rich), (2) rnicrolite (Ta-rich), and (3) betafite (Ti-rich). A fourth subgroup, rom6ite (Sb-rich), was recently suggested by Brugger et al [4]. Synthetic pyrochlore-group phases are also prominent actinide hosts in polyphase crystalline and certain glass-ceramic waste forms designed for the immobilization of high level nuclear waste [5,6].…”