GEOCHEMICAL ALTERATION OF PYROCHLORE GROUP MINERALS - BETAFITE SUBGROUP

Relatively low-temperature, secondary alteration is common in samples of metamict betafite and initially proceeds by the substitution mechan isms (NaF)-Na-A-F-Y --> (A) square(Y) square, (CaO)-Ca-A-O-Y --> (A) s quare(Y) square, and (CaO)-Ca-A-O-X --> (A) square(X) square. Alterati on is usually accompanied by hydration (similar to 10-15 wt% H2O) toge ther with minor increases in Al, K, Mn, Fe, Sr, and Ba. At this stage, U and Th remain relatively unaffected by the alteration process. Once Na and F are removed and the Ca content drops below about 0.2-0.3 ato ms per formula unit (similar to 2.5-3.5 wt% CaO), betafite bulk compos itions fall within the stability field of liandratite + uranpyrochlore + rutile (or anatase), thus promoting major element mobility (includi ng Th, U, Pb, and B-site cations), incipient recrystallization, and pa rtial dehydration. New phase assemblages occur as a function of decrea sing bulk U content in the order liandratite + rutile, liandratite + u ranpyrochlore + rutile, and uranpyrochlore + rutile. The same phase as semblages also occur in laboratory heating experiments performed in an inert atmosphere at 1000 degrees C, Up to 20-30% of the original amou nt of U may be lost during severe secondary alteration and recrystalli zation of betafite. Part of this U is retained by liandratite crystall ized in the adjacent host rock. Loss of radiogenic Pb results from bot h long-term diffusion and secondary alteration, aided by radiation dam age-induced volume expansion and microfracturing.