Geochemical alteration of pyrochlore group minerals: Betafite subgroup
- Australian Nuclear Science and Technology Organisation, Menai, New South Wales (Australia)
- Univ. of New Mexico, Albuquerque, NM (United States)
Relatively low-temperature, secondary alteration is common in samples of metamict betafite and initially proceeds by the substitution mechanisms {sup A}Na{sup Y}F{yields} {sup A}{open_square}{sup Y}{open_square}, {sup A}Ca{sup Y}O {yields} {sup A}{open_square}{sup Y}{open_square}, and {sup A}Ca{sup X}O {yields}{sup A}{open_square}{sup X}{open_square}. Alteration is usually accompanied by hydration ({approximately}10-15 wt% H{sub 2}O) together 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 atoms per formula unit ({approximately}2.5-3.5 wt% CaO), betafite bulk compositions fall within the stability field of liandratite + uranpyrochlore + rutile (or anatase), thus promoting major element mobility (including Th, U, Pb, and B-site cations), incipient recrystallization, and partial dehydration. New phase assemblages occur as a function of decreasing bulk U content in the order liandratite + rutile, liandratite + uranpyrochlore + rutile, and uranpyrochlore + rutile. The same phase assemblages also occur in laboratory heating experiments performed in an inert atmosphere at 1000 {degrees}C. Up to 20-30% of the original amount of U may be lost during severe secondary alteration and recrystallization of betafite. Part of this U is retained by liandratite crystallized in the adjacent host rock. Loss of radiogenic Pb results from both long-term diffusion and secondary alteration, aided by radiation damage-induced volume expansion and microfracturing. 33 refs., 7 figs., 5 tabs.
- OSTI ID:
- 543076
- Journal Information:
- American Mineralogist, Journal Name: American Mineralogist Journal Issue: 9-10 Vol. 81; ISSN AMMIAY; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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