Geochemical alteration of pyrochlore group minerals; pyrochlore subgroup

FERROTAPIOLITE AS A PSEUDOMORPH OF STIBIOTANTALITE FROM THE LASTOVICKY LEPIDOLITE PEGMATITE, CZECH REPUBLIC; AN EXAMPLE OF HYDROTHERMAL ALTERATION AT CONSTANT Ta/(Ta + Nb)

“…Values of Ta/(Ta + Nb) are typically found to be equal in the products of replacement of primary minerals such as stibiotantalite, simpsonite, manganocolumbite (Černý & Ercit 1985, Ercit 1986, Ercit et al 1986, Novák & Č erný 1998a, 2001, Smeds et al 1999, Cerný et al 2004, and they were also retained in pyrochlore-group minerals during geochemical alterations (Lumpkin & Ewing 1992. Uher et al (1998) and Baldwin (1989) are the only authors to have described minor changes in Ta/(Ta + Nb) between primary and secondary phases.…”

Hydroxycalciomicrolite, Ca1.5Ta2O6(OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

“…The general formula of the pyrochlore-supergroup minerals is A 2-m B 2 X 6-w Y 1-n , where m = 0 to 1.7, w = 0 to 0.7, n = 0 to 1 (Lumpkin and Ewing, 1995). In hydroxycalciomicrolite, the A, B and Y sites are dominated by Ca, Ta and OH, respectively.…”

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

“…Synthetic and natural pyrochlores, general formula A 2 B 2 O 7 , are of great interest for nuclear waste management because of their flexible crystal chemistry (Lian et al, 2001), chemical durability (Kamizono et al, 1991;Hayakawa and Kamizono, 1993;Yokoi et al, 1995;Icenhower et al, 2003;Roberts et al, 2000;Zhang et al, 2000) high waste loading capabilities (Lumpkin et al, 2000;Giere et al, 2002), and radiation resistance (Wang et al, 1999a,b;Sickafus et al, 2000;Weber and Ewing, 2000;Begg et al, 2001;Lian et al, 2001;Zhu et al, 2002;Ewing, 2005;Strachan et al 2005;Lumpkin, 2006). The pyrochlore supergroup has the space group Fd3m, and the chemical composition is described by the general formula of A 2-m B 2 X 6-w Y 1-n ·pH 2 O, where m = 0-1.7, w = 0-0.7, n = 0-1, and p = 0-2 (Hogarth, 1977;Lumpkin and Ewing, 1992;Lumpkin and Ewing, 1996;Atencio et al, 2010). The cubically coordinated A-site cations have larger ionic radii (0.086-0.155 nm; after Shannon, 1976) than the octahedrally coordinated B-site cations (ionic radii 0.060-0.083 nm).…”

“…The cubically coordinated A-site cations have larger ionic radii (0.086-0.155 nm; after Shannon, 1976) than the octahedrally coordinated B-site cations (ionic radii 0.060-0.083 nm). The A-site is usually occupied by Na, Ca, Sr, Pb, (Lumpkin and Ewing, 1995;Atencio et al, 2010). Classification of pyrochlore supergroup minerals is based on the relative proportions of Nb, Ta, and Ti in the B-site as follows: pyrochlore subgroup Ga in order to ascertain how pyrochlore-structured materials may behave as waste forms over geological time scales (Lumpkin and Ewing, 1988, 1996Giere et al, 2002;Hazen et al, 2008).…”

“…Classification of pyrochlore supergroup minerals is based on the relative proportions of Nb, Ta, and Ti in the B-site as follows: pyrochlore subgroup Ga in order to ascertain how pyrochlore-structured materials may behave as waste forms over geological time scales (Lumpkin and Ewing, 1988, 1996Giere et al, 2002;Hazen et al, 2008). Three different alteration stages of natural pyrochlore that characterize the secondary alteration stage (Lumpkin and Ewing, 1995;Lumpkin, 2001).…”