Experimental studies of Ta2O5 and columbite-tantalite solubility in fluoride solutions from 300 to 550oC and 50 to 100 MPaстатья
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Дата последнего поиска статьи во внешних источниках: 10 августа 2018 г.
Аннотация:It is well established that the fractionation of Li–F
granitic magmas at depth leads to the accumulation of flux
elements such as F and Li, and metal cations such as Ta and
Nb in residual melts. However, it remains to be determined
whethermagmatic fractionation is sufficient to concentrate Nb
and Ta into economically significant quantities, and what role
hydrothermal–metasomatic processes play in the formation of
such ore deposits. In the literature, reliable data about the
solubility of Ta and Nb in hydrothermal solutions is missing or
incomplete. This study provides a quantitative experimental
estimation of the possible contribution from hydrothermal
processes in Ta enrichment in cupolas of albitized and
greisenized Li–F granite. Experimental studies of Ta2O5
and columbite–tantalite (Mn,Fe)(Nb,Ta)2O6 solubility were
carried out in fluoride solutions consisting of HF, NaF, KF,
and LiF. At low fluoride concentrations (0.01 and 0.1 m),
Ta2O5 solubility at 550°C and 100 MPa under Co–CoO
oxidizing conditions is low (near 10−5–10−4 m) in all fluoride
solutions (HF, NaF, KF, LiF). At high fluoride concentrations
(1 and 2 m) the highest Ta2O5 concentrations (10−1 m) were
detected in HF solutions. In KF, NaF, and LiF solutions, the
Ta2O5 solubility is also high (10−3–10−2 m). The dependence
of columbite–tantalite (Nb2O5-59 wt. %, Ta2O5-18 wt. %)
solubility as a function of solution composition, T, and P has
also been investigated. Tantalum and Nb concentrations have
the highest values in HF solutions at reduced conditions (up
to 10−3 to 10−2 m Ta in 1 m HF). In 1 m NaF solutions, the
concentrations of Nb and Ta are, respectively, 2.5 and 3
orders of magnitude less than those in the 1 m HF solutions.
Solubility of Ta and Nb in KF solutions has intermediate
values. It is established that in NaF and KF solutions the
dependence of solubility on pressure is distinctly negative. The
Nb and Ta contents increase with increasing concentrations of
HF and KF in solution, however, they do not change with
increasing NaF concentration. In NaHCO3, Na2CO3, and HCl
solutions columbite–tantalite solubility is low. Even in 1 m
chloride solutions the content is within the limits of 10−5 m
for Nb and 10−6 to 10−8 m for Ta. We conclude that
hydrothermal transport of Ta and Nb is possible only in
concentrated fluoride solutions.