  The formation of lithium lines in the atmosphere of C-rich giants is
discussed. LTE and NLTE approximations are used to model lithium lines in the
spectra of super Li-rich AGB stars. The system of equations of the statistical
balance of neutral Li in plane-parallel model atmospheres is solved for a
20-level atom model. JOLA and line-by-line models of molecular absorption are
used to compute synthetic spectra as well as the opacity in the frequencies of
bound-bound and bound-free transitions of the lithium lines. Curves of growth
and synthetic spectra are computed in LTE and NLTE for several model
atmospheres of different Teff and C/O ratios for four lithium absorptions,
namely: the $\lambda 4603$, $\lambda 6104$, $\lambda 6708$ and $\lambda 8126$
{\AA} Li I lines. The sensitivity of the NLTE effects on Teff and the C/O ratio
is discussed. We found that NLTE mainly affects the resonance line doublet
($\lambda 6708$) while the impact of NLTE effects on the lithium subordinate
lines, formed in the inner regions of C-rich giant atmospheres, is rather weak.
Therefore the use of these lines is recommended for Li determination in AGB
stars. However, in no case can we properly account for the formation of Li
lines in AGB stars until sphericity, velocity stratifications, dust,
chromospheres and other related phenomena, which are in fact present in AGB
star atmospheres, are considered. Our results are used to derive Li abundances
in three super Li-rich C-stars taking into account NLTE effects. Finally, the
net Li yield from this class of stars into the interstellar medium is
reconsidered.
