  We present the chemical analysis of 49 giant stars of the globular cluster
NGC 2419, using medium resolution spectra collected with the multi-object
spectrograph DEIMOS@Keck. Previous analysis of this cluster revealed a large
dispersion in the line strength of the infrared Ca II triplet, suggesting an
intrinsic star-to-star scatter in its Fe or Ca content. From our analysis, we
assess that all the investigated stars share the same [Fe/H], [Ca/Fe] and
[Ti/Fe] abundance ratios, while a large spread in Mg and K abundances is
detected. The distribution of [Mg/Fe] is bimodal, with ~40% of the observed
targets having subsolar [Mg/Fe], down to [Mg/Fe] ~ -1 dex, a level of
Mg-deficiency never observed before in globular clusters. It is found that the
large dispersion in Mg abundances is likely the main origin of the observed
dispersion of the Ca II triplet lines strengths (that can be erroneously
interpreted in terms of Fe or Ca abundance scatter) because Mg plays a relevant
role in the atmosphere of giant stars as an electron donor. A strong depletion
in the Mg abundance leads to an increase of the line strength of the Ca II
triplet, due to the variation in the electronic pressure, at a constant Fe and
Ca abundance. Finally, we detect an anti-correlation between Mg and K
abundances, not easily explainable within the framework of the current
nucleosynthesis models.
