  The metallicity distribution function of globular clusters (GCs) in galaxies
is a key to understanding galactic formation and evolution. The calcium II
triplet (CaT) index has recently become a popular metal abundance indicator
thanks to its sensitivity to GC metallicity. Here we revisit and assess the
reliability of CaT as a metallicity indicator using our new stellar population
synthesis simulations based on empirical, high-resolution fluxes. The model
shows that the CaT strength of old ($>$ 10 Gyr) GCs is proportional to ${\rm
[Fe/H]}$ below $-0.5$. In the modest metal-rich regime, however, CaT does not
increase anymore with ${\rm [Fe/H]}$ due to the little contribution from
coolest red giant stars to the CaT absorption. The nonlinear nature of the
color--$CaT$ relation is confirmed by the observations of GCs in nearby
early-type galaxies. This indicates that the CaT should be used carefully when
deriving metallicities of metal-rich stellar populations. Our results offer an
explanation for the observed sharp difference between the color and $CaT$
distributions of GCs in the same galaxies. We take this as an analogy to the
view that metallicity--color and metallicity--Lick index nonlinearity of GCs is
primarily responsible for their observed "bimodal" distributions of colors and
absorption indices.
