  Aims. We use the spectra of more than 30,000 red giant branch (RGB) stars in
25 globular clusters (GC), obtained within the MUSE survey of Galactic globular
clusters, to calibrate the Ca II triplet (CaT) metallicity relation and derive
metallicities for all individual stars. We investigate the overall metallicity
distributions as well as those of the different populations within each
cluster.
  Methods. The Ca II triplet in the near-infrared at 8498, 8542, and 8662 AA is
visible in stars with spectral types between F and M and can be used to
determine their metallicities. In this work, we calibrate the relation using
average cluster metallicities from literature and MUSE spectra, and extend it
below the horizontal branch - a cutoff that has traditionally been made to
avoid a non-linear relation - using a quadratic function. In addition to the
classic relation based on V-VHB we also present calibrations based on absolute
magnitude and luminosity. The obtained relations are then used to calculate
metallicities for all the stars in the sample and to derive metallicity
distributions for different populations within a cluster, which have been
separated using so-called "chromosome maps" based on HST photometry.
  Results. We show that, despite the relatively low spectral resolution of MUSE
(R=1900-3700) we can derive single star metallicities with a mean statistical
intra-cluster uncertainty of ~0.12 dex. We present metallicity distributions
for the RGB stars in 25 GCs, and investigate the different metallicities of the
populations P3 (and higher) in so-called metal-complex or Type II clusters,
finding metallicity variations in all of them. We also detected unexpected
metallicity variations in the Type I cluster NGC 2808 and confirm the Type II
status of NGC 7078.
