  Presently unaccounted but quite probable "chemical factor" may be responsible
for the formation of old metal-rich globular clusters (MRGCs) in spheroids, as
well as of their conterparts, young (intermediate-age) massive star clusters
(MSCs) in irregulars. Their formation presumably occurs at the same stage of
the host galaxies' chemical evolution and is related to the essentially
increased SF activity in the hosts around the same metallicity, Zsol/3
([Fe/H]~-0.5). It is achieved very soon in massive spheroids, later in
lower-mass spheroids, and (much) more later in irregulars.
