  We discuss a method for classifying the singularity types of 1/2 Calabi-Yau
3-folds, a family of rational elliptic 3-folds introduced in a previous study
in relation to various U(1) factors in 6D F-theory models. A projective dual
pair of del Pezzo manifolds recently studied by Mukai is used to analyze the
singularity types. In particular, we studied the maximal rank seven singularity
types of 1/2 Calabi-Yau 3-folds. The structures of the singular fibers are
analyzed using blow-ups. Double covers of the 1/2 Calabi-Yau 3-folds yield
elliptic Calabi-Yau 3-folds and applications to six-dimensional $N = 1$
F-theory on the Calabi-Yau 3-folds are also discussed. The deduced singular
fibers have applications in studying the gauge groups formed in 6D F-theory
compactifications. The blow-up methods used to analyze the singular fibers and
sections utilized in this research might have applications in studying the U(1)
factors and hypermultiplets charged under U(1) in 6D F-theory.
