  In this paper we investigate the problem of finding the minimum of edge
density in families of convex mosaics with unit volume cells in $3$-dimensional
Euclidean space. We solve this problem for two families. In particular, we
prove that a translative convex mosaic with unit volume cells has minimal edge
density if and only if it is a face-to-face mosaic generated by unit cubes. We
also investigate decomposable mosaics, defined as mosaics whose cells are
Minkowski sums of cells of two lower dimensional convex mosaics, and show that
the edge density of every normal decomposable mosaic with unit volume cells is
at least as large as that of a face-to-face mosaic with regular triangle-based
right prisms as cells. In addition, we use our method to determine the minimal
total edge length of a $3$-dimensional unit volume parallelohedron.
