  Adhesion forces between submicrometer-sized silicate grains play a crucial
role in the formation of silicate dust agglomerates, rocky planetesimals, and
terrestrial planets. The surface energy of silicate dust particles is the key
to their adhesion and rolling forces in a theoretical model based on the
contact mechanics. Here we revisit the cohesion of amorphous silica spheres by
compiling available data on the surface energy for hydrophilic amorphous silica
in various circumstances. It turned out that the surface energy for hydrophilic
amorphous silica in a vacuum is a factor of 10 higher than previously assumed.
Therefore, the previous theoretical models underestimated the critical velocity
for the sticking of amorphous silica spheres, as well as the rolling friction
forces between them. With the most plausible value of the surface energy for
amorphous silica spheres, theoretical models based on the contact mechanics are
in harmony with laboratory experiments. Consequently, we conclude that silicate
grains with a radius of $0.1~\mu$m could grow to planetesimals via coagulation
in a protoplanetary disk. We argue that the coagulation growth of silicate
grains in a molecular cloud is advanced either by organic mantles rather than
icy mantles or, if there are no mantles, by nanometer-sized grain radius.
