  The poor stickiness of silicate dust grains is a major obstacle to the
formation of rocky planetesimals. In this study, we examine the possibility
that silicate grains with an organic mantle, which we call Organic-Mantled
Grains (OMGs), form planetesimals through direct coagulation. Organic mantles
are commonly found in interplanetary dust particles, and laboratory experiments
show that they are softer than silicates, in particular in warm environments.
This, combined with the theory of particle adhesion, implies that OMGs are
stickier than bare silicate grains. Because organic mantles can survive up to
400 K, silicate grains inside the water snow line in protoplanetary disks can
in principle hold such mantles. We construct a simple grain adhesion model to
estimate the threshold collision velocity below which aggregates of OMGs can
grow. The model shows that aggregates of 0.1 \micron-sized OMGs can overcome
the fragmentation barrier in protoplanetary disks if the mantles are as thick
as those in interplanetary dust particles and if the temperature is above
$\sim$ 200 K. We use this adhesion model to simulate the global evolution of
OMG aggregates in the inner part of a protoplanetary disk, demonstrating that
OMG aggregates can indeed grow into planetesimals under favorable conditions.
Because organic matter is unstable at excessively high temperatures, rocky
planetesimal formation by the direct sticking of OMGs is expected to occur in a
disk annulus corresponding to the temperature range $ \sim$ 200--400 K. The
organic-rich planetesimals may grow into carbon-poor rocky planetesimals by
accreting a large amount of carbon-poor chondrules.
