  Context: The formation of rocky planetesimals is a long-standing problem in
planet formation theory. One of the possibilities is that it results from
gravitational instability as a result of pile-up of small silicate dust
particles released from sublimating icy pebbles that pass the snow line. Aims:
We want to understand and quantify the role of the water snow line for the
formation of rock-rich and ice-rich planetesimals. In this paper, we focus on
the formation of rock-rich planetesimals. A companion paper examines the
combined formation of both rock-rich and ice-rich planetesimals. Methods: We
develop a new Monte Carlo code to calculate the radial evolution of silicate
particles in a turbulent accretion disk, accounting for the back-reaction
(i.e., inertia) of the particles on their radial drift velocity and diffusion.
Results depend in particular on the particle injection width (determined from
the radial sublimation width of icy pebbles), the pebble scale height and the
pebble mass flux through the disk. The scale height evolution of the silicate
particles, which is the most important factor for the runaway pile-up, is
automatically calculated in this Lagrange method. Results: From the numerical
results, we derive semi-analytical relations for the scale height of the
silicate dust particles and the particles-to-gas density ratio at the midplane,
as functions of a pebble-to-gas mass flux ratio and the $\alpha$ parameters for
disk gas accretion and vertical/radial diffusion. We find that the runaway
pile-up of the silicate particles (formation of rocky planetesimals) occurs if
the pebble-to-gas mass flux ratio is $> [(\alpha_{Dz}/\alpha_{acc})/3 \times
10^{-2}]^{1/2}$ where $\alpha_{Dz}$ and $\alpha_{acc}$ are the $\alpha$
parameters for vertical turbulent diffusion and disk gas accretion.
