  We determined the silicon abundances of 253 metal-poor stars in the
metallicity range $-4<\mathrm{[Fe/H]} <-1.5$, based on non-local thermodynamic
equilibrium (NLTE) line formation calculations of neutral silicon and
high-resolution spectra obtained with VLT-UT2/UVES. The $T_{\mathrm{eff}}$
dependence of [Si/Fe] noticed in previous investigation is diminished in our
abundance analysis due to the inclusion of NLTE effects. An increasing slope of
[Si/Fe] towards decreasing metallicity is present in our results, in agreement
with Galactic chemical evolution models. The small intrinsic scatter of [Si/Fe]
in our sample may imply that these stars formed in a region where the yields of
type II supernovae were mixed into a large volume, or that the formation of
these stars was strongly clustered, even if the ISM was enriched by single SNa
II in a small mixing volume. We identified two dwarfs with
$\mathrm{[Si/Fe]}\sim +1.0$: HE 0131$-$3953, and HE 1430$-$1123. These
main-sequence turnoff stars are also carbon-enhanced. They might have been
pre-enriched by sub-luminous supernovae.
