From interstellar abundances to grain composition: the major dust
  constituents Mg, Si and Fe
  We analyse observational correlations for three elements entering into the
composition of interstellar silicate and oxide grains. Using current solar
abundances (Asplund et al. 2009), we convert the gas-phase abundances into
dust-phase abundances for 196 sightlines. We deduce a sharp difference in
abundances for sightlines located at low ($|b|<30\degr$) and high
($|b|>30\degr$) galactic latitudes. For high-latitude stars the ratios Mg/Si
and Fe/Si in dust are close to 1.5. For disk stars they are reduced to ${\rm
Mg/Si} \sim 1.2$ and ${\rm Fe/Si} \sim 1.05$. The derived numbers indicate that
1) the dust grains cannot be the mixture of silicates with olivine and pyroxene
composition only and some amount of magnesium or iron (or both) should be in
another population and 2) the destruction of Mg-rich grains in the warm medium
is more effective than of Fe-rich grains. We reveal a decrease of dust-phase
abundances and correspondingly an increase of gas-phase abundances with
distance $D$ for stars with $D\ga 400$\,pc. We attribute this fact to an
observational selection effect: a systematic trend toward smaller observed
hydrogen column density for distant stars. We find differences in abundances
for disk stars with low ($E({\rm B-V}) \la 0.2$) and high ($E({\rm B-V}) \ga
0.2$) reddenings which reflect the distinction between the sightlines passing
through diffuse and translucent interstellar clouds. For Scorpius-Ophiuchus we
detect an uniform increase of dust-phase abundances of Mg and Si with an
increase of the ratio of total to selective extinction $R_{\rm V}$ and a
decrease of the strength of the far-UV extinction. This is the first evidence
for a growth of Mg-Si grains due to accretion in the interstellar medium.
