Silicate Composition of the Interstellar Medium
  The composition of silicate dust in the diffuse interstellar medium and in
protoplanetary disks around young stars informs our understanding of the
processing and evolution of the dust grains leading up to planet formation.
Analysis of the well-known 9.7{\mu}m feature indicates that small amorphous
silicate grains represent a significant fraction of interstellar dust and are
also major components of protoplanetary disks. However, this feature is
typically modelled assuming amorphous silicate dust of olivine and pyroxene
stoichiometries. Here, we analyze interstellar dust with models of silicate
dust that include non-stoichiometric amorphous silicate grains. Modelling the
optical depth along lines of sight toward the extinguished objects Cyg OB2 No.
12 and {\zeta} Ophiuchi, we find evidence for interstellar amorphous silicate
dust with stoichiometry intermediate between olivine and pyroxene, which we
simply refer to as "polivene." Finally, we compare these results to models of
silicate emission from the Trapezium and protoplanetary disks in Taurus.
