  We review the use of elemental depletions in determining the composition of
interstellar dust and present a new interpretation of the elemental depletion
patterns for the dust forming elements in a range of diffuse cloud types. We
discuss this within the context of dust processing in the ISM and show that Si
and Mg are selectively eroded from dust, with respect to Fe, as expected for a
sputtering erosion process. However, we find that Si is preferentially and
non-stoichiometrically eroded from dust with respect to Mg by some as yet
unidentified process that may act in conjunction with grain sputtering. On this
basis a new way of interpreting the depletions in terms of `continuous' dust
processing through erosion in the interstellar medium is presented. The
observed depletion patterns can then be understood in terms of a gradually
changing grain chemical composition as the erosion of the atoms proceeds
non-stoichiometrically in the low-density interstellar medium. The
stoichiometric erosion of multicomponent (e.g., core/mantle) single grains can
qualitatively explain the observed depletions but is not consistent with the
preferential erosion of Si from dust. We present suggestions for the usage of
mineralogical terms within the context of interstellar and circumstellar dust
mineralogy.
