Fig. 2.

The observed gas-phase column densities of Mg⁺, Fe⁺, and Si⁺ in the local ISM indicate that although there is a wide variation in the relative amount of Fe⁺ when compared to Mg⁺ (left), the primary dust component has (Mg + Fe)/Si ~ 2 such as expected for olivine silicates (right). The comparable value for pyroxene is (Mg + Fe)/Si ~ 1. Grain destruction returns more Mg than Fe to the gas-phase. The dashed line in the left figure shows the ratio predicted by photoionization models of the LIC (Slavin and Frisch, 2008), where it is also predicted that 15% of the Mg is doubly ionized (Slavin and Frisch, 2008). The dust composition on the right is obtained by comparing the gas-phase abundances of the elements with solar abundances. The presence of iron oxides in the dust would help explain the highest ratios (This figure is courtesy of Seth Redfield, using results from (Redfield and Linsky 2002, 2004) and Frisch et al. (2011).).