Counting Silicon in the 9.7 Micron Feature: A Band-Strength Budget for Three Kinds of Sightline

The silicate absorption feature near 9.7 microns is the most direct measure of the silicate grain population, and its strength can be converted into a silicon column. This paper performs that conversion with a stated laboratory band strength for the silicon-oxygen stretch on three classes of sightline distinguished by their visual extinction per unit feature depth. The diffuse medium, at a ratio of 18.5, gives 24.1 parts per million of silicon in silicates per hydrogen, 0.745 of the cosmic silicon abundance of about 32 parts per million; a Galactic-centre-like ratio of 9 gives 49.6 parts per million, 1.531 of cosmic, which is more silicon than the cosmic supply contains. The diffuse result leaves room for silicon in the gas and in other solids, while the Galactic centre profile, taken at face value with the same band strength, demands an impossible column, signalling that band strength, profile shape, or environment differs there rather than that the accounting can be ignored. All values are computed, not measured.