A large sample of 177 Miras, comprising 164 M and 13 S stars, has been examined to determine the dependence of 9.7 μm silicate emission, as revealed by their IRAS LRS Spectral class, on the visual light curve asymmetry factor, f. It is found that the silicate feature occurs not only in M (Vardya et al. 1986; Onaká & de Jong 1987) but in S Miras also only for f ≤ 0.45. This, however, is only a necessary condition, as about one fifth of Miras with f ≤ 0.45 do not show the 9.7 μm emission. This non-detection shows dependence on other parameters like the mean visual light amplitude. Non-detection is highest in the region 0.43 < f ≤ 0.45, as well as when mean amplitude is ≤ 5m.0. Though strong emission features in M Miras may occur for any value of f, very weak features are absent for small values of f, and the strongest feature tends to appear for large values of f. Infrared excess tends to increase with increase in the strength of the silicate emission and with decrease in the value of f.