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We have derived a mean profile of the 13 mu m emission feature from 11 ISO spectra of oxygen-rich AGB stars and present an overview of the mineral species that could account for it. Our results can be summarized as follows: i) Spherical particles of alpha-Al_2O_3 (corundum) have a sharp emissivity maximum peaking at 12.7 mu m. The difference of 0.3 mu m in the peak position compared to the mean observed band profile is a serious obstacle to assigning the 13 mu m dust feature to this dust species. ii) From a continuous distribution of ellipsoidal alpha-Al_2O_3-particles, a broad emissivity profile peaking at 13-14 mu m emerges. It is difficult to reconcile the large width of this profile with the width of the observed 13 mu m band. iii) The most prominent emissivity maximum of TiO_2 (rutile) is located at 13.5 mu m; its width is larger and its strength three times smaller than the emissivity maximum of corundum. iv …