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(abridged) We present spectroscopy data for a sample of 14 FUors and 2 TTauristars observed with the Spitzer Space Telescope or with the Infrared SpaceObservatory (ISO). Based on the appearance of the 10 micron silicate feature wedefine 2 categories of FUors. Objects showing the silicate feature inabsorption (Category 1) are still embedded in a dusty and icy envelope. Theshape of the 10 micron silicate absorption bands is compared to typical dustcompositions of the interstellar medium and found to be in general agreement.Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, buta superposed emission feature can explain the observed shape. We derive opticaldepths and extinction values from the silicate band and additional ice bands at6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at15.2 micron allows us to search for evidence for ice processing and constrainswhether the absorbing material is physically linked to the central object or inthe foreground. For objects showing the silicate feature in emission (Category2), we argue that the emission comes from the surface layer of accretion disks.Analyzing the dust composition reveals that significant grain growth hasalready taken place within the accretion disks, but no clear indications forcrystallization are present. We discuss how these observational results can beexplained in the picture of a young, and highly active accretion disk. Finally,a framework is proposed as to how the two categories of FUors can be understoodin a general paradigm of the evolution of young, low-mass stars. Only oneobject (Parsamian 21) shows PAH emission features. Their shapes, however, areoften seen toward evolved stars and we question the object's status as a FUorand discuss other possible classifications.

Evolution of dust and ice features around FU Orionis objects