Mid-infrared colors as a diagnostic tool of circumstellar envelopes in AGBstars

New observations of AGE circumstellar envelopes made with the mid-IR imagin g camera CAMIRAS are presented. Diagnostic tools based on mid-IR colors are proposed for the analysis of the observed objects, and tested against a la rger sample of AGE sources derived from IRAS observations. Radiative transf er modelling is used to calibrate our diagnostic procedures, in order to in vestigate the physical and chemical characteristics of both samples. The di fferent chemical signatures of C-rich and O-rich envelopes are put in evide nce by mid-IR color-color diagrams, and correlations are found between the observed colors and a mass loss parameter derived by radio observations. Ev idences for possible temporal variations in the mass loss rates are also su ggested. The possibility to derive direct information on the spatial struct ure and symmetry of the envelopes is finally investigated, and the image of the O-rich star WX Psc, as an example of a spatially resolved axisymmetric enveloper is presented.