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We compute the mass and composition of dust produced by stars with masses inthe range 1Msun<M<8 Msun and with a metallicity of Z=0.001 during their AGB andSuper AGB phases. Stellar evolution is followed from the pre-main sequencephase using the code ATON which provides, at each timestep, the thermodynamicsand the chemical stucture of the wind. We use a simple model to describe thegrowth of the dust grains under the hypothesis of a time-independent,spherically symmetric stellar wind. We find that the total mass of dustinjected by AGB stars in the interstellar medium does not increasemonotonically with stellar mass and ranges between a minimum of 10^{-6}Msun forthe 1.5Msun stellar model, up to 2x10^{-4} Msun, for the 6Msun case. Dustcomposition depends on the stellar mass: low-mass stars (M < 3Msun) producecarbon-rich dust, whereas more massive stars, experiencing Hot Bottom Burning,never reach the carbon-star stage, and produce silicates and iron. This is inpartial disagreement with previous investigations in the literature, which arebased on synthetic AGB models and predict that, when the initial metallicity isZ=0.001, C-rich dust is formed at all stellar masses. The differences are dueto the different modelling of turbulent convection in the super-adiabaticityregime. Also in this case the treatment of super-adiabatic convection shows upas the most relevant issue affecting the dust-formation process. We alsoinvestigate Super AGB stars with masses 6.5Msun<M<8 Msun that evolve over a ONecore.Due to a favourable combination of mass loss and Hot Bottom Burning, thesestars are predicted to be the most efficient silicate-dust producers, releasing[2 - 7]x 10^{-4} Msun masses of dust. We discuss the robustness of thesepredictions and their relevance for the nature and evolution of dust at earlycosmic times.

The transition from carbon dust to silicates production in low-metallicity AGB and SAGB stars