The evolution of metric parameters of 2:1 and 3:2 mullites have been measured between 4 K and 1320 K using neutron and X-ray powder diffraction. Negative thermal expansion was observed at low temperature for the a-cell parameter and consequently for the cell-volume, which is more pronounced for 2:1 mullite than those for 3:2 mullite. Each parameter is simulated using Gruneisen first-order approximation for the zero pressure equation of state at 0 K, where the vibrational energy was calculated using microscopic approach. While the b- and c-cell parameters require only one Debye term, a second Debye spectrum with negative Gruneisen parameter was required to fit the a-cell parameter as well as the cell volume. At 4 K, 300 K and 1320 K the model, respectively, calculates the volume thermal expansion coefficients of 0.09x10(-6) K-1, 9x10(-6) K-1, and 17.3x10(-6) K-1 for 2:1 mullite, and 0.09x10(-6) K-1, 8.7x10(-6) K-1, and 17.3x10(-6) K-1 for 3:2 mullite. Temperature-dependent Raman spectra and phonon density of states hint for the possible microscopic sources of the cell contraction at low temperature. A simple polynomial approach is presented to calculate the elastic stiffness coefficients of the 3:2 mullite, which are not available from experiments.