Using first principles quasi-harmonic theory, we have investigated the vibrational and thermodynamic properties of P21/c and C2/c MgSiO 3 clinoenstatites. Very good agreement between experimentally measured thermodynamic properties, such as equation of state, and those predicted by the local density approximation (LDA) functional has been found. The level of agreement found in these chain silicates is comparable to that found in framework silicates such as MgSiO3 perovskite. The phase boundaries calculated by the LDA and the generalized gradient approximation (GGA) bracket the experimentally measured ones following the established trend that the GGA functional overestimates the transition pressure while the LDA functional underestimates it. The calculated Clapeyron slope is 2.9 MPa K -1; and is used to constrain the position of the 3-pyroxene (P2 1/c, C2/c, and Pbca) triple point in MgSiO3, with the aid of room temperature experimental hysteresis in the P21/c to C2/c transition.