Using synthesized MgCO3 and reagent-grade MnCO3 as starting materials, a series of Mg1-xMnxCO3 carbonate solid solutions were synthesized by a simple solid reaction under high-temperature-pressure conditions of 3 GPa and 800 degrees C for 4 h. The phase compositions of as-synthesized Mg1-xMnxCO3 samples were investigated by powder X-ray diffraction (XRD); no impurities were observed. The lattice parameters were refined and showed a linear relationship as a function of the Mn2+ content, which is expected to be in accordance with the ideal solution model. Based on this, high-temperature XRD measurements were carried out to further study the thermal expansivity of Mg1-xMnxCO3. The axis thermal expansion coefficients (alpha(a) and alpha(c)) and the volumetric thermal expansion coefficient alpha(V) for Mg1-xMnxCO3 were quantified as alpha(a) = 7.41 x 10(-6)/degrees C, alpha(c) = 2.37 x 10(-5)/degrees C and alpha(V) = 3.86 x 10(-5)/degrees C for x = 0.0; alpha(a) = 6.67 x 10(-6)/degrees C, alpha(c) = 2.31 x 10(-5)/degrees C and alpha(V) = 3.67 x 10(-5)/degrees C for x = 0.1; alpha(a) = 6.16 x 10(-6)/degrees C, alpha(c) = 2.35 x 10(-5)/degrees C and alpha(V) = 3.59 x 10(-5)/degrees C for x = 0.3; alpha(a) = 5.91 x 10(-6)/degrees C, alpha(c) = 2.40 x 10(-5)/degrees C and alpha(V) = 3.58 x 10(-5)/degrees C for x = 0.5; alpha(a) = 5.47 x 10(-6)/degrees C, alpha(c) = 2.53 x 10(-5)/degrees C and alpha(V) = 3.61 x 10(-5)/degrees C for x = 0.7; alpha(a) = 4.76 x 10(-6)/degrees C, alpha(c)= 2.55 x 10(-5)/degrees C and alpha(V) = 3.52 x 10(-5)/degrees C for x = 0.9; alpha(a) = 4.18 x 10(-6)/degrees C, alpha(c) = 2.50 x 10(-5)/degrees C and alpha(V) = 3.35 x 10(-5)/degrees C for x = 1.0. The thermal expansion coefficients (alpha(a), alpha(c), and alpha(V)) can be fitted with a symmetric cubic function of the Mn2+ content as alpha(a) = 7.34 x 10(-6)-7.06 x 10(-6)x + 1.21 x 10(-5)x(2)-8.19 x 10(-6)x(3); alpha(c) = 2.37 x 10(-6)-7.94 x 10(-6)x + 2.57 x 10(-5)x(2) - 1.64 x 10(-5)x(3); alpha(V) = 3.85 x 10(-5) - 2.08 x 10(-5)x + 4.59 x 10(-5)x(2) - 3.01 x 10(-5)x(3).