The phase evolution of high purity natural aragonite from room temperature to 1 073 K and the thermal expansivity of both aragonite and calcite were studied by using high-temperature powder X-ray diffraction, and the difference and influencing factors of geological and biogenic aragonite were analyzed. The results show that orthorhombic aragonite transforms to trigonal calcite at 693~733 K with the critical temperature T_c=723 K and an increase in molar volume by 5.97%. The variation of volume thermal expansion coefficient of aragonite with the absolute temperature T(K) at 300~663 K is α(V) (10^-5/K)=3.59(79)+7.17(170)×10^-3 T. The expansion of aragonite is strongly anisotropic, with the axial expansion decreasing in order of α(c) > α(b) > α(a). The thermal expansion coefficients α (10^-5/K) of calcite at 733~973 K are α(V)=3.78(25), α(a)=-0.27(2), and α(c)=4.31(29), with negative thermal expansion for the a axis. The phase transition temperature of biogenic aragonite is 60~140 K lower than that of synthetic and geological aragonite, and that of pulverized aragonite is lower than that of single crystal and undisturbed aragonite. Pulverized biogenic aragonite and calcite have similar thermal expansion coefficients to non-biogenic aragonite and calcite; however, the uncrashed biogenic aragonite has a smaller thermal expansion coefficient.