All of the distances between the atoms Ca and O (of the CO3 ion) in aragonite are substantially greater than the distances Ca-O in calcite.
From quantitative thermodynamic data, calculations show that calcite should convert to aragonite at 25ºC at a pressure of 2900 atmospheres (= 2,94 kilobars), and at one atmosphere, aragonite could be stable at extremely low temperatures close to absolute zero. Later, the results were confirmed through thermodynamic calculations based on new data, with the experimental verification of the state of aragonite as a phase of high pressure.
When the phase limit extends out as a line, the low temperature ends close to absolute zero: this means that at this point the crystalline energy of calcite and aragonite are almost identical and the difference of energy in higher temperatures is because of different thermal vibration mechanisms. The elevated energetic content of calcite can be explained by the greater freedom (vibration) of the CO3 groups, in contrast to aragonite where the CO3 are more cohesive and restricted (due to the packaging, as stated above).
Looking at the phase diagram, it becomes obvious that the formation of stable aragonite at temperatures of the earth’s crust is only possible under a considerable pressure, of millions of bars.
According to the aragonite-calcite phase diagram, and judging the thermodynamic properties of these phases, calcite is the stable modification of calcium carbonate under conditions on the earth’s surface. All aragonite formed under sedimentary conditions and in organisms should be considered as thermodynamically metastable in relation to calcite, to which it should eventually convert.
