At roomtemperature and ambient pressure, aragonite is a metastable polymorph of calcium carbonate CaCO3 whose crystallization occurs frequently in nature (shells,corals, mineral sediments, etc.). It transforms into calcite that is the stable polymorphof calcium carbonate when heated. Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years.

Aragonite's more compact structure is composed of triangular carbonate ion groups (CO3), with a carbon at the center of the triangle and the three oxygens at each corner.

Aragonite is technically unstable at normal surface temperatures and pressures. It is stable at higher pressures, but not at higher temperatures such that in order to keep aragonite stable with increasing temperature, the pressure must also increase. If aragonite is heated to 400 degrees C, it will spontaneously convert to calcite if the pressure is not also increased. Since calcite is the more stable mineral, why does aragonite even form? Well under certain conditions of formation,

the crystallization of calcite is somehow discouraged and aragonite will form instead. The magnesium and salt content of the crystallizing fluid, the turbidity of the fluid and the time of crystallization are decidedly important factors, but there are perhaps others. Such areas as sabkhas and oolitic shoals tend to allow significant amounts of aragonite to form. Also metamorphism that includes high pressures and low temperatures (relatively) can form aragonite. After burial, given enough time, the aragonite will almost certainly alter to calcite.

Sedimentologists are very interested in aragonite and calcite stability fields because the conversion of aragonite to calcite after deposition has a distinct effect on the character of the sedimentary rocks. The experimental result shows that aragonite may already be formed out of calcite at a depth of 15 km if the temperature is not much higher than 100° C.

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