The aragonite–calcite and calcite–lime transformations are observed in situ in the high voltage transmission electron microscope. Above 400°C aragonite single crystals begin to transform into calcite through heterogeneous nucleation on dislocations, twin boundaries, and modulated microstructures. Subsequent growth is generally topotactic with (100)A = (1120)C, (010)A ∼ (1104)C, (011)A ∼ (0001)C, and [001]A ∼ [r2:f1]C in the aragonite–calcite transition. Above 500°C calcite decomposes into lime and CO2. This reaction is also topotactic with (001)L ∼ (1104)C, and (111)L ∼ (0001)C. Movement of partial dislocations and propagation of stacking faults are not observed during the transformations. In situ observations therefore support a heterogeneous nucleation and interface-controlled growth mechanism rather than a martensitic mechanism for the aragonite–calcite transformation under the conditions of the experiment. Decomposition may proceed initially by loss of CO2 from a relatively rigid framework.

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