Table 2.
Calorimetric data
| Sample description | Acid calorimetry in 5 N HCl, 26 °C | DSC | |||
| Type | ID | ΔHsln (kJ/mol) | ΔHcryst at 26 °C (kJ/mol) (ACC to calcite) | Tcryst (°C) | ΔHanhydACC-cryst at Tcryst (kJ/mol) * (anhydrous ACC to calcite) |
| Synthetic calcite | Calcite | −28.75 ± 0.26 (7) | - | - | - |
| More disordered synthetic ACC( more metastable) | ACC-1 | −50.70 ± 1.78 (5) | −21.47 ± 1.80 | 326 | −15.4 ± 0.4 |
| ACC-2 | −53.33 ± 1.38 (3) | −23.95 ± 1.40 | 327 | −13.3 ± 0.7 | |
| Less disordered Synthetic ACC (less metastable) | ACC-3 | −46.33 ± 0.93 (4) | −17.00 ± 0.97 | 329 | −15.2 ± 0.3 |
| ACC-4 | −46.38 ± 0.60 (5) | −17.09 ± 0.65 | 327 | −14.8 ± 0.9 | |
| ACC-5 | −46.81 ± 0.85(4) | −17.61 ± 0.89 | 313 | −12.8 ± 0.5 | |
| ACC-spicules (biogenic ACC) | spicule-48-1 to 48-3 | −42.50 ± 0.47 (3) (−43.84 ± 0.68) † | −12.86 ± 0.76 ‡ (−14.29 ± 0.97) ‡† | ||
| Calcite-spicules (biogenic calcite) | Spicule-72-1 and 72-2 | −29.60 ± 0.60 (5) (−29.54 ± 0.60) § | - | ||
| spicule-48-5 and 48-6 | −29.81 ± 0.48 (4)(−29.67 ± 0.48) § | - | |||
Extra decimal places are retained to prevent round-off error. Uncertainties are given as two standard deviations of the mean. Values in parentheses are the number of experiments. The symbols ΔHsln and ΔHcryst stand for the enthalpies of dissolution and crystallization respectively.
*Enthalpy of crystallization of anhydrous ACC to calcite (ΔHanhydACC-cryst) from exothermic DSC peak at crystallization temperature Tcryst.
†Values with 10% calcite and water corrections.
‡Enthalpy of crystallization of biogenic-ACC to biogenic-calcite (ΔHcryst-bio).
§Values with water correction (ΔH = -0.4 kJ/mol (50)).